Displays

A detailed 120 km deep electromagnetic joint inversion model for the ultra-slow Mohns Ridge was constructed combining controlled source- and magnetotelluric data. About one third of mid-ocean ridges have a spreading rate less than 20 mm/yr¹, but due to lack of deep imaging, factors controlling melting and mantle upwelling, depth to the lithosphere – asthenosphere boundary (LAB), crustal thickness and hydrothermal venting are not well understood for this class of ridges. Modern electromagnetic data have significantly improved understanding of fast-spreading ridges, but have not been available for the ultra-slow ridges. The new inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Though the upwelling pattern shows several of the characteristics of a dynamic system, instead it likely reflects passive upwelling controlled by slow and asymmetric plate movements.

Upwelling asthenosphere and melt are enveloped by the 100 Ωm contour denoted the electrical LAB (eLAB). This transition may represent a rheological boundary defined by a minimum melt content. We also find that a model where crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates can explain the thin crust below the ridge. Fluid convection extends for long lateral distances exploiting high porosity at mid crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultra-slow ridges. Further, active melt emplacement into ca 3 km thick oceanic crust culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at Loki´s Castle hydrothermal field.

How to cite: Johansen, S., Panzner, M., Mittet, R., Amundsen, H., Lim, A., Vik, E., Landrø, M., and Arntsen, B.: Electrical imaging of the Mohns Ridge in the Greenland Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22586, https://doi.org/10.5194/egusphere-egu2020-22586, 2020.

The active seismic method is a standard tool for studying the Earth’s lithosphere. On scales from centimetres to kilometres, academic research is generally interested in highly complex geological targets such as volcanic edifices, crustal faults or salt environments. In order to properly image these structures, large and expensive multichannel acquisitions with a high offset-to-target depth ratio are required. In practice, however, these are often hardly affordable for academic institutions, with the result that reflections often only poorly illuminate laterally variable structures, which in turn compromises imaging and interpretation. As in common practice, most of the processing and interpretational steps are tailored to the reflected wavefield, faint diffracted contributions are typically considered as an unwanted by-product.

In recent works, however, it has been shown that diffractions possess unique properties which bear the potential to overcome the aforementioned limitations. Wave diffraction occurs at geodynamically important features like faults, pinch-outs, erosional surfaces or other small-scale scattering objects and encodes sub-wavelength information on the scattering geometry. Since diffracted waves do not obey Snell’s Law, they provide superior illumination compared to reflected waves. Moreover, due to their passive-source like radiation, they encode their full multichannel response in prominent data subsets like the zero-offset section. In order to explore what can be learned from the faint diffracted wavefield, we use academic seismic data from the Santorini-Amorgos Tectonic Zone (SATZ) in the Southern Aegean Sea. This is an area well known for its local complexity, indicated by the occurrence of extended fault systems and volcanic edifices as well as a complex acoustic basement. As the available seismic data in this region were acquired using a relatively short streamer, the SATZ represents a classical example for the need of innovative methods for seismic processing and interpretation.

By means of a robust and computationally efficient scheme for the extraction of diffractions that models and adaptively subtracts the reflected wavefield from the data, we reveal a rich diffracted wavefield from zero-offset data. On the one hand, we use the diffraction-only sections for analysing the small-scale structural complexity and demonstrate that the geological interpretation can benefit from these observations. On the other hand, we use the diffractions to estimate insightful wavefront attributes in the zero-offset domain. Based on these attributes, we perform wavefront tomography to obtain depth-velocity models. Compared to depth-velocity models derived from the reflected contributions, the diffraction-based velocity model fits the data significantly better. After refining this velocity model, we perform prestack depth migration and obtain highly valuable depth converted seismic sections. Concluding our results, we strongly encourage the incorporation of diffractions in standard processing and interpretational schemes.

How to cite: Preine, J., Schwarz, B., Bauer, A., Gajewski, D., and Hübscher, C.: Seismic diffraction imaging - a case study from the Southern Aegean Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-538, https://doi.org/10.5194/egusphere-egu2020-538, 2020.

Investigations of the deep lithosphere aiming at the reconstruction of the geological models remain one of the key sources of the knowledge about the processes shaping the outer shell of our planet. Among different methods, the active seismic Ocean-Bottom Seismometer (OBS) experiments conducted in wide-angle configuration are routinely employed to better understand these processes. Indeed, long-offset seismic data, combined with computationally efficient travetime tomographic methods, have a great potential to constrain the macro-scale subsurface velocity models at large depths. 

On the other hand, decades of development of acquisition systems, more and more efficient algorithms and high-performance computing resources make it now feasible to move beyond the regional raytracing-based traveltime tomography. In particular, the waveform inversion methods, such as Full-Waveform Inversion (FWI), are able to exhaustively exploit the rich information collected along the long-offset diving and refraction wavepaths, additionally enriched with the wide-angle reflection arrivals. So far however, only a few attempts have been conducted in the academic community to combine wide-angle seismic data with FWI for high-resolution crustal-scale velocity model reconstruction. This is partially due to the non-convexity of FWI misfit function, which increases with the complexity of the geological setting reflected by the seismograms. 

In its classical form FWI is a nonlinear least-squares problem, which is solved through the local optimization techniques. This imposes the strong constraint on the accuracy of the starting FWI model. To avoid cycle-skipping problem the initial model must predict synthetic data within the maximum error of half-period time-shift with respect to the observed data. The criterion is difficult to fulfil when facing the crustal-scale FWI, because the long-offset acquisition translates to the long time of wavefront propagation and therefore accumulation of the traveltime error along the wavepath simulated in the initial model. This in turns increases the possibility of the cycle-skipping taking into account large number of propagated wavelengths.

Searching to mitigate this difficulty, here we investigate FWI with a Graph-Space Optimal Transport (GSOT) misfit function. Comparing to the classical least-squares norm, GSOT is convex with respect to the patterns in the waveform which can be shifted in time for more than half-period. Therefore, with proper data selection strategy GSOT misfit-function has potential to reduce the risk of cycle-skipping. We demonstrate the robustness of this novel approach using 2D wide-angle OBS data-set generated in a GO_3D_OBS synthetic model of subduction zone (30 km x 175 km). We show that using GSOT cost-function combined with the multiscale FWI strategy, we reconstruct in details the highly complex geological structure starting from a simple 1D velocity model. We believe that further developments of OT-based misfit functions can significantly reduce the constraints on the starting model accuracy and reduce the overall risk of cycle-skipping during FWI of wide-angle OBS data.

How to cite: Górszczyk, A., Métivier, L., and Brossier, R.: Relaxing the initial model constraint for crustal-scale full-waveform inversion with graph-space optimal transport misfit function, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7743, https://doi.org/10.5194/egusphere-egu2020-7743, 2020.

Can we image and monitor the internal ocean structure at global scales? Can we monitor in vast expanses of the Earth’s cryosphere subsurface with meter-length resolution? Can we characterize the interior structures of asteroids and comets out in space efficiently and with high confidence? At the core of these questions lies the understanding and development of wave-based imaging systems, based on seismic or radar, that rely on highly-sparse, high-quality data, but whose output image quality is comparable to that of densely sampled, wide aperture array-based data. Traditionally, exploration seismology has long relied on wide aperture, dense data sets together with high-end imaging such as reverse-time migration and full-waveform inversion to produce high resolution subsurface models. Given the recent rise of drone-like, autonomous systems, in this talk, we present approaches that can take highly-sparse data as would be recorded by autonomous platforms, into accurate high-resolution images as if they had been acquired by densely-sampled, wide aperture source and receiver arrays. We demonstrate two approaches that could achieve this goal. The first is the use of sparse multicomponent sources and receivers capable of exciting/recording fields and their spatial gradients, together with a gradient-based wavefield reconstruction approach and subsequent imaging. The second approach relies on a new deep learning architecture, the so-called Recurrent Inference Machine, designed specifically for inverse problems – showing that it can surpass the capabilities of deterministic approaches to data reconstruction and imaging. We illustrate these approaches using a numerical model for oceanic turbulence, where we show the compressive sensing potential of these acquisition, reconstruction and imaging methods for acoustic imaging of the ocean's internal structure – overcoming current limitations in data acquisition and processing for seismic oceanography. Finally, we postulate that these approaches, though still in their early days, will pave the way in enabling breakthrough imaging systems at the frontiers of geo-imaging, e.g., for oceanography at global scales, in imaging the Earth’s cryosphere or for planetary exploration.

How to cite: Vasconcelos, I., Ruan, J., Kuijpers, D., Ravasi, M., and Putzky, P.: Towards highly-sparse, autonomous imaging systems: high-resolution wavefield imaging for frontier exploration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18782, https://doi.org/10.5194/egusphere-egu2020-18782, 2020.

Coda-wave interferometry, introduced by Snieder and co-workers, employs the relative high sensitivity of the scattering coda in an acoustic or seismic response to time-lapse changes of the propagation velocity and/or structure. It has been successfully applied at many scales, ranging from inferring temperature changes in granite samples, via structural health monitoring of bridges, to monitoring the minute changes in the interior of a volcano prior to eruption. Whereas in most situations the velocity changes are assumed to take place in a large region, it has been shown that coda-wave interferometry can also be used to image a local perturbation of the propagation velocity or structure. The latter approach assumes diffuse waves and employs an array of receivers that surrounds the perturbation.

We investigate the application of coda-wave interferometry for monitoring of fluid-flow processes in aquifers, geothermal reservoirs, CO₂-storage reservoirs and hydrocarbon reservoirs. In these applications the velocity perturbation is local, but the medium is probed with deterministic seismic body waves from the surface only. The location of the velocity perturbation is usually reasonably well known, but it is practically impossible to identify events in the coda that are directly related to the local perturbation. Recently we introduced the Marchenko method, which retrieves information about multiple scattering from reflection data at the surface in a data-driven way. Here we propose to use the Marchenko method to remove the response from the areas above and below the local velocity perturbation. In this way we isolate the scattering coda of the local velocity perturbation, which enables the application of coda-wave interferometry to monitor the fluid-flow process.

How to cite: Wapenaar, K. and van IJsseldijk, J.: Coda-wave interferometry and the Marchenko method , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20123, https://doi.org/10.5194/egusphere-egu2020-20123, 2020.

The goal of most seismic experiments is to use data readily available at the surface of the Earth to characterise the inaccessible interior. In order to solve this inverse problem, we generally make a number of assumptions about either the data or the Earth to simplify the physics. For example, we often assume that the Earth is an acoustic medium rather than an elastic medium, which for data without S-waves makes the problem far more tractable computationally than the full elastic problem.

One of the most common assumptions made about the data is the single-scattering assumption, widely known as the Born approximation. Clearly this is invalid in the presence of multiple scattering, which occurs in all seismic experiments. Despite this, the majority of imaging and inversion methods applied to seismic data are dependent on this assumption, including most full waveform inversion algorithms. As a consequence, seismic data processing requires a great deal of effort to remove multiply scattered waves from data.

A key justification for making this assumption is that a priori we can only estimate a relatively smooth Earth model that does not predict multiply scattered waves. However, with the recent emergence of so-called Marchenko methods, we now have access to full Green’s functions between sources and receivers at the Earth’s surface and virtual source or receiver locations inside the Earth’s interior, Green’s functions which can be estimated using only recorded reflection data and an estimate of the direct (non-scattered) wavefield travelling into the subsurface. As Marchenko methods become more commonplace, our justification for the single-scattering assumption diminishes, and hence we require new methods to use this information.

By iterating the Lippmann-Schwinger equation, we define a new compact form of the Frechét derivative of the Green’s function that involves all orders of scattering. In combination with Green’s functions obtained by a Marchenko method, these may be used for imaging and inversion of seismic data. We will describe an example of such a scheme, which we call “Marchenko Lippmann-Schwinger Full-Waveform Inversion”, to demonstrate how our redefined Green’s function derivative may be applied to solve seismic inverse problems for the Earth’s subsurface structure.

How to cite: Cummings, D. and Curtis, A.: Defining the Green’s function derivative for imaging & inversion without the Born approximation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10804, https://doi.org/10.5194/egusphere-egu2020-10804, 2020.

It has been widely recognized that the cross-correlation function (CCF) of ambient noise data recorded at two seismic stations approximates to the part of Green’s Function between these two stations. Theoretically, the CCF should include the higher modes, apart from the fundamental mode. However, currently well-known and mature methods that can extract dispersion curves are not pretty proficient in extracting higher modes. Fortunately, our newly proposing method, the Frequency-Bessel Transform Method (F-J Method), has presented its obvious advantage in extracting higher modes. This study applied F-J method to seismic ambient noise data for the east of South China, including Jiangnan Orogen and South China Fold System. We have acquired higher modes, not to mention the fundamental mode with wider frequency than previous studies. Combining both fundamental mode and higher modes, we used L-BFGS inversion method to inverse and acquire more accurate crustal and upper-mantle structure than previous studies only adopting fundamental mode for the east of South China. As shown in this study for the east of South China, we can use F-J method  to conveniently and precisely extract multimodes from ambient noise data and thus add more constrains for inversion results, which can significantly improve the preciseness of imaging crustal and upper-mantle structure.

How to cite: Chen, J. and Chen, X.: Extracting higher-mode dispersion curves from ambient noise data by using F-J method to acquire more accurate crustal and upper-mantle structure for the east of South China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5407, https://doi.org/10.5194/egusphere-egu2020-5407, 2020.

