SM5.1

More often than not, important geologic processes occur at micro-scales, e.g., fluid flow, mineral-phase changes, chemically-induced alteration, rock-frame compaction, or even mechanical ruptures/instabilities leading to large earthquakes. However, reliably imaging material properties at such scales from remote long-wavelength information contained in either seismic or EM fields has long been a challenge to the geophysical, engineering and material science communities. In this talk, we present a general framework for the estimation of sub-wavelength material properties from long-scale waves, building on recent advances on statistical microstructure descriptors (SMDs) within the field of material science.

In geoscience, traditional approaches to describing material microheterogeneity rely on either analytical inclusion-based models, or in sample-based digital rocks: each of these having their pros and cons. Here, we instead rely on SMDs, namely two-point correlation and polytope functions, to describe microheterogeneous geo-materials in a manner that is capable of generalizing complex geometrical information hidden in microstructures, while also retaining realism and sample fidelity. Using SMDs, we rely on wave-equation-based Strong Contrast Expansions (SCEs) to predict frequency/scale-dependent effective wave properties for acoustic, elastic and EM waves.  We briefly discuss how SMD-described microstructures affect long-wave properties – and in particular how they not only predict frequency-dependent attenuation due to sub-wavelength scattering, but that attenuation is particularly sensitive to microstructure when compared to effective wavespeeds.

When it comes to the estimation of microstructure properties from wave observations, the problem becomes substantially more difficult because realistic microscale parameters could in principle have far too many degrees of freedom than what is observable from finite-frequency wave data. As such, it is key that any method that aims at realistically retrieving microstructure information from long-scale wave data accounts for uncertainty, while also handling the highly nonlinear nature of microstructure-dependent effective wave properties. To that end, we combine our SMD and SCE approaches for effective wave properties with the supervised machine-learning method of Random Forests to construct a Bayesian approach to infer microstructure properties from effective wave parameters as observables. This method yields full posterior distributions for microstructure parameters (e.g., property contrast, volume fraction, and geometry information) from frequency-dependent observations of wave velocities and attenuation. We present several examples of inference scenarios, showing, for example, that i) attenuation is key to microstructure imaging, and ii) microgeometry information can only be reliably retrieved if contrast and volume fraction are relatively well known a priori. We illustrate of inference approach with several examples of analytical and real microstructures, including data from a  laboratory compaction experiment controlled by microscale CT imaging.

How to cite: Vasconcelos, I., Klessens, W., Jiao, Y., Niemeijer, A., and Hangx, S.: Imaging the Earth’s small structures: AI-driven, Bayesian inference of microstructure descriptors from finite-frequency waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10058, https://doi.org/10.5194/egusphere-egu21-10058, 2021.

About 25% of the Earth’s mid-ocean ridges spread at ultraslow rates of less than 20 mm/yr. However, most of these ultraslow spreading ridges are located in geographically remote areas, which hamper investigation. Consequently, how the crust forms and ages at such spreading centres, which traditional models predict to be magma-starved and cold, remains poorly understood.

CAYSEIS project was proposed to survey the Cayman Trough area in order to obtain new data that constraints the nature of the crust, tectonic structures, lithologies outcropping and hydrothermal processes taking place in this area, which includes the Mid Cayman ultra-slow spreading centre (MSCS) with spreading rates of ~15-17 mm/yr. Understanding the sub-seabed geophysical structure of the MCSC is key to understanding not only the lithologies and structures exposed at the seabed, but more fundamentally, how they are related at depth and what role hydrothermal fluid flow plays in the geodynamics of ultraslow spreading. CAYSEIS was a joint and multidisciplinary programme of German, British and US American top tier scientists designed for the obtaining of a new high-quality dataset, including 3D Wide-Angle Seismic (WAS), magnetic, gravimetric and seismological data.

We took leverage of the CAYSEIS dataset to invert a 3D velocity model of the Cayman Trough lithosphere using the Tomo3D code (Meléndez et al., 2015; 2019). This is one of the first times that the Tomo3D code is used for 3D inversion of real datasets. Thus, we are checking our results comparing them with travel time tomographic inversions of 2D lines and testing the different parameters to obtain the more accurate and higher resolution model as possible. The results of this experiment show not only the lithospheric structure along and across the MSCS, including the exhumed Ocean Core Complexes in the surrounding areas, but the 3D lithospheric configuration of the region which is important to understand the crustal formation processes and evolution of ultra-slow spreading settings.