In this work, we present results from waveform tomography conducted in the Mediterranean and Southeast Asia. Whilst computationally more expensive than ray-based imaging methods, the advantage of waveform methods lies in their ability to incorporate in a consistent manner all the information contained in seismograms – not just the arrivals of certain, specified phases. We can therefore naturally and coherently exploit body and multimode surface waves, and take into account source effects, frequency-dependence, wavefront healing, anisotropy and attenuation.

Here, we look at applications of this method in two geologically complex regions: the Mediterranean and Southeast Asia. Both are characterised by broadscale convergence and a complicated pattern of interactions between larger and smaller-scale tectonic plates.

The Mediterranean is historically one of the best studied areas in the world, with an impressive density of seismic stations which greatly aids the detailed imaging of the region. We have been able to image the Central and Eastern Mediterranean down to the mantle transition zone, thereby illuminating the complex slab structures and geometries within the domain. We identify several main slabs that correspond to major current and former subduction zones.

In Southeast Asia, we work at a larger scale, with a model domain encompassing the Sunda arc (which gives rise to some of the world’s most significant natural hazards), the Banda arc with its spectacular 180° curvature and various smaller-scale features, such as the tectonically complex island of Sulawesi. To date, sparse instrument coverage in the region has led to a heterogeneous path coverage, in particular around Borneo which is located in an intra-plate setting. A recent series of temporary seismometer deployments in Sabah (North Borneo), Kalimantan, Sulawesi and the Celebes Sea allows us to fill the gaps in the publicly available data, thereby providing new opportunities to investigate the region's complexity using waveform tomography.

In this presentation, we will also discuss a number of features and “best practices” that can significantly influence waveform tomography results. In particular, we highlight how we can optimise sensitivity to deep structure by combining long-period data with a window selection approach that specifically targets body wave signals, and we discuss the effect of uncertainties in earthquake source parameters on the seismic inversion process.

How to cite: Blom, N., Fichtner, A., Gokhberg, A., Rawlinson, N., and Wehner, D.: Waveform tomography in the Mediterranean and Southeast Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-967, https://doi.org/10.5194/egusphere-egu2020-967, 2020.

Body-wave tomography with data from regional networks, also called ACH tomography, has provided a lot of information on the upper mantle structure, especially in continental settings. A key factor in this technique is the usage of relative residuals, not absolute ones. It is based on the assumption that travel times at stations in a regional network are affected in a similar way by errors in source location and origin time, as well as by large-scale heterogeneities in the lower mantle, and that demeaning the residuals strongly reduces the influence of these factors on the inverse regional model. This results in the well-known fact that the final velocity model is relative to an unknown vertically varying reference model. This also prevents combining data obtained with networks in the vicinity of each other but operating at different times, even though we may have stations, for example permanent stations, which are common to these networks. This is because the residuals at the two networks are measured with respect to different unknown averages and cannot be inverted together. This is very unfortunate as we have numerous examples of asynchronous network deployments which together would provide a much more useful station coverage and model than independently.

I will analyse how a simple change in the formulation of the direct problem allows to take into account that the residuals are demeaned, and, most importantly, how this can be used to remedy the limitations of regional body-wave tomographic methods in terms of asynchronous station deployment. I will first illustrate this with a very simple example and then present the results of a more realistic synthetic test combining data from two neighboring asynchronous networks in a single inversion.

How to cite: Maupin, V.: Combining asynchronous data sets in regional body-wave tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8571, https://doi.org/10.5194/egusphere-egu2020-8571, 2020.

The Receiver Function (RF) technique, that aims to isolate P to S teleseismic converted waves, is largely used to image seismic discontinuities at depth. In particular, in subduction zones, the subducting crust has often be identified on RF as a Low Velocity Layer (LVL) embedded between the mantle of the overriding plate and the mantle of the subducting lithosphere. In several subduction zones, a high Vp/Vs ratio inside this LVL has been estimated from the arrival times of the primary and backscattered P to S converted waves at the top and at the base of the LVL. However seismograms are filtered to enhance the signal over noise ratio and this processing step can dramatically reduce the resolution of the converted waves. In order to check if the signal periods associated to common filters could lead to an overestimation of the Vp/Vs ratio, a wavelet response in conversion for primary and backscattered converted waves is developed for a LVL typical of an oceanic crust. This multiscale analysis allows to illustrate that the LVL characteristics can be misinterpreted for the common frequency range due to interferences between the converted waves at the top and at the base of the LVL. For a dominant period of about 3s, the Vp/Vs of a typical oceanic crust can be largely overestimated (about Vp/Vs=2.8 instead of Vp/Vs=1.8) and its thickness underestimated (about 5 km instead of 7 km). The characteristics of a typical oceanic crust can be reliably retrieved only in the non interaction domain that corresponds to a constant spacing between the converted waves at the top and at the base of the LVL. This non-interaction domain corresponds to dominant signal period smaller than 1 s for the primary converted waves and 3 s for the backscattered. As the Vp/Vs is generally estimated based on the interpretation of both primary and backscattered waves, the period of 1 s is required for a reliable interpretation. The multiscale approach is applied to a real data example of teleseismic events recorded at a 3-component seismometer in order to reliably constrain the Vp/Vs ratio and the thickness of the oceanic crust at the top of the Hellenic subduction.

How to cite: Gesret, A.: How resolving are teleseismic forward and backscattered P to S converted waves?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14071, https://doi.org/10.5194/egusphere-egu2020-14071, 2020.

A Receiver Function Analysis was carried out in the Mjøsa area, Eastern Norway, in order to better image this tectonically complex area, understand the crustal contrasts and complement geological analysis that were made previously in the area. For this, we used seismic traces received for seven broadband stations from the NORSAR permanent array. The H-K (depth vs Vp/Vs) stacking procedure and a Reversible jump Markov chain Monte Carlo (Rj-McMC) inversion were developed independently. The first analysis allows us to obtain a model with the Mohorovicic discontinuity values under each seismic station and the average Vp/Vs crustal ratio. With the inversion, it was possible to develop a 1D local velocity model. Applying the Nafe-Drake relationship, a 2D density model was obtained and tested against observed gravity. Results indicate the presence of a low anomalous density layer that is located to the NNW of the study area, which is probably related to low-density meta-sediments in the Åsta Basin located above the basement. A main crustal fault is also indicated from the density model, spatially coinciding with faults grown during the Sveconorwegian orogenic process.

How to cite: Pavez, C., Brönner, M., Olesen, O., and Bjørlykke, A.: Moho Topography and Velocity/Density Model for the Hedmarken area, Eastern Norway, using Receiver Function Analysis and Rj-McMC, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4468, https://doi.org/10.5194/egusphere-egu2020-4468, 2020.

We determine the mantle attenuation (1/Q) structure beneath 70 Myr seafloor in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: in the shallow layer, in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric measured at higher frequency (~3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (V_S) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (f_e) and determine, at each depth, which values of f_e predict and V_S that can fit the NoMelt and V_S values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in f_e by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in f_e reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.   

How to cite: Ma, Z., Dalton, C., Russell, J., Gaherty, J., Hirth, G., and Forsyth, D.: Shear attenuation and anelastic mechanisms in the central Pacific upper mantle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5302, https://doi.org/10.5194/egusphere-egu2020-5302, 2020.

Cape Verde is an intraplate archipelago located in the Atlantic Ocean about 560 km west of Senegal, on top of a ~130 Ma sector of the African oceanic lithosphere. Until recently, due to the lack of broadband seismic stations, the upper-mantle structure beneath the islands was poorly known. In this study we used data from two temporary deployments across the archipelago, measuring the phase velocities of Rayleigh-waves fundamental-modes in a broad period range (8–250 s), by cross-correlating teleseismic earthquake data between pairs of stations. Deriving a robust average, phase-velocity curve for the Cape Verde region, we inverted it for a shear-wave velocity profile using non-linear gradient search.

Our results show anomalously low velocities of ∼4.2 km/s in the asthenosphere, indicating the presence of high temperatures and, eventually, partial melting. This temperature anomaly is probably responsible for the thermal rejuvenation of the oceanic lithosphere to an age as young as about 30 Ma, which we inferred from the comparison of seismic velocities beneath Cape Verde and the ones representing different ages in the Central Atlantic.

The present results, together with previously detected low-velocity anomalies in the lower mantle and relatively He-unradiogenic isotopic ratios, also suggest a hot plume deeply rooted in the lower mantle, as the origin of the Cape Verde hotspot.

The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and FIRE project Ref. PTDC/GEO- GEO/1123/2014.

How to cite: Carvalho, J., Bonadio, R., Silveira, G., Lebedev, S., Custódio, S., Mata, J., Arroucau, P., Meier, T., and Celli, N.: High temperature in the upper mantle beneath Cape Verde as a possible cause for the oceanic lithosphere rejuvenation inferred from Rayleigh-wave phase-velocity measurements., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5754, https://doi.org/10.5194/egusphere-egu2020-5754, 2020.

The northeast Asia region exhibits complex tectonic settings caused by interactions between Eurasian, Pacific, and Philippine Sea plates. Distributed extensional basins, intraplate volcanoes and other heterogeneous features in the region marked results of the tectonic processes, and their mechanisms related to mantle dynamics can be well understood by estimating radial anisotropy in the lithospherie and asthenospherie. We constructed a three-dimensional radial anisotropy model in northeast Asia using hierarchical and transdimensional Bayesian joint inversion techniques with different types of dispersion data up to the depth of the upper mantle (~ 160 km). Thick and deep layers with positive radial anisotropy (V_SH > V_SV) were commonly found at depths between 70 and 150 km beneath the continental regions. On the other hand, depths and sizes of layers with positive radial anisotropy become shallower and thinner (30 ~ 60 km) respectively beneath regions where experienced the Cenozoic extension. These variations in positive radial anisotropy for different tectonic regions can be understood with the context of extensional geodynamic processes in back arc basins within the East Sea (Japan Sea). Interestingly, the most predominant positive radial anisotropy is imaged along areas with large gradient of the litheosphere-asthnosphere boundary beneath intraplate volcanoes. These observations favor the mechanism of edge-driven convection caused by the difference in lithosphere thickness and localized sublithospheric lateral flow from the continental region to back arc basins.

How to cite: Lee, S.-J., Kim, S., and Rhie, J.: Anisotropic upper mantle structures in northeast Asia from Bayesian inversions of ambient noise data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6466, https://doi.org/10.5194/egusphere-egu2020-6466, 2020.

Parker–Oldenburg algorithm was applied to gravity data in the western USA to map the Moho discontinuity. F-H Parasnis method was also applied to the gravity data to estimate the Bougeur reduction density for calculation of the Bougeur gravity anomaly. The inversion process uses the Oldenburg equation (Eq.2: Oldenburg, 1974), which is a rearrangement of Parker’s equation (Eq.1: Parker, 1973), to estimate the depth to the undulating interface from the gravity anomaly by means of an iterative process. These formulas are shown as follows,

where F(Δg) is the Fourier transform of the gravity anomaly,  G is the gravitational constant, ρ is the density contrast across the interface,  K is the wave number,  h(x) is the depth to the interface (positive downwards) and z₀ is the mean depth of the horizontal interface.

The resulted Moho depth map shows depths ranging from 9 km to 50 km. Moho anomalies showed good spatial correlation with the major physiographic provinces in the study area. The subduction trench of the Farallon remnants (Juan de Fuca and Gorda) was mapped at the north of the Mendocino Triple Junction (MTJ). The subducting plates show north-east dipping direction with low dipping angle. The effect of the subduction appears in the structure at the northern part (i.e. Cascade Mountain, Walla-Walla plateau and Northern Rocky Mountains), whereas the southern part is affected by the transform movement of the Pacific Plate yielding a set of basins (Central Valley, Great Basin and Wyoming Basin). Results of this research, in conjunction with other information of the area, provide a new information for the analysis of the tectonic framework of the western North-America.

References

Oldenburg, D.W., 1974. The inversion and interpretation of gravity anomalies. Geophysics 39 (4), 526–536.

Parker, R.L., 1973. The rapid calculation of potential anomalies. Geophysical Journal of the Royal Astronomical Society 31, 447–455

How to cite: Shehata, M. and Mizunaga, H.: Application of Parker-Oldenburg Algorithm to map Moho Discontinuity using Gravity Data in Western USA , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-643, https://doi.org/10.5194/egusphere-egu2020-643, 2020.

We collected new seismological and gravity data in the Val Sesia and Lago Maggiore regions in NW Italy to constrain the geometry and properties of the Ivrea Geophysical Body. This piece of lower Adriatic lithosphere is known to be at anomalously shallow depth along the inner arc of the Western Alps, yet existing seismological constraints (vintage seismic refraction data, local earthquake tomography) are spatially sparse. With the aim to reach higher spatial resolution in imaging the structure of the IGB, we analyze the seismological data with various receiver function approaches to map the main velocity discontinuities, followed by joint inversion with gravity data to fill the bulk properties of bodies with densities.

The new data acquisition consisted of two type of campaigns. For seismology, we deployed 10 broadband seismic stations (MOBNET pool, IG CAS Prague) along a linear West-East profile at 5 km spacing along Val Sesia and across the Lago Maggiore. This network continuously recorded seismic data for 27 months at 100 Hz sampling rate. For gravimetry, we compiled existing datasets and then completed the spatial gaps by relative gravity surveys, tied to absolute reference points, to achieve 1 gravity point every 1-2 km along the profile.