How to cite: Gómez de la Peña, L., Grevemeyer, I., Dannowsky, A., Meléndez, A., Peirce, C., and van Avendonk, H.: 3D tomographic imaging of the Cayman Trough lithosphere: challenges, ongoing work and first results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1545, https://doi.org/10.5194/egusphere-egu21-1545, 2021.

How to cite: González Álvarez, I., Rost, S., Nowacki, A., and Selby, N.: Small-scale lithospheric heterogeneity characterization using Bayesian inference, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11115, https://doi.org/10.5194/egusphere-egu21-11115, 2021.

The North Anatolian Fault (NAF), a right-lateral strike-slip fault spanning 1500 km in length, stretches across northern Turkey and it marks the boundary between the Eurasian and Anatolian plates. Nucleating in the east, at the Karliova triple junction and reaching the Aegean Sea at the west, it is a particularly active fault zone with a series of migrating high-magnitude earthquakes. Using 6445 Z-component waveforms from a temporary seismic network in the area (Dense Array for North Anatolia – DANA), this study aims to investigate the western part of the NAF, which splays into a northern and southern branch. Coda attenuation imaging is utilised for imaging the absorption characteristics of the area, as it can be used as a marker for source and dynamic Earth processes due to its higher sensitivity to small variations of lithospheric properties compared to seismic velocity. The absorption structure is recovered by inverting for the coda attenuation quality factor, Q_c, at frequencies between 3-18 Hz, using sensitivity kernels. The extensive seismicity in the area, as well as the density of the seismic stations, provide high-resolution models of 0.04-0.05 degrees in spacing. The scattering structure of the region is imaged using peak-delay time, which is used as a direct measure for multiple forward scattering. Preliminary results show a clear change in scattering between the Istanbul and Sakarya zones, north and south of the fault respectively, with the scattering increasing from north to south at lower frequencies and decreasing at higher frequencies. At a smaller scale, absorption and scattering anomalies appear to outline contrasting geological units beneath the DANA network.

How to cite: Sketsiou, P., De Siena, L., and Cornwell, D. G.: Coda-attenuation imaging of the North Anatolian Fault Zone, northern Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15749, https://doi.org/10.5194/egusphere-egu21-15749, 2021.

Between August and October 2016, the Central Apennines in Italy have been struck by a long-lasting seismic sequence, known as the Amatrice (Mw 6.0) - Visso (Mw 5.9) - Norcia (Mw 6.5) sequence. The cascading ruptures occurred in this sequence have been considered connected to the fluid migration in the fault network, as suggested by previous studies. The behaviour of fluids in the crust is crucial to understand earthquakes occurrence and stress changes since fluids reduce fault stability. It has long been understood that the seismic attenuation is strongly controlled by the structural irregularity and heterogeneities; micro-cracks and cavities, either fluid-filled or dry, temperature and pressure variations cause a decrease in seismic wave amplitude and pulse broadening. Hence seismic attenuation imagining is a powerful tool to be a relevant provenance of information about the influence and abundance of fluids in a seismic sequence.

The aim of this work is to separate scattering and absorption contributions to the total attenuation of coda waves and to provide their spatial and temporal variations at different frequency bands of these quantities using two datasets: the first one comprising 592 earthquakes occurred before the sequence (March 2013-August 2016) and the second one comprising 763 events (ML > 2.8) from the Amatrice-Visso-Norcia sequence. Scattering and absorption have been measured through peak-delay and coda-wave attenuation parameters (the latter inverted using frequency-dependent sensitivity kernels).

The preliminary results show a clear difference between the pre-sequence and sequence images, mainly at low frequencies (1.5 Hz), where we can define a spatial increase of scattering with time attributed to rock fracturing and fluid circulation. The coda attenuation tomography also demonstrates a clear variation between the pre-sequence and the sequence over series of time windows being before and after the largest main shocks of the seismic sequence, with an increase of the attenuation in space with decreasing time. The peak delay indicates a high scattering area corresponding to the Gran Sasso massif and L’Aquila zone, where an important seismic sequence (Mw 6.3) occurred in 2009.