The receiver function (RF) analyses aim at detecting velocity increases with depth: primarily the Moho and the shallow IGB interfaces and their crustal reverberations (multiples), together with their potential dip by analyzing the transverse component RFs. Furthermore, we aim at investigating the sharpness of the velocity gradient across the discontinuities by analyzing the frequency dependence of the corresponding RF peaks. We aim at reproducing the observations by simple synthetic models.

The 2D joint inversion combines S wave velocity V_S and bulk density as physical parameters to match both the seismological and gravimetry data. The relationship between the two parameters is initially chosen from the literature, but depending on the first results the relation itself may be inverted for, considering the various high-grade metamorphic rocks observed at the surface in the area, whose properties may not align with classical V_S–density equations. In conclusion, we propose new constraints on the IGB, demonstrating the advantage of using multi-disciplinary geophysical observations and improved data coverage across the study area.

How to cite: Scarponi, M., Hetényi, G., Plomerová, J., Solarino, S., and Baron, L.: Receiver function analyses and joint inversion with gravity data: new constraints on the Ivrea geophysical body along a high-resolution profile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19375, https://doi.org/10.5194/egusphere-egu2020-19375, 2020.

A novel geotomography technique has been applied in and around Macedonia using selected earthquakes that occurred over a period of 40 years and were recorded on 47 seismograph stations. The aim was to test this new tomography method for the first time in investigation of the crustal shape and structures in that specific tectonic environment with an extensive dataset.

A three-dimensional velocity model and many cross-sections of the crust were produced by this methodology and compared with the previous models of Macedonia. They show the potential of the tomography application in revealing geological features on local and regional scale. 

The new images will contribute towards a better understanding of the seismicity and tectonics in that part of the Balkans and assist in the process of integrated seismic hazard assessment.

Keywords: Geotomography, Earthquakes, Seismic Imaging

How to cite: Sinadinovski, C., Pekevski, L., Abdullah, A., and McCue, K.: Seismic Tomography of Macedonia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1817, https://doi.org/10.5194/egusphere-egu2020-1817, 2020.

The Ordos block is located on the west side of the North China Carton, adjacent to the northeastern part of the Tibetan Plateau. Affected by two tectonic movements, Ordos block internal structure remains relatively stable structure, but surrounded by active tectonic belts. With the development of the second and third part of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been greatly improved. This study will use the new seismic observation data of "Array III", and combined with the phase observation data of "Array II" to form a more complete seismic phase travel time data set. The regional seismic body-wave travel time tomography will figure out a more reliable three-dimensional velocity structure of P waves in Ordos.

Our study area spans from 32°N to 42°N and 108°E to 114°E , which includes the Ordos block and its adjacent structures . The seismic data we used for inversion were recorded by 1244 stations including: 198 permanent stations and 1043 temporary stations (ChinArray II and III), from November 2013 to August 2017. After manual labeled the seismic phase, we select events with more than ten phase records of individual seismic events. The epicentral distance is less than 200km. Finally, we obtained about 22,500 phase records of 1882 local seismic events.

The preliminary results are consistent with previous studies and surface structures of a wide range of velocity distributions. However, in the middle-upper crust under the Liupan Mountain west, the low-speed anomaly extending downward is shown, which may be caused by the shallow crustal damage caused with the continuous eastward compression of asthenosphere in the northeastern margin of the Qinghai-Tibet Plateau during the Cenozoic. It is worth noting that there is an EW-trending low-velocity zone under the Dingbian-Suide fault beneath the Ordos Basin, with a depth form lower crust to 50 km in upper mantel. This low-velocity anomaly divides the high-speed disturbance in the Ordos block into two parts，indicate the depth of the fault can reach the upper mantel. In the Taihang Mountains in the west of the study area, low-velocity anomalies extending to the upper layer of the mantle are shown. We initially believe that this anomaly is related to the volcanic thermal motion that once existed on the area.

How to cite: Liu, Y. and Wu, J.: 3D Crustal P-Wave Velocity Structure for the Ordos block and Its Adjacent Area based on travel time tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21080, https://doi.org/10.5194/egusphere-egu2020-21080, 2020.

We present a 3D P-wave velocity model of the crust and uppermost mantle below Ireland. In the absence of local earthquakes, we used quarry and mining blasts recorded on permanent stations in the Irish National Seismic Network (INSN) and during various temporary deployments. We compiled a database of 1,100 events and around 20,000 P-wave arrivals, with each event associated with a known quarry. The source location uncertainty is therefore minimal. Both source and receiver locations are fixed in time and we used repeating events to estimate the travel time uncertainty for each source-receiver combination. We created a starting 1D velocity model from previously available data, and then used VELEST to calculate a preliminary minimum 1D velocity model. The 1D velocity model enabled us to remove outliers from the data set, and to calculate the final minimum 1D model used as the initial model in the 3D tomographic inversion. The resulting 3D P-wave velocity model will shed new light on the 3D crustal structure of Ireland.

How to cite: Subašić, S., Rezaeifar, M., Piana Agostinetti, N., Lebedev, S., and Bean, C.: 3D P-wave velocity model of Ireland's crust from controlled source tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18510, https://doi.org/10.5194/egusphere-egu2020-18510, 2020.

The Alpine Fault along the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth.  

Here we present results from a 3D seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole penetrated almost down to the fault surface. Within the glacial valley, we collected 3D seismic data to constrain valley structures that were obscured in previous 2D seismic data. The new data consist of a 3D extended vertical seismic profiling (VSP) survey using three-component receivers and a fibre optic cable in the DFDP-2B borehole as well as a variety of receivers at the surface.

The data set enables us to derive a reliable 3D P-wave velocity model by first-arrival travel time tomography. We identify a 100-460 m thick sediment layer (average velocity 2200±400 m/s) above the basement (average velocity 4200±500 m/s). Particularly on the western valley side, a region of high velocities steeply rises to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3D structures implied by the velocity model on the upthrown (Pacific Plate) side of the Alpine Fault correlate well with the surface topography and borehole findings.

A reliable velocity model is not only valuable by itself but it is also required as input for prestack depth migration (PSDM). We performed PSDM with a part of the 3D data set to derive a structural image of the subsurface within the Whataroa Valley. The top of the basement identified in the P-wave velocity model coincides well with reflectors in the migrated images so that we can analyse the geometry of the basement in detail.

How to cite: Lay, V., Buske, S., Bodenburg, S. B., Kleine, F., Townend, J., Kellett, R., Savage, M., Schmitt, D., Constantinou, A., Eccles, J., Lawton, D., Bertram, M., Hall, K., Kofman, R., and Gorman, A.: The structural architecture of the Whataroa Valley at the Alpine Fault (New Zealand) from first-arrival tomography and reflection imaging using an extended 3D VSP survey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8538, https://doi.org/10.5194/egusphere-egu2020-8538, 2020.

The current knowledge of the structure of the Bohemian Massif (BM) crust is mostly based on interpretation of refraction and reflection seismic experiments performed along 2D profiles. The recent development of ambient noise tomography, in combination with dense networks of permanent seismic stations and arrays of passive seismic experiments, provides unique opportunity to build the high-resolution 3D velocity model of the BM crust from long sequences of ambient seismic noise data.

The new 3D shear-wave velocity model is built from surface-wave group-velocity dispersion measurements derived from ambient seismic noise cross-correlations by conventional two-step inversion approach. First, the 2D fast marching travel time tomography is applied to regularise velocity dispersions. Second, the stochastic inversion is applied to compute 1D shear-wave velocity profiles beneath each location of the processing grid.

We processed continuous waveform data from 404 seismic stations (permanent and temporary stations of passive experiments BOHEMA I-IV, PASSEQ, EGER RIFT, ALPARRAY-EASI and ALPARRAY-AASN) in a broader region of the BM (in an area of 46-54⁰ N 7-21⁰ E). The overlapping period of each possible station-pair and cross-correlation quality review resulted in more than 21,000 dispersion curves, which further served as an input for surface-wave inversion at high-density grid with the cell size of 22 km.

We present the new high-resolution 3D shear-wave velocity model of the BM crust and uppermost mantle with preliminary tectonic interpretations. We compare this model with a compiled P-wave velocity model from the 2D seismic refraction and wide-angle reflection experiments and with the crustal thickness (Moho depth) extracted from P-wave receiver functions (see Kampfová Exnerová et al., EGU2020_SM4.3). 1D velocity profiles resulting from the stochastic inversions exhibit regional variations, which are characteristic for individual units of the BM. Velocities within the upper crust of the BM are ~0.2 km/s higher than those in its surroundings. The highest crustal velocities occur in its southern part (Moldanubian unit). The velocity model confirms, in accord with results from receiver functions and other seismic studies, a relatively thin crust in the Saxothuringian unit, whilst thickness of the Moldanubian crust is at least 36 km in its central and southern parts. The most distinct interface with a velocity inversion at the depth of about 20 to 25 km occurs in the Moldanubian unit. The velocity decrease in the lower crust reflects probably its transversely isotropic structure.

How to cite: Kvapil, J., Plomerová, J., Babuška, V., Kampfová Exnerová, H., Vecsey, L., Working Group, A.-E., and Working Group, A.: Shear-Wave Velocity Model of the Bohemian Massif Crust from Ambient Noise Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7916, https://doi.org/10.5194/egusphere-egu2020-7916, 2020.

The tectonics of the Sicily Channel, located in the Central Mediterranean, are thought to be driven by the Calabrian back-arc system moving south-eastwards and the north moving African plate. The Channel is characterized by a seismically and volcanically active rift zone, which extends for more than 600 km in length offshore from the south of Sardinia to the south-east of Malta. Much of the observations we have today are either limited to the surface and the upper crust, or are broader and deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. The project GEOMED () addresses this issue by processing all the seismic data available in the region in order to understand better the geodynamics of the Central Mediterranean.

We use seismic ambient noise recorded on more than 50 stations located on Algeria, Italy (Lampedusa, Linosa, Pantelleria, Sardinia (LISARD seismic network), Sicily), Libya, Malta, and Tunisia to generate high-resolution seismic tomography maps for the region at different depths. We measure Rayleigh-wave phase velocities with periods ranging from 5 to 100 seconds sampling through the entire lithosphere. We find that at short periods (<25 s), paths of station-pairs crossing across Africa and Italy have slower velocities than those crossing the Tyrrhenian and Ionian basins indicating that these paths are sampling thick continental crust. However, station pairs limited to the Sicily Channel Rift Zone (SCRZ) have faster phase velocities for periods > 20 s comparable to those beneath the basins suggesting that the SCRZ has a thinner crust. The seismic velocity maps are compared with the regional tectonics, seismicity, volcanic activity and other geophysical studies to present a more holistic understanding of the processes involved.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 843696.

How to cite: Agius, M., Cammarano, F., Magrini, F., Faccenna, C., Funiciello, F., and van der Meijde, M.: Imaging the Sicily Channel Rift Zone (Central Mediterranean) with seismic ambient noise tomography., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8404, https://doi.org/10.5194/egusphere-egu2020-8404, 2020.

We perform an adjoint waveform tomography using a combined data set consisting of regional earthquake waveforms and Rayleigh wave ambient-noise Green's function to construct a new 3-D wave velocity model of the crust and uppermost mantle beneath the Iranian plateau. The earthquake waveforms come from 250 regional events with magnitudes of 4.5-6.5 recorded by 136 broadband seismic stations. The EGFs are derived from cross correlations of more than three years of continuous seismic noise. The inversion starts with an initial model derived from the global Crust1 model locally modified using information from previous studies. Adjoint tomography refines the initial model by iteratively minimizing the frequency-dependent travel-time misfits between real and synthetic earthquake data and EGFs and synthetic Green’s functions measured in different period bands. Our new model covers the known tectonic units such as the Central Iranian Block, Zagros fold-and-thrust belt, Sanandaj-Sirjan metamorphic zone and Urumieh-Dokhtar magmatic arc. Overall, the adjoint tomography provides images with more real resolutions and amplitudes due to the finite-frequency consideration. Using the numerical spectral-element solver in adjoint tomography provides accurate structural sensitivity kernels, which help generate more robust images rather than those generated by ray-theory tomography. Our study also demonstrates improvement of lateral resolution and depth sensitivity using combined data set instead of only earthquake data.

How to cite: Komeazi, A., Yaminifard, F., Kaviani, A., Rümpker, G., Tatar, M., Liu, Q., and Wang, K.: 3D structure beneath Iranian plateau and Zagros using adjoint tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3506, https://doi.org/10.5194/egusphere-egu2020-3506, 2020.