How to cite: Gabrielli, S., Akinci, A., Napolitano, F., De Siena, L., Del Pezzo, E., and Ventura, G.: Spatio-temporal variation of Anaelastic and Scattering Seismic Attenuation during the 2016-2017 Central Italy Seismic Sequence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8802, https://doi.org/10.5194/egusphere-egu21-8802, 2021.

Strong lateral variations in medium properties affect the response of seismic wavefields. The Tyrrhenian Sea is ideally suited to explore these effects in a mixed continental-oceanic crust that comprises magmatic systems. The study aims at investigating the effects of crustal thinning and sedimentary layers on wave propagation, especially the reverberating (e.g., Lg) phases, across the oceanic basin. We model regional seismograms (600-800 km) using the software tool OpenSWPC (Maeda et al., 2017, EPS) based on the finite difference simulation of the wave equation. The code simulates the seismic wave propagation in heterogeneous viscoelastic media including the statistical velocity fluctuations as well as heterogeneous topography, typical of mixed settings. This approach allows to evaluate the role of interfaces and layer thicknesses on phase arrivals and direct and coda attenuation measurements. The results are compared with previous simulations of the radiative-transfer equations. They provide an improved understanding of the complex wave attenuation and energy leakage in the mantle characterizing the southern part of the Tyrrhenian Sea and the Italian peninsula. The forward modelling is to be embedded in future applications of attenuation, absorption and scattering tomography performed with MuRAT (the Multi-Resolution Attenuation Tomography code – De Siena et al. 2014, JVGR) available at https://github.com/LucaDeSiena/MuRAT.

How to cite: Nardoni, C., De Siena, L., Cammarano, F., Mattei, E., and Magrini, F.: Finite difference forward modelling across the Tyrrhenian basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1745, https://doi.org/10.5194/egusphere-egu21-1745, 2021.

This work puts in light the several steps followed to obtain a 3D velocity model in Arraiolos, a region located in central Portugal. After the earthquake of January 2018 occurred, a set of stations were deployed around the main shock area and has recorded the aftershock sequence during a period of six months.

The first stage of this study used a set of data recorded along the 1^st month by 21 temporary seismological stations. 317 aftershocks were used to invert a 3D P and S  velocity model, using LOTOS program, and showing an agglomeration of events in one local point leading to a poor resolution. Therefore, we added more stations and data to the second stage of study by integrating 437 aftershocks recorded during a period of 6 months by a set of 34 stations. The tomographic inversion of this extended aftershock sequence has shown a significant improvement of the 3D velocity model resolution and suggesting an alignment of the seismic events cluster. However, the imaged crustal volume was still too small and possessing low resolution on the edges of the area. To fix this issue, additional data and seismological stations were integrated to the study in order to increase the area of interest and cover it entirely in terms of ray density.

The step which we are currently conducting concerns the location of new events followed by their integration to the tomographic study using IPMA and DOCTAR station network records. Since the later phases PmP and SmS has the potential to increase the ray coverage as similarly as the resolution of an area, we will hopefully obtain, after their integration, significant improvements in terms of accuracy and reliability of the crustal image. The main purpose of this new stage of study is to finally provide significant interpretations and figure out precisely the tectonic processes having generated the Arraiolos seismicity.

Thanks are due to FCT for the financial support to the ICT project (UID/GEO/04683/2013) with the reference POCI-01- 0145-FEDER-007690, to the IDL project (UIDB/50019/2020 – IDL).

How to cite: Hamak, I., Wachilala, P., Borges, J., Dias, N., Rio, I., and Bezzeghoud, M.: Improved three-dimensional image of the tomographic inversion of the Arraiolos aftershock sequence., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8079, https://doi.org/10.5194/egusphere-egu21-8079, 2021.