We use 4695 local waveforms from 1206 earthquakes (epicentral distance < 350 km and 2.0 ≤ Mw ≤ 5.5) recorded by IISER Kolkata network (IK) at 22 stations (32°N to 35°N latitude and 74°E to 77°E longitude), located within the North-Western Himalaya (28°N to 39°N latitude and 68°E to 81°E longitude). We study the coda waves which are generally the tail of a seismogram and arrive after the main seismic waves. We use the temporal decay of coda amplitude to calculate the coda quality factor (Q_c) from which we estimate the attenuation (Q_c^-1). We consider the single back-scattering model (Aki & Chouet, 1975) where both the scattering (Q_sc^-1) and intrinsic (Q_i^-1) component of the attenuation are included in the measurement. We use a lapse time of 2t_s (t_s is the S-wave arrival time) as the starting point of the coda window. Then, we consider multiple forward-scattering model, where the attenuation (Q_c^-1) is dominantly dependent on the intrinsic (Q_i^-1) component. In this model we use lapse time greater than 2t_s so that the coda waves encounter multiple scatterers and enter the diffusive regime. We calculate the frequency dependent quality factor for each earthquake-receiver path at frequencies 1 to 14 Hz using the linear least squares approach on temporal decay of coda amplitude. We calculate Q₀ (quality factor at a reference frequency f₀ which is chosen to be 1 Hz for the analysis) and its frequency dependence (η) using weighted least squares approach on the power law dependence of Q_c on frequency. To see the lateral variation of Q in our study area, we have produced 2-D maps by combining the Q_c measurements together in a tomography. For single back-scattering model we use the back-projection algorithm which is based on the calculation of area overlap of ellipses with the gridded region. For multiple forward-scattering model, the same back-projection algorithm is modified to calculate the length overlap of traces with the gridded region. To understand the spatial resolution of the 2-D Q_c maps, we use the point spreading function test which quantifies the recovery of Q_c perturbation. In addition to this, we also perform a standard checkerboard resolution test to ensure simultaneous recovery of Q_c perturbation. We observe low Q in the Kashmir basin and Lesser Himalaya and high Q in surrounding northeastern Higher Himalaya which clearly correspond to the coda wave attenuation signatures in the older Tethyan sedimentary rocks and crystalline igneous rocks in these regions respectively.

How to cite: Bera, A., Agrawal, H., Mitra, S., and Sharma, S.: Seismic attenuation tomography of the North-Western Himalaya using Coda waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-979, https://doi.org/10.5194/egusphere-egu2020-979, 2020.

For the past 40 years, the Geophysical Institute of Israel has been in charge of the recording, monitoring and relocating of local earthquakes. Due to the variety of data analysts and data sources, as well as several network upgrades, the resulting bulletin data has to be completed and homogenised, and station metadata needs to be tracked down, and sometimes corrected. For those reasons, as well as because of the lack of consensus on an accurate model for seismic velocities in the area, published source locations are often poorly constrained. We present a homogenised Israeli bulletin, including natural and man-made explosion data. We extract sets of seismic sources with location accuracy greater than 5 km (GT5), as well as GT0 explosions.

We select a set of events with the highest network coverage, comprising (1) natural earthquakes, (2) man-made quarry or mine blasts, (3) GT5 earthquakes or explosions, and (4) GT0 explosions. We relocate them altogether using the BayesLoc package, a Bayesian, hierarchical, multi-event locator which produces, after source relocation, event-, station- and phase-specific correction terms. We put different a priori constraints on the different categories of seismic events, allowing poorly constrained origin parameters to improve thanks to the more accurate GT locations. BayesLoc also produces traveltime correction terms that can be used to correct systematic errors in the dataset, as well as error estimates.

Eventually, we invert this homogenised local traveltime dataset in order to invert for a P-wave crustal velocity model of Israel and its surroundings. To do so, we use the Fast Marching Tomography package, which allows the representation of a wide variety of input structures (starting model and geometry of layer boundaries) and can take many different types of input data. We show preliminary inversion tests and results that are in good agreement with past local studies.

This crustal model of Israel is ultimately to be used as a starting model in a larger tomographic study of the Eastern Mediterranean and Middle East region, where the Regional Seismic Travel Time approach is to be expanded, in order to improve the CTBT’s capabilities in monitoring the regional seismicity. Eventually, such a velocity model could also be used to relocate the whole earthquake catalogue more accurately, and improve the Earthquake Early Warning System currently in development in Israel.

How to cite: Schardong, L., Ben-Horin, Y., Ziv, A., Wust-Bloch, H., and Radzyner, Y.: P-wave tomographic model from local bulletin data for improved seismic location in and around Israel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13371, https://doi.org/10.5194/egusphere-egu2020-13371, 2020.

The Basque-Cantabrian Zone was one of the most subsident areas between the European plate and the Iberian sub-plate during the Mesozoic rifting process that gave birth to the Bay of Biscay. Since the latest Cretaceous and during the Cenozoic, a change to a contractional setting driven by the northward drift of the African Plate made this hyperextended rift basin to be inverted and incorporated into the Pyrenean-Cantabrian mountain belt. The resulting crustal structure shows a high complexity, as evidenced by the many existing geophysical observations pointing to the presence of intracrustal high-velocity bodies, deep transfer structures, and sharply-varying Moho depths across the area.

In this work, we use data provided by the dense SISCAN and MISTERIOS seismic networks, deployed in the region between 2014 and 2018, to obtain a detailed 3D shear-wave velocity model. We use the continuous recordings to compute seismic noise cross-correlation functions, from which we extract surface wave dispersion measurements. We use these measurements to obtain a set of phase velocity maps, and then perform a non-linear inversion at regularly spaced locations for the 1D shear-wave velocity structure. In the non-linear inversion, the forward modeling accounts for the presence of higher modes of surface waves, which have been shown to be more sensitive to velocity decreases with depth than the fundamental model. In order to better constrain the deeper layers of the model, we complement the seismic noise observations with an analysis of teleseismic receiver functions, which allows us to better constrain the depths of the major crustal discontinuities. Our results agree with previous geophysical studies, but significantly improve the availability of high-resolution information in the Basque-Cantabrian Zone.

How to cite: Olivar-Castaño, A., Pilz, M., Pedreira, D., Pulgar, J. A., Díaz-González, A., and González-Cortina, J. M.: Crustal structure of the Basque-Cantabrian Zone (N Spain) from seismic noise and receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9603, https://doi.org/10.5194/egusphere-egu2020-9603, 2020.

A receiver function analysis was carried out along two profiles located in north- and southwestern Norway. We selected and processed 801 teleseismic events registered by twelve seismic stations belonging to the 2002-2005 Geofon/Aarhus temporary network. The HK (depth vs Vp/Vs) stacking procedure and a Reversible jump Markov chain Monte Carlo (Rj-McMC) inversion were applied independently with the objective to reveal new crustal and crust-mantle transitional contrasts gaining a better understanding of the geology. In the southern profile, the most noticeable feature corresponds to a Moho offset of about ~5 km ca. 85 km to the east of the Norwegian coast: That feature was previously observed in several occasions and is also well-supported from this research. Furthermore, a very deep Moho discontinuity – at between 45 – 50 km depth - was found beneath the northern profile, approximately 70 km inland from the coast, and dipping about 30° to the northwest. Even when this deep structure was previously inferred through other methods, its presence was not certainly confirmed and so far, the origin of this feature is still disputed. We discuss two hypotheses, which are valid to explain the occurrence of the noticeable anomaly. First, a gradual and wide crust-mantle transition zone, which is also reflected in the velocity model or second, the presence of a paleo-slab of Fennoscandian basement subducted and deformed during the Caledonian Orogen (490-390 Ma).

How to cite: Brönner, M. and Pavez, C.: 2D velocity profiles along south- and northwestern Norway: An approach using receiver function analysis and Markov chains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5537, https://doi.org/10.5194/egusphere-egu2020-5537, 2020.

Cratons are representative of the oldest cores of continental crusts. Study of cratons is important  as they preserve the pristine nature of continental crusts as well as they have economic significance as a major source of the world's mineral deposits. The crustal thickness, crustal composition, structure and physical properties of crust-mantle transition (the Moho) are the key parameters for understanding the formation and evolution of continental crust. The ratio of  seismic P-wave and S-wave velocity (Vp/Vs) is used as a parameter to understand the petrologic nature of the Earth's crust. Using these parameters, we address the crustal properties of all Archean cratons. The teleseismic P-wave receiver function analysis reveals that all the Eoarchean (4-3.6 Ga) cratons (Superior, North Atlantic Craton, North China Craton, Yilgarn, Zimbabwe, Kaapvaal) have crustal thickness ranges between 34-42 km and Vp/Vs ratio 1.68-1.79, the Paleoarchean (3.6-3.2 Ga) cratons (Baltic shield, Pilbara, Tanzania, Grunehogna) have 29-52 km crustal thickness and Vp/Vs ratio 1.7-1.85, the Mesoarchean (3.2-2.8 Ga) cratons (Sao Francisco, Guapore, Yangtze, Antananarivo) have 36-53 km thickness and Vp/Vs ratio 1.7-1.9, and Neoarchean (2.8-2.5 Ga) cratons (Guiana, Anabar, Gawler, Napier, Tarim) have 36-59 km thickness and Vp/Vs ratio 1.64-1.95. The nature of crust-mantle transition is overall sharp and flat.  We also found that the crusts which are stabilized earlier, are thinner compared to the later stabilized crusts. Our findings are well-correlated with the craton evolution process predicted by Durrheim and Mooney (1994), where older crusts are thin due to delamination process and relatively younger crusts are thick due to basaltic underplating. Our result of higher Vp/Vs ratio in the relatively younger crusts corroborates with the mafic nature of the crust whereas the older crusts are felsic-intermediate resulting lower Vp/Vs ratio. Our study is unique as it includes most of the global cratons and suggests a global model of continental crust formation and evolution process.

How to cite: Roy, P. and Borah, K.: Seismic P-wave receiver function modelling of Archean cratonic crust: A global perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9666, https://doi.org/10.5194/egusphere-egu2020-9666, 2020.

We present a new detailed map of the Moho in the Bohemian Massif (BM) derived from P-to-S conversions calculated from broad-band waveforms of teleseismic events recorded at 325 temporary and permanent stations operating in a region framed in 10–19º E and 48–52º N during last two decades. We processed data collected from running AlpArray Seismic Network (2015 – 2019) (http://www.alparray.ethz.ch/) and its complementary experiment AlpArray-EASI (2014 – 2015), as well as from previous passive seismic experiments in the region – BOHEMA I-IV (2001 – 2014), PASSEQ (2006 – 2008) and EgerRift (2007 – 2013). The study aims at upgrading the current knowledge of structure of the BM crust and providing a homogeneous estimate of Moho depths, particularly for the use in deep Earth studies, e.g., the upper mantle tomography. Different velocity models, including the new one retrieved from the ambient-noise study (see Kvapil et al., EGU2020_SM4.3), are tested in the time-depth migration procedures. Regional variations of the Moho depth correlate with main tectonic units of the BM. The crust thickens significantly in the Moldanubian part of the BM and thins along the Eger Rift in the western part of the massif. Detailed variations of the Moho depth from the receiver functions along several profiles are compared with crustal sections retrieved from the ambient noise tomography.

How to cite: Kampfová Exnerová, H., Plomerová, J., Kvapil, J., Babuška, V., Vecsey, L., Working Group, T. A.-E., and Working Group, T. A.: Mapping the Moho in the Bohemian Massif with P-receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7845, https://doi.org/10.5194/egusphere-egu2020-7845, 2020.

Seismic interferometry can be used to turn earthquakes into virtual seismic sensors. Cross-correlation of seismic traces from two earthquakes recorded at the same actual sensor and summation of cross-correlations from many actual sensors result in the (strain) Green’s function between the two earthquakes. This simple concept provides an exciting new way for high-resolution imaging of subsurface structures in areas with poor instrument coverage. We test and apply this method to the Central Alborz region of Iran where we can compare results with regular local earthquake tomography. We first extracted the Rayleigh wave group velocities (U) from the Green’s functions and then inverted them for the upper crustal Rayleigh wave maps. We used vertical seismograms from 819 well-located Mw<4 earthquakes with vertical and horizontal location uncertainties of less than 2.5 km recorded between January 2006 and May 2019. The recordings are from seismic stations operated by the Iranian Seismological Center, the International Institute of Earthquake Engineering and Seismology, and the Tehran Disaster Management and Mitigation Organization. Pre-processing of each seismic trace consisted of removing the mean, detrending, instrument response correction, and 1-20 s band-pass filtering. We cross-correlated event-pairs at all available actual sensors and stacked the cross-correlations to obtain the inter-event empirical Green’s function (EGF); we used the phase weighted stacking procedure to enhance the signal-to-noise ratio of the EGF. We then applied the classical FTAN method to calculate the group velocities from the Rayleigh wave dispersion measurements. Cross-correlations, stacking and dispersion analysis were performed for all available event-pairs. Finally, we inverted the dispersion measurements to obtain group velocity maps using the Fast Marching Surface-Wave Tomography method. The resulting group velocity maps indicate a thick layer of low velocity material near the junction of the Mosha-North Tehran faults, a low velocity anomaly related to the Damavand Volcano, and abrupt transitions from high to low velocity anomalies associated with the Mosha fault.

How to cite: Afra, M., Shirzad, T., Braunmiller, J., Rahimi, H., and Naghavi, M.: Crustal structure of the Central Alborz, Iran from inter-event interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13331, https://doi.org/10.5194/egusphere-egu2020-13331, 2020.

Rockall trough lies to the west of Ireland in NE Atlantic, it has a complex geology and has been debated for controversial geology for more than two decades. We have performed Full waveform inversion (FWI) on 2D seismic data set that is recorded in 2013-14 by using 10 km long streamer, this 2D seismic line is situated near the North-West margin in the Rockall Bank area. Full waveform inversion (FWI) is a powerful technique for obtaining elastic properties of the sub-surface from the seismic data. FWI provides properties of the sub-surface at the scale of the wavelength of the data set. We used travel time tomography on downward extrapolated data set to obtain a smooth starting velocity model for FWI. Downward continuation is a technique that enhances the first arrival and also reduces the computation time for forward modelling in FWI. The velocity model obtained from refraction travel time tomography, indicates the velocity from 1.6-4 km/s for the sediments and we have also observed very high velocity ~ 6-7.5 km/s just 3 km below sea-floor. We have performed FWI using these TTT velocity model as a starting model and inverted the refractions along with the wide angle reflections in the frequency range of 3-10 hz. FWI results gives the velocity of 6-7.2 km/s as well as defines geological structures that can be seen in the migrated seismic section. These high velocity structures could be a part of the continental crust and/or lower oceanic crustal igneous rocks like Gabbro.

How to cite: Tomar, G., J. Bean, C., and C. Singh, S.: Lower crustal structures at Rockall Trough (west of Ireland) by Full waveform inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18907, https://doi.org/10.5194/egusphere-egu2020-18907, 2020.

Detailed knowledge about geometry and physical properties of magmatic systems at arc volcanoes promises to better constrain models of magma differentiation, transit and storage in the crust, and to help assess volcanic hazard.

Unfortunately, low-velocity zones associated with melt accumulation are particularly difficult to image by conventional travel-time tomography due to its limited resolving power, resulting in blurred boundaries and underestimated velocity contrasts.

Here we alleviate these issues by applying full-waveform inversion (FWI) to study a magmatic system of Santorini - an active, semi-submerged volcano with a known record of large, caldera-forming eruptions.

We use a 3D wide-angle, multi-azimuth seismic dataset from the recent PROTEUS experiment acquired with ca. 150 ocean-bottom/land seismic stations and ca. 14,000 air-gun shots. We implement a finite-difference immersed boundary method to simulate reflections off the caldera’s irregular topography, and pressure-velocity conversion to take full advantage of the multi-component data. We perform inversion with careful data-selection, increasing frequency up to 6 Hz, and extensive quality-control based on a phase spatial-continuity criterion.

A final P-wave velocity model of the upper crust offers a high-resolution image of Santorini magmatic and hydrothermal systems with pronounced low-velocity zones due to a high melt and water content respectively. The features are better resolved and the velocity contrasts distinctly sharper than in the starting model obtained with travel-time tomography. We also recover a previously undetected low velocity anomaly of >40% beneath Kolumbo - a submarine volcanic cone to the NE of Santorini caldera. We interpret this anomaly as a magmatic sill.

How to cite: Chrapkiewicz, K., Paulatto, M., Morgan, J., Warner, M., Heath, B., Hooft, E., McVey, B., Toomey, D., Nomikou, P., and Papazachos, C.: Full-waveform inversion at Santorini volcano, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19836, https://doi.org/10.5194/egusphere-egu2020-19836, 2020.

We present an interpretation of a 3-D velocity model resulting from a regional analysis of earthquake waveforms. This model contains 3-D structure of the crust and upper mantle beneath the Arabian-Eurasian collision zone in eastern Turkey and Iran. We use full-waveform inversion (FWI) of three-component recordings from permanent networks. FWI can exploit all parts of a seismogram, including body and multi-mode surface waves in a broad range of frequencies. This allows us to constrain seismic structure of both the crust and the upper mantle.

In our method we simulate 3-D visco-elastic wavefields using a spectral-element method (Fichtner et al,2018). Our numerical mesh honors topography of the surface. We compare observed and synthetic waveforms using time-frequency phase misfits. Using adjoint techniques, we then compute sensitivity kernels with respect to the model parameters, which are V_SV, V_SH, V_PV, and V_PH. Finally, the kernels enable the iterative solution of the nonlinear inverse problem with the help of the L-BFGS algorithm and without a need for crustal corrections.

For this study we obtained seismic waveform data of 59 earthquakes within the magnitude range of Mw 4.5 to 6.3 that occurred in the region between 2012 and 2016. These events were recorded by 398 broadband seismic stations belonging to the two national Iranian networks and freely available seismic stations of the Turkish Network, made available by IRIS.

Starting from the first generation of the Collaborative Seismic Earth Model (Afanasiev et al.2019), we first constrained longer-wavelength structure. To this end, we considered 3-component recordings from a subset of 37 events in the period range from 50 to 80 s. This band was successively broadened by reducing the shorter period from 50 s to 40 s, and finally to 20 s. For each period band, the number and the length of measurement windows are increased; the number of events is also increased to 59 to use the complete dataset. After 46 iterations our model can explain recordings of events, which were not used in the inversion. The results provide to discuss about high-velocity anomaly beneath the Zagros and the shallow low velocities beneath Central Iran using cross-sections to investigate lateral variation of seismic velocity in the lithosphere.

REFERENCES

Afanasiev, M., Boehm, C., van Driel, M., Krischer, L., Rietmann, M., May, D. A., Knepley, M. G., Fichtner, A., 2019. Modular and flexible spectral-element waveform modelling in two and three dimensions. Geophysical Journal International 216, 1675-1692, doi: 10.1093/gji/ggy469.

Fichtner, A., van Herwaarden, D.-P., Afanasiev, M., Simute, S., Krischer, L., Cubuk-Sabuncu, Y., Taymaz, T., Colli, L., Saygin, E., Villasenor, A., Trampert, J., Cupillard, P., Bunge, H.-P., Igel, H., 2018. The Collaborative Seismic Earth Model: Generation I. Geophysical Research Letters 45, doi: 10.1029/2018GL077338.

How to cite: Masouminia, N., van Herwaarden, D.-P., Thrastarson, S., Rahimi, H., Krischer, L., Afanasiev, M., Böhm, C., and Fichtner, A.: 3-D seismic full-waveform inversion of the Taurus-Zagros region in Iran and Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13470, https://doi.org/10.5194/egusphere-egu2020-13470, 2020.

Iceland, situated in the North Atlantic Ocean between Greenland and Norway, is located above the Mid-Atlantic Ridge. It is a part of the oceanic crust forming the floor of the Atlantic Ocean. Its tectonic structure is characterized by various seismically and volcanically active centers. We focused on active seismicity in the SW part, where the Reykjanes Ridge segment of the Mid-Atlantic Ridge is located in Reykjanes Peninsula as the landward ridge continuation connecting it to the Western Volcanic Zone. The seismicity in this area is monitored by REYKJANET seismic network stations operated by IGF CAS. The earthquakes are released in form of swarms and are largely confined to the upper few kilometers of the oceanic layer related to a large number of faults and fissures with the high seismic activity at depths of 2 to 6 km, however, some events may be as deep as 13 km.

Since knowledge of a detailed crustal structure is essential for all advanced studies of seismicity and focal parameters of the earthquakes, we concentrated on velocity model and prominent discontinuity depth retrieval in the area. We selected the best located events of the 2017 swarm in Reykjanes Peninsula and refined their locations by manual picking. This resulted in processing of waveforms from earthquakes with magnitudes >1 recorded at 15 REYKJANET seismic network stations. The waveforms typically displayed dominant direct P and S waves followed by converted and reflected waves secondarily generated at shallow and deeper subsurface structure. We tested a multi-azimuthal approach in data processing of Hrubcová et al. (2013; 2016) to increase resolution of these phases in the waveforms. We applied the waveform cross-correlation of the P and S waves, and rotated, aligned and stacked the seismograms to extract the reflected/converted phases. In the interpretation, we focused on the most prominent interface at the crust/mantle boundary, the Moho, and processed its reflected phases. These phases were inverted for laterally varying Moho depth by ray tracing and a grid search inversion algorithm and verified by modeling of full waveforms computed by the discrete wave number method.

How to cite: Hrubcová, P., Doubravová, J., and Horálek, J.: Prominent crustal discontinuities in Reykjanes Peninsula, Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18042, https://doi.org/10.5194/egusphere-egu2020-18042, 2020.

The Pear River Estuary (PRE) area is located in the northern margin of South China Sea (SCS), which is a typical rifted passive continental margin between the South China Block and SCS Basin. The Littoral Fault Zone (LFZ) crossing the PRE is an important seismogenic and boundary fault. Strong earthquakes occurred in both the west and east segments. While, the middle segment of the LFZ in PRE area is lack of large earthquake and possibly a seismic gap. Imaging the fine structure of the PRE area is helpful to understand the background of the spatial heterogeneity of seismicity. To explore the crustal structure of the PRE area, we carried an active-source experiment in July 2015. During the experiment, an airgun array composed of four individual airguns with total volume of 6000 in³ mounted on SCSIO’s R/V Shiyan II was used as the seismic source. A total of 12200 shots were fired every 300 meters along 10 NW and 3 NE-trending shooting profiles. Six dynamite sources with a charge range from 1000 to 2500 kg were also shot on the land. During the experiment, 431 receivers including 29 ocean bottom seismographs (OBS), 256 short period seismometers, and 146 broadband seismometers were available. We manually picked Pg arrivals from the airgun and dynamite sources. We calculated a minimum 1D velocity model by VELEST. We then obtained the upper crust Vp structure using three-dimensional seismic tomography. Our preliminary result reveals that the Vp is consistent with local geological settings. There are low velocity anomaly beneath the LFZ and obvious velocity anomalies across the NW- and NE-trending active faults, which maybe potential threat to the Greater Bay Area.

How to cite: Wang, L., Ye, X., Wang, X., Lv, Z., Zhang, Y., and Wang, B.: Three dimensional crustal Vp structure beneath the Pearl River Estuary from joint onshore-offshore seismic experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12225, https://doi.org/10.5194/egusphere-egu2020-12225, 2020.

Fitting subduction zone guided waves with synthetics is an ideal choice for studying the velocity structure of the oceanic crust. After an earthquake occurs in subduction zones, seismic waves can be trapped in the low-velocity oceanic crust and propagated as guided waves. The arrival time and frequency characteristics of the guided waves can be used to image the velocity structure of the oceanic crust. The analysis and modeling based on guided wave observations provide a rare opportunity to understand the velocity structure of the oceanic crust and the variations in oceanic crustal materials during the subduction process.

High-frequency guided waves have been observed in the subduction zone of eastern Alaska. On several sections, observed seismograms recorded by seismic stations show low-frequency (<2Hz) onsets ahead of the main high-frequency (>2Hz) guided waves. Differences in the arrival times and dispersion characteristics of seismic phases are related to the velocity structure of the oceanic crust, and the characteristics of coda waves are related to the distribution of elongated scatters in the oceanic crust. Through fitting the observed broadband waveforms and synthetics modeled with the 2-D FDM (Finite Difference Method), we obtain the preferred oceanic crustal velocity models for several sections in the subduction zone of eastern Alaska. The preferred models can explain the seismic phase arrival times, dispersions, and coda characteristics in the observed waveforms. With the obtained P- and S- wave models of velocity structures on several sections, the material compositions they represent are deduced, and the variations of oceanic crustal materials during subducting can be understood. This provides new evidence for studying the details of the subduction process in the subduction zone of eastern Alaska.

How to cite: Guan, X., Zhou, Y., and Furumura, T.: Modeling guided waves to constrain the velocity structure of the oceanic crust in the subduction zone of eastern Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3268, https://doi.org/10.5194/egusphere-egu2020-3268, 2020.

Rayleigh wave ellipticity depends, in theory, only on the Earth structure below a seismic station, offering the advantage of a “single-station” method to infer crustal properties. Therefore, ellipticity measurements can be used to construct pseudo 3-D shear velocity models of the earth structure using even seismic stations that did not record simultaneously.  

Based on that, we carried-out ellipticity measurements by using teleseismic waveforms recorded by the OPS seismic network we deployed at the western flank of the North Tanzanian Divergence between June 2016 and May 2018, covering 17 sites. We then expanded our measurements on the waveforms recorded by the adjacent CRAFTI seismic network from January 2013 and December 2014, available on IRIS, which comprised more than 30 sites. 

While the OPS network covers the transition between the Tanzania Craton and North Tanzanian Divergence, the CRAFTI network is entirely contained in the North Tanzanian Divergence. Therefore, the imaging that can be obtained by integrating the two asynchronous passive seismology experiments will help to better understand the dynamics of this segment of the eastern branch of the Eastern African Rift.

Preliminary results show heterogeneity structure that are in agreement with previous tomographic studies based on ambient noise cross-correlation and body-waves arrival-times. In regions where previous seismological studies are not available, results match the known geological structure of the transition between the Tanzanian Craton and the North Tanzanian Divergence. This demonstrates that measurements of ellipticity can be a useful and integrative tool for earth structure imaging, especially at the edges of the active rifts where the seismicity is scarce.

How to cite: Parisi, L., Berbellini, A., and Mai, P. M.: Rayleigh wave ellipticity measurements in the North Tanzanian Divergence (Eastern African Rift), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7976, https://doi.org/10.5194/egusphere-egu2020-7976, 2020.

Southeast Asia (SEA) is one of the most active tectonic regions on the planet due to its convergent setting, which accommodates rapid northward motion of the Indo-Australian plate and westward motion of the Philippines Sea plate. This activity gives rise to intense seismicity along the convergent plate margins in and around SEA, including the Sunda Arc, which wraps its way around the southern margin of the Indonesian archipelago.

Borneo is located at the centre of SEA, on the leading edge of the Sundaland block of the Eurasian plate, and exhibits lower rates of seismicity when compared to the surrounding regions due to its intraplate setting. Sulawesi, an island which lies adjacent to Borneo in the east, is characterised by intense seismicity due to multiple subduction zones in its vicinity. The tectonic relationship between the two islands is poorly understood, as is the provenance of their lithosphere, which may have Eurasian or East Gondwana origin.