The collision between the Eurasian and Indian plates since about 65 Ma caused extensive deformation in the Tibetan Plateau and surrounding areas. Northern Vietnam located in the southeastern Himalayan syntaxis is directly influenced by the collision and extrusion through the Red River shear zone that runs from southeastern Tibet to the South China Sea. Knowledge of crustal structure characteristics in northern Vietnam and across the Red River shear zone is crucial to improve our understanding not only of seismic hazards in the region but also of the regional Himalayan tectonic evolution as a whole. Seismic tomography is one of few methods that allows to study the subsurface structures effectively. In this study, we perform a joint tomographic inversion for northern Vietnam integrating the P- and S-direct waves traveling in the crust and the head waves along the Moho waves arrival time from more than 1000 earthquakes observed by Vietnamese networks. The obtained velocity model shows a good correlation with shallow geological features but also some complexity at crustal-scale. Several velocity anomalies bounded by and across the fault zones are revealed and discussed

Keyword: Traveltime tomography, Northern Vietnam, VpVs ratio, crustal structure, Vietnam seismic network

How to cite: Long, H. V., Huang, H.-H., Nguyen, L. M., Nguyen, V. D., Le, Q. K., Ha, T. G., Van, D. Q., Huang, B.-S., and Le, T. S.: Crustal seismic structure of Red-River shear zone and surrounding area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2001, https://doi.org/10.5194/egusphere-egu21-2001, 2021.

We present results from a joint inversion study of new seismic and gravity data to constrain a 2D high-resolution image of one of the most prominent geophysical anomalies of the European Alps: the Ivrea geophysical body (IGB). Our work exploits both new data and multidisciplinary a priori constraints, to better resolve the shallow crustal structure in the Ivrea-Verbano zone (IVZ), where the IGB is known to reach anomalously shallow depths and partially outcrop at the surface.

A variety of previous studies, ranging from gravity surveys to vintage refraction seismics and recent local earthquake tomographies (Solarino et al. 2018, Diehl et al. 2009), provide comprehensive but spatially sparse information on the IGB structure, which we aim at investigating at higher resolution, along a linear profile crossing the IVZ. To this purpose, we deployed 10 broadband seismic stations (MOBNET pool, IG CAS Prague), 5 km spaced along a linear West-East profile, along Val Sesia and crossing Lago Maggiore. This network operated for 27 months and allowed us to produce a new database of ca. 1000 seismic high-quality receiver functions (RFs). In addition, we collected new gravity data in the IVZ, with a data coverage of 1 gravity point every 1-2 km along the seismic profile. The newly collected data was used to set up an inversion scheme, in which RFs and gravity anomalies are jointly used to constrain the shape and the physical property contrasts across the IGB interface.

We model the IGB as a single interface between far-field constraints, whose geometry is defined by the coordinates of four nodes which may vary in space, and  density and V_S shear-wave velocity contrasts associated with the interface itself, varying independently. A Markov chain Monte Carlo (MCMC) sampling method with Metropolis-Hastings selection rule was implemented to efficiently explore the model space, directing the search towards better fitting areas.

For each model, we perform ray-tracing and RFs migration using the actual velocity structure both for migration and computation of synthetic RFs, to be compared with the observations via cross-correlation of the migration images. Similarly, forward gravity modelling for a 2D density distribution is implemented and the synthetic gravity anomaly is compared with the observations along the profile. The joint inversion performance is the product of these two misfits.

The inversion results show that the IGB reaches the shallowest depths in the western part of the profile, preferentially locating the IGB interface between 3 and 7 km depth over a horizontal distance of ca. 20 km (between Boccioleto and Civiasco, longitudes 8.1 and 8.3). Within this segment, the shallowest point reaches up to 1 km below sea level. The found density and velocity contrasts are in agreement with rock physics properties of various units observed in the field and characterized in earlier studies.

How to cite: Scarponi, M., Hetényi, G., Plomerová, J., and Solarino, S.: Joint receiver function and gravity inversion for new constraints on the Ivrea body structure: a 2D high-resolution view along the Val Sesia profile (N. Italy)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9882, https://doi.org/10.5194/egusphere-egu21-9882, 2021.

Following the M_D4.0 (Mw3.9) earthquake of August 21 2017 which occurred on the Ischia island (Naples, southern Italy), the local monitoring seismic network was significantly improved in terms of both number of stations and instrumentation performance. Due to the considerable amount of collected data, in particular of seismic noise recorded at broadband stations, some efforts have been addressed in particular to the definition of a 1D average velocity model effective for the whole island. This is an important scientific step because, in complex volcanic areas, the use of reliable velocity models is essential for an accurate localization of local earthquakes. In this work, the main target is to retrieve a pseudo-3D velocity model of the Ischia island. Specifically, we inverted H/V curves and frequency peaks evaluated at about twenty sites to obtain a velocity profile for each of the investigated measurement points. Taking into account that the H/V frequency peak depends on both velocity and thickness of layers, for each site we applied an inversion process fixing the velocities and modifying the thicknesses in order to obtain the corresponding 1D velocity models. We are quite enough confident about the robustness of models, since during the inversion process, we achieved a good convergence towards the best-fit solutions. Then, a pseudo-3D velocity model was obtained by contouring the 1D models of each station site to highlight possible lateral variations of the layer thicknesses and to reconstruct the morphology of the deeper interface characterized by a high impedance contrast. A good correspondence between the pseudo-3D model and the geological features of the island was observed, especially in the northern sector where most of the stations is installed. In particular, the top of the high-impedance contrast interface appears deeper in the northern coastal areas and shallower in the central sector. This is in agreement with the structural setting of the island likely due to the resurgence of Mount Epomeo.

How to cite: Manzo, R., Nardone, L., Gaudiosi, G., Martino, C., Galluzzo, D., Bianco, F., and Di Maio, R.: A Pseudo-3D Vs Velocity Model of Ischia Island (Italy) by HVSR spectral ratio inversion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13610, https://doi.org/10.5194/egusphere-egu21-13610, 2021.

The Gloria fault is a strike-slip oceanic plate boundary fault, which has remained poorly studied due mostly to its remote location in the north Atlantic Ocean. The fault has hosted some of the largest strike-slip earthquakes in the oceanic domain, notably the 1941 M8.3 and the 1975 M8.1 earthquakes, and generating tsunamis is the surrounding areas.

The seismic data used for this study was recorded by 12 broadband ocean bottom seismometers (OBSs) located about 100 km north of the Gloria fault during a 10-month experiment. The dataset has been used before to image crustal and mantle discontinuities using receiver function analysis and to infer the S-wave velocity structure of the oceanic lithosphere north of the Gloria fault from P-wave polarization. These studies indicate a slight crustal thickening towards the Gloria fault, as well as an increase in uppermost mantle S-wave velocities towards the fault.

In this study, we use ambient noise surface wave tomography to find the velocity structure beneath the OBS deployment. First, we present a 1D shear-velocity model obtained from inversion of the average fundamental mode Rayleigh and Love wave group and phase velocities. In addition, the hydrophone is also used to better constrain the inversion at shallow depths, because the hydrophone shows a clear fundamental mode without interference of the first higher mode. Because of the short interstation distances of the array, it is not possible to extract the dispersion curves at periods longer than ~16 s. To compute the Vs inversions, we used the code SURF96 (Herrmann and Ammon, 2004) and consider a water layer in the initial model for Rayleigh waves, because these waves are affected by the water layer. Our results show an upper mantle low-velocity zone, which may be related to serpentinization due to the proximity of the Gloria Fault. Finally, we present the lateral variations of group and phase velocities, as a function of period obtained using FMST (Rawlinson and Sambridge, 2005), which show strong contrast velocity anomalies at the center of the array at short periods (shallow depths).

The authors acknowledge support from the Portuguese FCT – Fundação para a Ciência e a Tecnologia, I.P., within the scope of project UTAP-EXPL/EAC/0056/2017 and with the FCT grant PD/BD/135069/2017 - IDL.

How to cite: Pinzon, J., Custódio, S., Silveira, G., Matias, L., and Krüger, F.: Ambient noise surface wave inversion using OBS data from the north Atlantic, north of the Gloria fault., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7188, https://doi.org/10.5194/egusphere-egu21-7188, 2021.