The aim of this presentation is to showcase recent receiver function results from temporary and permanent broadband seismic stations in the region, which can be used to help improve our understanding of the structure of the crust and the mantle lithosphere beneath Borneo and Sulawesi. H-K stacking, receiver function migration and inversion are all applied in an effort to determine robust crustal thickness estimates and variations in shear wavespeed with depth. Our preliminary results from Borneo indicate that the crust beneath Sabah (northern Borneo), which is a post-subduction setting, appears to be much more complex than the rest of the island. Furthermore, we find that crustal thickness varies between different tectonic blocks defined from surface mapping, with the thinnest crust (24 km thick) occurring beneath Sarawak in the northwest.

How to cite: Linang, H. T., Gilligan, A., Jenkins, J., Greenfield, T., and Rawlinson, N.: Seismic Structure and Tectonic Evolution of Borneo and Sulawesi, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12007, https://doi.org/10.5194/egusphere-egu2020-12007, 2020.

Southeast Asia is one of the most complex tectonic regions on Earth. This is mainly a result of its location within the triple junction of the Australian, Eurasian and Philippine Sea plates which has created a complicated configuration of active plate tectonic boundaries. High plate velocities have generated thousands of kilometers of subducted material and ongoing subduction along the Sunda Arc represents a significant natural hazard (such as the 2004 Sumatra-Andaman earthquake, 2012 Indian Ocean earthquakes and 2018 Anak Krakatoa eruption). However, recent tectonic activity around Borneo may be related to postsubduction processes which could be the key to understanding how the tectonic subduction cycle terminates. Further east, the region is dominated by several minor tectonic plates and the spectacular 180-degree curvature of the Banda Arc. Our work aims to further improve the understanding of this area by providing detailed images of the upper mantle.

Adjoint waveform tomography is especially suitable for imaging such complex regions. By simulating the 3D wavefield, it is possible to directly compare observed and simulated seismograms, thereby taking into account both body and surface waves. The method can account for the effects of anisotropy, anelasticity, wavefront healing, interference and (de)focusing that can hamper other seismological methods, and is thus especially suitable for strongly heterogenous areas such as Southeast Asia.

To date, sparse instrument coverage in the region has contributed to a heterogeneous path coverage. In this project, we make use of publicly available data as well as our recently deployed networks of broadband seismometers on Borneo and Sulawesi. This, in addition to access to national permanent networks promises a significant improvement in data coverage around the Banda Arc, Borneo and Sulawesi, thereby providing new opportunities to untangle the region’s complexity.

We compiled a catalogue of well-constrained earthquakes, optimising for coverage, signal-to-noise ratio and data availability across a wide frequency band, and compared our observed data to synthetics generated from an initial model. In the first part of the inversion, we use long periods of 100 - 150 s to update our initial model using a gradient-based optimisation scheme. We use adjoint methods to obtain sensitivity kernels as the corresponding gradients and initial results will be documented in this presentation. In subsequent iterations, we permit increasingly shorter periods in order to progressively recover finer scales structure and avoid cycle skipping issues.

How to cite: Wehner, D., Blom, N., and Rawlinson, N.: Towards 3D multiscale adjoint waveform tomography of the crust and upper mantle beneath Southeast Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-694, https://doi.org/10.5194/egusphere-egu2020-694, 2020.

    The Middle-Lower Yangtze River Metallogenic Belt, known as the eastern “industrial corridor”, is an important polymetallic mineral resource base in eastern China. It has a significant influence on economic development in eastern China and its deep dynamic process has been the focus of debate for deposit scientists.

    Why does such a narrow zone contain so rich mineral resources? What is the deep dynamics? Many scholars put forward different explanations, such as “ continental extension mode ”, “ subduction mode ” and “ reverse L-shaped collision mode ”. Although these views reach a consensus in some aspects, they are different essentially. Thus, the deep structure information has become the key to distinguish such different views.

    In this study, we conduct teleseismic P wave receiver function, ambient noise tomography and teleseismic two-plane-wave tomography to probe the crustal and uppermost mantle structures in the Middle-Lower Yangtze River region. The data include (1) continuous seismic data from June 2012 to July 2013 recorded from Chinese provincial networks; (2) seismic event data from 2010 to 2011 recorded from Chinese provincial networks; (3) continuous seismic data from November 2009 to August 2010 deployed by Chinese Academy of Geological Sciences; (4) continuous seismic data from June 2012 to July 2013, from August 2014 to June 2015 and from July 2015 to November 2015 deployed by China University of Geosciences(Beijing). First, we apply receiver function to get the Moho depth below 191 seismic stations and the results are in good accordance with previous researches. We then conduct joint inversion of receiver function and surface wave dispersion to generate shear wave velocity structures below 191 seismic stations. The result shows that in the upper crust, the basin regions, including the JiangHan, HeHuai, SuBei, HeFei and NanYang basin, are all featured with low velocities, and the mountain regions with high velocities. In the northeastern of the Middle-Lower Yangtze River Metallogenic Belt, the result shows ore districts are clearly characterized with the strongest low velocity anomaly in the uppermost mantle at ~70km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the northeast to southwest.

How to cite: Li, J.: Crustal and uppermost mantle velocity structure beneath the Mid-lower Yangtze metallogenic belt and its adjacent regions from joint inversion of receiver function and surface wave, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2007, https://doi.org/10.5194/egusphere-egu2020-2007, 2020.

The Ordos Block located in the center of China mainland, which is one of the oldest and most stable cratons in Asia. It is contiguous to the Yinshan Block, the North China Craton, Alex Block, Yangze Block, and Northeast Tibet. Numerous geologic and geophysical studies engaged in the mechanics of the Ordos Block deformation and evolution, but the detail structure and deformation style of the Ordos Block remains uncertain due to poor geophysical data coverage. During 2013 and 2018, China Earthquake Administration developed XMLY Seismic Array in Ordos Block and adjacent area, which operated more than 1000 broadband seismic stations with an average station spacing of 35km. Using the P-wave Travel time data recorded by the array and multi-scale seismic traveltime tomography technique, we obtained a high-resolution P-wave velocity structure beneath Ordos Block. The seismic tomography algorithm employs sparsity constrains on the wavelet representation velocity model via the L1-norm regularization. This algorithm can efficiently deal with the uneven-sampled volume, and give multi-scale images of the model. Our preliminary results can be summarized as follows: 1, the crustal and upper mantle P-wave velocity structure is strongly inhomogeneous and consistent with the surface geological setting; 2, significant low-velocity anomalies exist beneath the northwestern margin of Ordos Block, which suggested that there exist upper mantle upwelling; 3, There have obvious boundary between Alex and Ordos Block along 104ºE at upper mantle;  4, Along 38ºN tectonic line, there exist different structure between south part and north part of Ordos upper mantle, the south part of Ordos show high-velocity feature and the upper mantle show low-velocity anomalies in north part of Ordos Block. This feature can be interpreted that the two parts of the Ordos Block undergone different Tectonic evolution processes.

How to cite: Guo, B., Chen, J., Li, X., and Li, S.: Crustal and upper mantle Structure Beneath the Ordos Block by Multi-scale seismic tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13043, https://doi.org/10.5194/egusphere-egu2020-13043, 2020.

South-East Asia is one of the most tectonically complex regions on Earth. North Borneo in particular is home to a number of intriguing features, the formation of which is not fully understood. These include the North-West Borneo trough, the rapidly uplifted 4000m high Mt Kinabalu and the uplifted circular sedimentary basins such as the Maliau Basin. To study North Borneo's tectonics in depth we deployed a new dense temporary network of 46 broadband seismometers across the regionin a semi-regular grid pattern with approximately 40km spacing. This closely spaced network, which operated for 22 months, allows a high-resolution seismic analysis of the crust and mantle under North Borneo. Here, we use ambient noise cross-correlations to measure phase velocities of surface waves. We then use the phase velocities to analyze the crustal and upper mantle structure.

How to cite: Volk, O., Bacon, C., Tongkul, F., and Rawlinson, N.: Crust and upper mantle structure of North Borneo explored using surface waves from ambient noise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21108, https://doi.org/10.5194/egusphere-egu2020-21108, 2020.

We present preliminary P-wave tomographic images of the upper mantle beneath northern Borneo (Sabah) using teleseismic earthquake data. Sabah underwent diachronous double-polarity subduction, one dipping to the southeast (terminated in the early Miocene) and the other to the northwest (terminated 5-6 Ma). With the goal of better understanding post-subduction processes in Sabah, 24 permanent seismic stations of MetMalaysia were augmented by the deployment of 46 temporary stations of the nBOSS network, which ran from March 2018 to January 2020. Relative P-wave traveltime residuals from nearly a thousand teleseismic events have been extracted from the continuous records using an adaptive stacking technique, which uses the coherency of global phases across the entire network. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival time residuals are mapped as 3-D P-wave perturbations.

The most intriguing feature of the final tomographic model is a north-east trending lithospheric structure running across northern Borneo and separating relatively low to high wavespeeds to the west and east, respectively. This structure possibly indicates the suture between pre-Cenozoic lithosphere to the east and the Cenozoic accreted material to the west.

Results from receiver function analysis (i.e., crustal thickness) and crustal velocities from ambient noise tomography will be in the future incorporated in the tomographic inversion in order to obtain an integrated view of the crust-mantle system beneath Sabah.

How to cite: Pilia, S., Rawlinson, N., Tongkul, F., Gilligan, A., and Cornwell, D.: Preliminary results from teleseismic tomography of the upper mantle beneath northern Borneo, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4148, https://doi.org/10.5194/egusphere-egu2020-4148, 2020.

The Bransfield Strait is a tectonically active region located between the South Shetland archipelago (SSI) and the Antarctic Peninsula (AP), characterised by the presence of an incipient back-arc spreading ridge driven by on-going slab rollback of the Phoenix plate under the Antarctic and Shetland plates. Twelve broad-band seismic stations deployed in the region are used to obtain P-wave receiver functions from teleseismic earthquakes to improve the current understanding of the crust and upper mantle structures. This includes the depth and spatial variability of the Moho discontinuity, the average crustal Vp/Vs ratio and the thickness of the Mantle Transition Zone (MTZ). Results reveal a highly variable crustal thickness in the South Shetland block, ranging from ~30 km near the SW and NE ends of the South Shetland Trench to ~15 km in the central Bransfield Basin (Deception Island), where the highest Vp ⁄ Vs ratios in the region are reached (> 2). In contrast, the AP displays typical and homogeneous continental crust characteristics with an average crustal thickness of ~34 km and Vp/Vs ~1.77. A low velocity zone (LVZ) is identified under all stations suggesting partial melting in the upper mantle beneath the lithosphere, which is widespread throughout the region and not only confined to the mantle wedge above the subducted Phoenix oceanic slab. There is evidence of magmatic underplating under the SSB in accordance with the LVZ together with the active volcanism and the high crustal Vp/Vs ratio in the area. The Phoenix oceanic slab is inferred to subduct steeply, as the MTZ appears already thickened under the AP.

How to cite: Parera-Portell, J. A., Mancilla, F. D. L., Morales, J., and Almendros, J.: Structure of the crust and upper mantle beneath the Bransfield Strait (Antarctica) using P-wave Receiver Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12344, https://doi.org/10.5194/egusphere-egu2020-12344, 2020.

We present evidence of significant diversity in the Indian cratonic lithosphere mantle based on the analysis of 3-D shear wave velocity maps. These images are obtained through the inversion of 21600 fundamental mode Rayleigh wave group velocity dispersion data retrieved from ambient noise and from earthquake waveforms. The velocity model is constructed using two step approach-firstly generating group velocity maps at 1^° square grid at time periods from 10s to 100s; and subsequently inversion of dispersion data at each grid node to a depth of 200 km in terms of velocity-depth model. Analysis of velocity images suggest a bipolar characteristics of lithospheric mantle. We observe a two layer-lithospheric mantle correlated with the Eastern Peninsular India comprising of Archean cratons like east Dharwar, Bastar, Singhbhum, Chotanagpur, Bundelkhand and Proterozoic Vindhyan Basin. The intra lithospheric mantle boundary is at a depth of ~90 km where Vs increases from 4.5 km/s to over 4.7 km/s. The positive velocity gradient continues to a depth of 140-180 km beyond which it reverses the trend and mapped as layer with lower velocity Vs of 4.3-4.4 km/s, as which could be possibly defined as the lithosphere-asthenosphere boundary. Geologically, the region correlates with the kimberlite fields with the xenoliths showing presence of eclogite in them. The other group of Precambrian terrains like 3.36 Ga western Dharwar, eastern Deccan Volcanics, southern Granulite terrane and the Marwar block in western India are characterized by an almost uniform mantle with shear wave velocity of 4.4-4.5 km/s, also supported by other seismological studies. We do not observe any low-velocity layer underlying these terrains. Presence of such a uniform lower than expected mantle velocity could be due to its fertilization through an early geodynamic process. The velocity imprint of Deccan volcanism is best preserved in term of the thinned lithosphere (100-120 km) restricted to the westernmost part of Deccan Volcanic Province (DVP). This suggests that the plume-Indian lithosphere interaction was primarily confined to the western most Deccan volcanic province and possibly extending into the Indian ocean.

How to cite: Saha, G. K. and Rai, S. S.: Diversity in the Indian lithosphere revealed from ambient noise and earthquake tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1102, https://doi.org/10.5194/egusphere-egu2020-1102, 2020.

S wave reflectors in the crust may be caused by strong heterogeneous structures such as ones containing  fluid. Especially, fluid around an earthquake fault could play an important role for initiation of the earthquake rupture as a mechanism for reducing fault strength. The location and geometry of the reflector can be determined from the travel time of the reflected phases. For detecting the reflections, we need to observe at stations located close to the hypocentre because of sufficient phase separation of the small lapse time of the reflected phases due to a reflector in the crust from direct S wave. In this study, we attempted to detect reflected waves in observed seismograms at the seismic stations in and around the 2016 Kaikoura earthquake (Mw7.6). Seismic records were obtained from the permanent GeoNet stations as well as from seismic stations deployed before the Kaikoura earthquake in the northern South Island. We applied reflection seismology techniques to the data obtained by the network. We used seismograms with smaller epicentral distance than 30 km and obtained dip move-out sections for each station. We detected several reflectors in the mid and lower crust from the sections. Strong reflected phases were observed at the southern edge of the focal area (from a reflector with depth about 20 km). Weak reflectors were detected in/beneath the aftershock area (in the mid- to lower-crust). In addition, the subducting slab might be imaged with dipping angle 20 degree.  Reflectors parallel to the slab were also found below the interface.

How to cite: Matsumoto, S., Kawamura, Y., Okada, T., Matsuno, M., Iio, Y., Sibson, R., Savage, M., Graham, K., Suzuki, M., and Bannister, S.: Detection of S-wave reflectors beneath aftershock area of the 2016 Kaikoura earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3843, https://doi.org/10.5194/egusphere-egu2020-3843, 2020.

The Central and East Java region is part of the Sunda Arc which has an important role in producing destructive earthquakes and volcanic complexes as a result of the subduction of the Indo-Australian plate under the Eurasian plate. Seismic tomography is one geophysical tool that is adaptable to understanding the mechanism process related to tectonic activity, seismicity, and volcanism. We collected a series of waveforms from 1,519 events in the period January 2009 to September 2017 and re-picked 11,192 phases for P- and S-waves at 34 stations of the BMKG network. We determined the 3-D P- and S-wave velocity structure beneath this high-risk region down to a depth of 200 km. In this study, we compare the tomographic images and relocated seismicity in order to represent the subducted slab geometry and the features in the seismic zones, i.e. the 2006 Yogyakarta earthquake zone (Opak fault), south of the mainland, and the 1994 Banyuwangi earthquake zone. Low-velocity anomalies beneath the volcanoes, i.e. Merapi, Merbabu, Kelud, Semeru, Bromo, and Ijen also imply the existence of fluid material and possible partial melting of the upper mantle which migrated from the subducted slab.

How to cite: Muttaqy, F., Nugraha, A. D., Puspito, N. T., Mori, J. J., Daryono, D., Rohadi, S., and Supendi, P.: P- and S-wave Velocity Structure beneath Central and East Java, Indonesia: Preliminary Result, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4299, https://doi.org/10.5194/egusphere-egu2020-4299, 2020.

Seismic scattering and absorption parameters provide valuable information about the propagation of seismic waves within the lithosphere. For the analysis of the seismic scattering and absorption parameters in the oceanic lithosphere of the Eastern North Atlantic, we use a seismological array which was installed in 5000 m water depth about 100 km North of the Gloria fault, defining the plate boundary between the Eurasian and African plate at this location. During our seismological experiment, more than 350 local and regional earthquakes were identified within 10 month for epicentral distances up to 900 km. The acquired regional earthquake recordings show up to 30 Hz P and S wave arrivals with long codas lasting tens to hundreds of seconds. Modelling results suggest that these long codas originate from scattering in the oceanic lithosphere. The waves travel with upper mantle apparent velocities and are therefore referred to as oceanic Pn (Po) and Sn (So) waves. We use direct So waves and their coda of pre-selected earthquakes to estimate frequency-dependent seismic scattering and intrinsic attenuation parameters. The results for the analysed events show that intrinsic attenuation is stronger than scattering attenuation and the estimated transport mean free path lengths between 30-800 km indicate that the So wave coda is weakly influenced by the oceanic crust. Furthermore, the calculated parameters show higher attenuation for western to northern event azimuths than for eastern to southern ones which might be related to differences in lithospheric ages for example of the Eurasian and African plates, or the influence of the Gloria fault itself.

How to cite: Hannemann, K., Eulenfeld, T., Krüger, F., and Dahm, T.: Seismic scattering and absorption of oceanic lithospheric S waves in the Eastern North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5790, https://doi.org/10.5194/egusphere-egu2020-5790, 2020.

Lithosphere-asthenosphere boundary (LAB) has generally been estimated using surface tomography (Priestley and McKenzie, 2006; Auer et al., 2014), surface waves anisotropy (Plomerova et al, 2002; Burgos et al., 2014) and receiver function methods (Kumar and Kawakatsu, 2011; Rychert et al., 2012), indicating a change in the S-wave velocity. Recently, some limited studies have shown that LAB could be imaged using seismic reflection imaging method (Stern et al., 2015; Mehouachi and Singh, 2018), requiring a sharp P-wave velocity contrast. Here, we present a continuous seismic reflection image of the LAB from 2-to-75-Ma in the Equatorial Atlantic Ocean over the African plate along a 1400 km long profile using a 12 km long offset multi-channel multi-component (hydrophone and three component acceleration) seismic data. Optimal de-ghosting by summation of pressure (P) and vertical acceleration (A_z) components has improved the bandwidth of the data by removing the notches (Vassallo et al., 2013). Further advanced processing consisted of noise removal in shot and receiver domains followed by low frequency boost-up. Trace interpolation followed by wavenumber filtering was performed in the common-midpoint domain to achieve a higher signal-to-noise ratio and flat events enhancement. Post-stack processing mainly consisted of frequency-space deconvolution, dip filtering and data-adaptive edge-preserving modified Kuwahara filter. We find two prominent reflections: The upper varies from ~12 km (3.5 s) two-way time (TWT) below the seafloor at 2 Myr to ~78 km (20 s) TWT below the seafloor at 75 Myr. For 27-to-47-Myr old lithosphere, we image a second almost flat continuous reflection from 21 s to 22 s TWT below the seafloor (~82 km). This event is not prominent for 49-to-75-Myr old lithosphere, probably due to the influence of the mantle thermal anomalies from St.Helena/Cameroon. There is also evidence of some other reflections between these two reflections, which also dip towards older age. The crust-mantle boundary or the Moho is also fairly imaged throughout the profile with crustal thickness ranging between 1.4 – 2.7 s TWT. We interpret the uppermost reflection as the base of lithosphere, the top of the LAB, and the lower reflection as the base of the LAB, while the reflections between these as sheared melt sheets. Our results provide the very first image of the whole LAB system continuously as it deepens and thins away from the ridge axis. In this presentation, we will discuss different implications of these images and provide a comprehensive model of the oceanic LAB over normal oceanic lithosphere.

How to cite: Audhkhasi, P. and Singh, S.: Continuous seismic reflection image of the Lithosphere-Asthenosphere Boundary (LAB) from 2-75 Ma on the African plate in the Equatorial Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9622, https://doi.org/10.5194/egusphere-egu2020-9622, 2020.

The maximum achievable resolution of a tomographic model varies spatially and depends on the data sampling and errors in the data. Adaptive parameterization schemes match the spatial variations in data sampling but do not address the effects of the errors. The propagation of systematic errors, however, is resistant to data redundancy and results in models dominated by noise if the target resolution is too high. This forces us to look for smoother models and thus limits the imaging resolution.

We develop a surface-wave tomography method that finds optimal lateral resolution at every point by means of error tracking. We first measure inter-station phase-velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Unlike in the classical approach, multiple versions of the maps with varying smoothness constraints are computed, so that the maps range from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can then be inverted for shear-velocity (V_s) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. Very smooth V_s models computed from very smooth phase-velocity maps will be the most robust, but at a cost of a loss of most structural information. At the other extreme, models that are too rough will be dominated by noise. We define the optimal resolution at a point such that the error of the local phase-velocity curve is below an empirical threshold. The error is estimated by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure. A 3D V_s model is then computed by the inversion of the phase-velocity maps with the optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, optimal resolution does not scale with the density of the data coverage: some of the best-sampled locations require relatively low lateral resolution, probably due to systematic data errors. We apply the method to image the Ireland’s and Britain’s upper mantle, using our large, new regional dataset. We report a pronounced thinning of the lithosphere beneath the British Tertiary Igneous Province, with important implications for the Paleogene uplift and volcanism in the region.

How to cite: Bonadio, R., Lebedev, S., Arroucau, P., Schaeffer, A., Licciardi, A., Agius, M., Horan, C., Collins, L., O'Reilly, B., Readman, P., and Working Team, T. I. A.: Optimal resolution seismic tomography: Lithospheric thinning beneath the British Tertiary Igneous Province and other new observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11942, https://doi.org/10.5194/egusphere-egu2020-11942, 2020.

The distribution of magmatic structures within lithosphere beneath Mt. Baekdu is important to understand the origin, evolution history, and current status of the volcano. A previous study using ambient noise surface wave dispersions (Kim et al., 2017) suggested a feature of magma fractionation and cooling with layering of mafic and felsic structures in deeper crust. However, the existence of melt directly beneath the Mt. Baekdu and corresponding modification of lithosphere are still unclear. In this study, we additionally calculate P-wave receiver functions for stations in Mt. Baekdu and surrounding regions to confirm the previously defined structures and to check the existence of partial melting. We obtain data from three temporary arrays distributed over areas in Northeast China and DPRK. A harmonic decomposition approach is used to account for anisotropy and to obtain isotropic receiver functions. A series of joint inversions is performed for isotropic receiver functions and surface wave dispersion data using a hierarchical transdimensional Bayesian method. Retrieved isotropic radial receiver functions from stations near (<50 km) the Mt. Baekdu show a consistent negative signal between P and Ps conversion, which indicates low velocity layers in the crust. In addition, relatively high energy of tangential receiver functions with two- and four-lobe patterns indicate that effects of azimuthal anisotropy and/or tilted interfaces are also significant beneath the volcano. On the other hand, stations away from the volcano show features of more isotropic and homogeneous crustal structures. Inverted models show a clear pattern of thicker crust (35-40 km) with slower S-wave velocities (3.2-3.4 km/s in the crust and 4.0-4.2 km/s in the upper mantle) beneath the Mt. Baekdu compared to other regions with relatively shallow Moho (33-35 km) and high velocities (>3.4 km/s in the crust and >4.2 km/s in the upper mantle). The result indicates that a localized structure with elevated temperature and potentially partial melting is exist directly beneath the volcano. With further analysis, more detailed variations of whole lithosphere structures will be presented.

How to cite: Lim, Y. and Kim, S.: Variations of lithospheric structures around the Mt. Baekdu (Changbaishan) volcano from Bayesian joint inversions with receiver functions and surface wave dispersions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19226, https://doi.org/10.5194/egusphere-egu2020-19226, 2020.

Imaging the subsurface structure through seismic data needs various information and one of the most important information is the subsurface P-wave velocity. The P-wave velocity structure mainly influences on the location of the reflectors during the subsurface imaging, thus many algorithms has been developed to invert the accurate P-wave velocity such as conventional velocity analysis, traveltime tomography, migration velocity analysis (MVA) and full waveform inversion (FWI). Among those methods, conventional velocity analysis and MVA can be widely applied to the seismic data but generate the velocity with low resolution. On the other hands, the traveltime tomography and FWI can invert relatively accurate velocity structure, but they essentially need long offset seismic data containing sufficiently low frequency components. Recently, the stochastic method such as Markov chain Monte Carlo (McMC) inversion was applied to invert the accurate P-wave velocity with the seismic data without long offset or low frequency components. This method uses global optimization instead of local optimization and poststack seismic data instead of prestack seismic data. Therefore, it can avoid the problem of the local minima and limitation of the offset. However, the accuracy of the poststack seismic section directly affects the McMC inversion result. In this study, we tried to overcome the dependency of the McMC inversion on the poststack seismic section and iterative workflow was applied to the McMC inversion to invert the accurate P-wave velocity from the simple background velocity and inaccurate poststack seismic section. The numerical test showed that the suggested method could successfully invert the subsurface P-wave velocity.

How to cite: Jun, H., Jou, H.-T., Kim, H.-J., and Lee, S. H.: Iterative P-wave velocity inversion using Markov chain Monte Carlo method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6298, https://doi.org/10.5194/egusphere-egu2020-6298, 2020.

In a variety of geoscientific applications we require maps of subsurface properties together with the corresponding maps of uncertainties to assess their reliability. Seismic tomography is a method that is widely used to generate those maps. Since tomography is significantly nonlinear, Monte Carlo sampling methods are often used for this purpose, but they are generally computationally intractable for large data sets and high-dimensionality parameter spaces. To extend uncertainty analysis to larger systems, we introduce variational inference methods to conduct seismic tomography. In contrast to Monte Carlo sampling, variational methods solve the Bayesian inference problem as an optimization problem yet still provide fully nonlinear, probabilistic results. This is achieved by minimizing the Kullback-Leibler (KL) divergence between approximate and target probability distributions within a predefined family of probability distributions.