Passive seismic interferometry (ambient-noise seismology) is an increasingly popular, eco-friendly, relatively inexpensive exploration geophysics tool, to map S-wave velocity in the Earth’s crust. This method has not yet been applied widely to marine exploration. The purpose of this study is to investigate the crustal structure of a quasi-amagmatic portion of the Southwest Indian Ridge by interferometry, and to examine the performance and reliability of interferometry in marine exploration. To achieve this goal, continuous vertical-component recordings from 43 ocean bottom seismometers (OBS) deployed during the SISMO-SMOOTH cruise (2014) were utilized. Recorded signals span frequencies between 0.1Hz and 3Hz. We show that reliable estimates of the Green’s function are obtained for many station pairs, by cross-correlation in the frequency domain. The comparison of the cross-correlations with the theoretical Green’s (Bessel) function provides one Rayleigh-wave dispersion curve per station pair; dispersion curves are then averaged, and inverted through a conditional neighborhood algorithm to determine a 1D S-wave velocity model, that we estimate to be well constrained within the crust. Our S-wave velocity model is analyzed and interpreted with geological information, and independent geophysical studies in the region of interest, as well as other areas characterized by similar tectonically-dominated, quasi amagmatic spreadings.

How to cite: Mohamadian Sarvandani, M., Kästle, E., Boschi, L., Leroy, S., and Cannat, M.: Passive seismic interferometry of the ultraslow-spreading Southwest Indian Ridge, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6132, https://doi.org/10.5194/egusphere-egu21-6132, 2021.

Long-duration stacks of ambient seismic noise cross-correlation can be used to generate high-resolution images of the lithosphere. In this study, we investigate the crustal structure beneath southeastern Canada and the northeastern USA, using an ambient noise tomography technique. Our study area covers the Phanerozoic northern Appalachians and the Proterozoic eastern Grenville Province, recording a complex tectonic history since ~1 Ga. Our datasets include continuous records of vertical component time series, recorded by 69 stations belonging to 7 seismograph networks over a more than two-year period. The ambient seismic noise directionality and seasonality variations of our datasets are analyzed in detail, and possible noise source locations are proposed in the Atlantic and Pacific oceans. Our analysis suggests strong variations of dominant seismic noise sources at both Primary (11-20 s) and Secondary (5-10 s) bands in various months, with different observed patterns at these passband periods. Our tomographic models indicate complex and strong variations of Rayleigh wave phase velocities across the study area, providing us evidence to discuss tectonic implications. The resulting Rayleigh wave phase velocity maps suggest generally slower velocities beneath the Appalachians than the Grenville province. A sharp velocity contrast is observed across the Grenville Province-Appalachian domain boundary at periods sensitive to the lower crust, suggesting a step-like geometry of the Moho interface beneath this area.

How to cite: Bagherpur Mojaver, O. and Darbyshire, F.: Crustal structure, seasonal and directional variations of ambient seismic noise in SE Canada and the NE USA, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6124, https://doi.org/10.5194/egusphere-egu21-6124, 2021.

The dynamics of crustal extension and the crust-mantle interaction in the Central-Western Mediterranean and Italian peninsula (i.e. Liguro-Provençal and Tyrrhenian Basin), and plate convergence (i.e. Alpine and Apennines chains) are key for the understating of the current geodynamics setting and its evolution in the region. However, open questions such as the style, depth and extent of the deformation still exist despite the wealth of seismological and non-seismological data acquired in the past decades. In this context, it is necessary to provide improved subsurface models in terms of seismic velocities, from which better constraints on the geodynamic models can be derived.

We use seismic ambient noise for retrieving phase velocities of Rayleigh and Love waves in the 4-35 s period range, using private (LiSard network in Sardinia island) and publicly available continuous recordings from more than 500 seismic stations. Considering the excellent coverage and the short period of recovered phase velocities, our study aims to provide an unprecedented, high-resolution image of the shallow crust and uppermost mantle.

We employ a Bayesian trans-dimensional, Monte Carlo Markov chain inversion approach that requires no a-priori model nor a fixed parametrization. In addition to the (isotropic) shear wave velocity structure, we also recover the values of radial anisotropy (ξ=(V_SH/V_SV)²) as a function of depth, thanks to the joint inversion of both Rayleigh and Love phase velocities.

Focusing on radial anisotropy, this appears clearly uncoupled with respect to the shear wave velocity structure. The largest negative anisotropy anomalies (V_SH<V_SV, ξ<0.9) are found in the Liguro-Provençal and western Tyrrhenian basins in the top 10-15 km, suggesting a common structural imprint inherited during the extensional phases of such basins. Conversely, the eastern Tyrrhenian basin shows positive radial anisotropy (V_SH>V_SV, ξ>1.1) within the same depth range. This evidence, combined with the observed shear wave velocities typical of the uppermost mantle, corroborates the presence of a sub-horizontal asthenospheric flow driving the current extension and oceanization of the eastern Tyrrhenian basins.