We introduce two variational inference methods: automatic differential variational inference (ADVI) and Stein variational gradient descent (SVGD). In ADVI a Gaussian probability distribution is assumed and optimized to approximate the posterior probability distribution. In SVGD a smooth transform is iteratively applied to an initial probability distribution to obtain an approximation to the posterior probability distribution. At each iteration the transform is determined by seeking the steepest descent direction that minimizes the KL-divergence.

We apply the two variational inference methods to 2D travel time tomography using both synthetic and real data, and compare the results to those obtained from two different Monte Carlo sampling methods: Metropolis-Hastings Markov chain Monte Carlo (MH-McMC) and reversible jump Markov chain Monte Carlo (rj-McMC). The results show that ADVI provides a biased approximation because of its Gaussian approximation, whereas SVGD produces more accurate approximations to the results of MH-McMC. In comparison rj-McMC produces smoother mean velocity models and lower standard deviations because the parameterization used in rj-McMC (Voronoi cells) imposes prior restrictions on the pixelated form of models: all pixels within each Voronoi cell have identical velocities. This suggests that the results of rj-McMC need to be interpreted in the light of the specific prior information imposed by the parameterization. Both variational methods estimate the posterior distribution at significantly lower computational cost, provided that gradients of parameters with respect to data can be calculated efficiently. We therefore expect that the methods can be applied fruitfully to many other types of geophysical inverse problems.

How to cite: Zhang, X. and Curtis, A.: Seismic Tomography Using Variational Inference Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7389, https://doi.org/10.5194/egusphere-egu2020-7389, 2020.

Seismic waves lose energy during propagation in heterogeneous Earth media. Their decrease of amplitude, defined as seismic attenuation, is central in the description of seismic wave propagation. The attenuation of coherent waves can be described by the total quality factor, Q, and it is defined as the fractional energy lost per cycle, controlling the decay of the energy density spectrum with lapse time. The coda normalization (CN) method is a method to measure the attenuation of P- or S-waves by taking the ratio of the direct wave energy and late coda wave energy in order to remove the source and site effects from P- and S-wave spectra. One of the main assumptions of the CN method is that coda attenuation, i.e. the decay of coda energy with lapse time measured by the coda quality factor Q_c is constant. However, several studies showed that Q_c is not uniform in the crust for the lapse times considered in most attenuation studies. In this work, we propose a method to overcome this assumption, measuring coda attenuation for each source-station path and evaluating the effect of different scattering regimes on the corresponding imaging. The data consists of passive waveforms from the fault network in the Pollino Area (Southern Italy) and Mount St. Helens volcano (USA).

How to cite: Sketsiou, P., De Siena, L., Gabrielli, S., and Napolitano, F.: Seismic attenuation tomography using body-wave energy normalised by the heterogeneous coda, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19859, https://doi.org/10.5194/egusphere-egu2020-19859, 2020.

Lateral variations in crustal structure may affect the propagation of Lg phases, guided waves that propagate efficiently only in the continental crust. Seismic paths crossing continental-oceanic transitions are characterized by Lg blockage due to the drastic decrease in crustal thickness. Here, we investigate the effects of crustal thinning on wave propagation in the Tyrrhenian basin using radiative transfer theory. We first model regional coda envelopes (600-800km) using the software tool Radiative3D (Sanborn & Cormier 2018, GJI). It allows to synthesize seismograms envelopes produced by earthquakes by propagating energy packets through a deterministic structure, taking into account the crustal layers, including Moho transition depth, and parameters describing the medium heterogeneities.  Then, we approach the complex problem of meshing, including measured Moho depths, for simulations based on spectral elements (Komatitsch D. et al., 2012, SPECFEM3D, Computational Infrastructure for Geodynamics) and finite differences methods (Maeda et al., 2017, OpenSWPC). The results aim at understanding complex wave attenuation and leakage in the mantle, for future implementations into the Multi-Resolution Attenuation Tomography code (MuRAT – De Siena et al. 2014, JVGR)

How to cite: Nardoni, C., De Siena, L., Cammarano, F., and Mattei, E.: Coda wave simulations across the Tyrrhenian Basin using radiative transfer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9823, https://doi.org/10.5194/egusphere-egu2020-9823, 2020.

Intrinsic attenuation plays an important role in investigating the interior structure of Earth, especially for the Lithosphere-asthenosphere system, the best place to understand the physical mechanics of plate tectonic. The dissipation, the high attenuation of seismic waves in the low-velocity zones, and the frequency dependence are the characteristic of intrinsic attenuation. However, N. Takeuchi, et al. measured the Northwestern Pacific Ocean’s lithosphere-asthenosphere system, and state the attenuation of the asthenosphere is 50 times larger than the attenuation of lithosphere attenuation. The attenuation of the lithosphere shows strong frequency dependency, but the attenuation of the asthenosphere does not. Previous theories of attenuation failed to explain this phenomenon. Here we demonstrate an explicit attenuation formulation to explain the high attenuation of seismic waves in the low-velocity zones and to show the mechanisms of spectral of teleseismic body waves rapidly fall off as frequency bigger than 1 Hz by perturbing the wave equation with the novel method we proposed. The result also indicates that the difference between the attenuation of the lithosphere and asthenosphere is because their attenuation governs by different physics mechanisms and mathematical models. Moreover, we illustrate the explicit formulation of the relationship between apparent t*, wave velocity, and frequency.

How to cite: Wu, Y.-C. and Wu, C.-J.: The Nature of Intrinsic Attenuation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4220, https://doi.org/10.5194/egusphere-egu2020-4220, 2020.

There is the problem that application of controlled-source seismic exploration is not always possible in nature protected areas. As an alternative, application of passive seismic techniques in such areas can be proposed. In our study we show results of application of passive seismic interferometry for mapping the uppermost crust in the area of active mineral exploration in northern Finland using the data recorded during XSoDEx (eXperiment of SOdankylä Deep Exploration) project. The objectives of the project were to obtain a structural image of the upper crust in the Sodankylä area of Northern Finland in order to achieve a better understanding of the mineral system at depth. Within XSoDEx, a combined seismic reflection and refraction survey was organised by Geological Survey of Finland, University of Oulu, Finland (Oulu Mining School and Sodankylä Geophysical Observatory) and TU Bergakademie Freiberg, Germany. The vibrotrack of TU BAF was used as a source. The experiment was performed during July and August 2017 resulting in an approximately 80 km long seismic profile line. The seismic refraction data were simultaneously recorded by 60 vertical- and 40 three-component wireless autonomous receivers along an extended line around the reflection spread with maximum offsets of around 10 km. During night time, the receivers were recording passive seismic data. Thus the XSoDEx experiment provided a good opportunity to verify results of passive seismic interferometry with controlled-source seismic data, to identify limitations of this technique in areas of generally low level of high-frequency anthropogenic noise and to propose possible improvements of known techniques. Analysis of the data and theoretical modelling demonstrated that the dominating sources of ambient noise are non-stationary and have different origin in different parts of XSoDEx lines. In addition, the length of passive data for cross-correlation was limited to several hours and the long data recording period is usually considered as one of the main conditions for seismic interferometry applications. In order to obtain reliable Empirical Green Functions (EGF) from such short-term and non-stationary data, we applied a special technique (signal-to-noise ratio stacking). The calculated EGFs were inverted in order to obtain S-wave velocity models along XSodEx lines down to a depth of several hundreds metres. The obtained results are S-wave seismic velocity models of the upper crust in Northern Finland that agree well with geological data and complement the results of reflection seismic data interpretation.

How to cite: Kozlovskaya, E., Afonin, N., Karjalainen, J., Heinonen, S., and Buske, S.: Passive seismic interferometry in XSoDEx experiment in northern Finland , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12921, https://doi.org/10.5194/egusphere-egu2020-12921, 2020.

We developed a new method where teleseismic P-to-S converted waves are used to construct a fully 3-D shear-wave velocity model of the crust. The method differs from ambient noise and local earthquake tomography in its ray-paths being closer to vertical. Our approach requires a dense seismological network, and we first focus on the Central Alps considering the available permanent and temporary station datasets (e.g., Hetényi et al., 2018, Surv. Geophys.).

We implemented an accurate ray-propagator which respects Snell’s law in 3-D at any interface geometry. Following a teleseismic P ray propagator (Knapmeyer, 2004) from event to station which uses a 1-D global velocity model (iasp91), P-to-S conversion at the Moho is calculated for the crustal S ray considering the true local dip. The corresponding arrival to the surface is typically several km away from the station, which we then adjust by changing the ray-parameter. In the Central Alps, using the 3-D P-velocity structure of Diehl et al. (2009) and the local Moho geometry of Spada et al. (2013), the mean distance between the arriving S-wave and the station is about 150 m (median ca. 40 m).

For our approach we adopt a new model parameterization of velocities. It is rectangular in map view (nodes at 25x25 km in the Alps), while in depth we define a 2-layer model with separate velocities above and below each discontinuity. The introduction of this flexibility allows us to accommodate a velocity gradient within each layer and investigate velocity jumps across discontinuities.

The inversion proceeds iteratively, by visiting every node of the map following a Travelling Salesman Path. At each node, receiver function rays in the surrounding volume are considered for inversion, and bundled into sub-blocks and ranges of back-azimuth (5x5 km size, 45° or 60° bins for the Central Alps). The velocity model at the given node is inverted using the technique of Simulated Annealing, followed by a pattern search algorithm to avoid falling in a local minimum. During iterations of the Simulated Annealing, individual velocity model corresponding to each receiver function is extracted from the 3-D model along its ray path.

The inversion proceeds for 4 or 5 independent parameters: Moho and a hypothetical intra-crustal discontinuity depth, Vp/Vs ratio (either full crust, or separately for upper and lower crust) and the P-wave velocity jump at the intra-crustal discontinuity. Finally, the velocity structure is updated with the result obtained at the given node. We observe that a few rounds of Travelling Salesman Paths improve the overall misfit.

First results on the Central Alps show that the Moho depth generally reflects well the roots of the Alpine orogen. Resolving crustal Vp/Vs ratio is more stable when considering the full crust, instead of two separate layers. The Conrad discontinuity remains difficult to resolve. The obtained velocity structure is compared along profiles to recent Vs results from 3-D ambient noise tomography (Lu et al., 2018).

How to cite: Colavitti, L., Hetényi, G., and Working Group, A.: A new inversion method to construct a 3-D crustal shear-wave velocity model from P-to-S converted waves and application to the Central Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10386, https://doi.org/10.5194/egusphere-egu2020-10386, 2020.

Reducing multiple contaminations in reflection seismic data remains one of the greatest challenges in seismic processing and its effectiveness is highly dependent on geologic settings. We undertook two-dimensional reflection seismic data crossing the upper and lower accretionary wedge slopes off SW Taiwan to test the efficiency of various multiple-attenuation scenarios. The area has resulted from an incipient arc-continent collision between the northern rifted margin of the South China Sea and the Luzon volcanic arcs. The wedge extends from shallow water to deep water bathymetries, hence promoting both short-period and long-period multiples within the seismic records. The multichannel seismic data were achieved under 468 hydrophones, 4-ms sampling rate, 12.5-m channel spacing, 50-m shot spacing and 15-second recording length. Preprocessing flow includes swell noise removal, direct wave mute, and missing channel and shot restoration. A subset of demultiple methods based on the periodicity nature and the spatial move-out behavior of multiples were explored to attenuate multiples energy under different geologic environments. The first step relies on the simultaneous subtraction of surface-related multiples, which combined wave-equation multiple attenuation (WEMA) and surface-related multiple elimination (SRME). WEMA is a shot domain multiple attenuations based on a combination of numerical wave extrapolation through the water layer and the water bottom reflectivity. This method was capable to partially suppress the water layer multiples. SRME was applied to attenuate the residual multiple energy at near-offset. This method assumes surface-related multiples can be kinematically predicted by convolution of prestack seismic traces at possible surface multiple reflection locations. Some primary reflections seem to be better retained after the combined subtraction process than using WEMA or SRME filtering independently. The second step lies on parabolic Radon transform to attenuate far-offset multiples by subtracting the noise energy in tau-p on input gathers that have been corrected for normal move-out and inverse transform the remaining primary energy back to CMP-offset domain. Predictive deconvolution in the x-t domain was performed to attenuate low-frequency reverberations in the upper wedge slope. A double-gap deconvolution operator was extended to predict reverberations with correct relative amplitudes, followed by time-variant bandpass filtering to reduce much of residual multiple energy. In general, WEMA and predictive deconvolution were more effective in attenuating the multiples energy at the upper wedge slope where the water depths are shallower; whereas SRME and parabolic Radon were capable of reducing the energy of multiples at the lower wedge slope. Nevertheless, multiples energy could not be fully eliminated due to several factors. The dependency of some demultiple methods (e.g. parabolic Radon, WEMA, SRME) on velocity function may perturb the forward multiple predictions before subtraction as primary velocities might not be present due to the highly tilted strata in the thrust belts domain. Furthermore, parabolic Radon may not perform well in shallow water and area with slowly increasing velocities with depth (e.g. the upper wedge slope). Since the reflection seismic dataset spans various tectonic environments and water depth, results suggest there was no single demultiple method capable to suppress multiples in all environments.

How to cite: Dirgantara, F., Lin, A. T.-S., Liu, C.-S., and Chen, S.-C.: Demultiple strategies in the submarine slope of Taiwan accretionary wedge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5178, https://doi.org/10.5194/egusphere-egu2020-5178, 2020.