Moving towards the Italian mainland, a strong anomaly of negative anisotropy appears in the eastern portion of the Apennines chain. We relate such an anisotropic signal with the ongoing compressive regime affecting the area. Here, the high-angle thrust faults and folds, that accommodates the horizontal shortening, obliterate the horizontal layering of the sedimentary deposits, currently constituting the flanks of the fold system.

Our results suggest that the combination of radial anisotropy and shear wave velocities can unravel key characteristics of the crust and uppermost mantle, such as inherited or currently active structures resulting from past or ongoing geodynamic processes.

How to cite: Diaferia, G., Magrini, F., Agius, M., and Cammarano, F.: Seismic radial anisotropy in Central-Western Mediterranean and Italian peninsula from ambient noise recordings, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15238, https://doi.org/10.5194/egusphere-egu21-15238, 2021.

The Central Mediterranean, the area encompassing Italy, Sardinia, Tunisia and Libya, is characterised by multiple tectonic processes (plate convergence, subduction, and backarc extension). The evolution and interaction of the plate margins within this relatively small area are still being unravelled particularly at the adjacent region known as the Sicily Channel located between Sicily, Tunisia, Libya and Malta. This Channel is characterised by a seismically and volcanically active rift zone. Much of the observations we have today for the southern parts of the Calabrian arc 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 (https://geomed-msca.eu) 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 measure Rayleigh- and Love-wave phase velocities from ambient seismic noise recordings to infer the structures of the Central Mediterranean, from the Central Apennines to the African foreland, with a special focus on the Sicily Channel Rift Zone (SCRZ). The phase-velocity dispersion curves have periods ranging from 5 to 100 seconds and sample through the entire lithosphere. We invert the dispersion data for isotropic and polarised shear velocities with depth and infer crustal thickness and patterns of radial anisotropy. We find that continental blocks have thick crust (30-50 km), whereas beneath the SCRZ the crust is thin (<25 km), and thinner beneath the Tyrrhenian Sea. Beneath the SCRZ and the Tyrrhenian Sea, the crustal shear velocities are characterised by positive radial anisotropy (V_SH>V_SV) indicative of horizontal flow or extension, whereas the uppermost mantle is characterised by slow shear velocities indicative of warmer temperatures and strong negative radial anisotropy (V_SH>V_SV) indicative of vertical flow. We discuss the relevance of these findings together with other geophysical studies such as the regional seismicity and GPS velocity vectors to identify the rifting process type of the SCRZ.

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., Magrini, F., Diaferia, G., Cammarano, F., Faccenna, C., Funiciello, F., Kästle, E., and van der Meijde, M.: Geodynamics of the Central Mediterranean (GEOMED) inferred from ambient seismic noise, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6028, https://doi.org/10.5194/egusphere-egu21-6028, 2021.

Wavefield gradient instruments, such as rotational sensors and DAS systems, are becoming more and more accessible in seismology. Their usage for Full Waveform Inversion (FWI) is in sight. Nevertheless, local small-scale heterogeneities, like geological inhomogeneities, surface topographies, and cavities are known to affect wavefield gradients. This effect is in fact measurable with current instruments. For example, the agreement between data and synthetics computed in a tomographic model is often not as good for rotation as it is for displacement.

The theory of homogenization can help us understand why small-scale heterogeneities strongly affect wavefield gradients, but not the wavefield itself. It tells us that at any receiver measuring wavefield gradient, small-scale heterogeneities cause the wavefield gradient to couple with strain through a coupling tensor J. Furthermore, this J is 1) independent of source, 2) independent of time, but 3) only dependent on the receiver location. Consequently, we can invert for J based on an effective model for which synthetics fit displacement data reasonably well. Once inverted, J can be used to correct all other wavefield gradients at that receiver.

Here, we aim to understand the benefits and drawbacks of wavefield gradient sensors in a FWI context. We show that FWIs performed with rotations and strains are equivalent to that performed with displacements provided that 1) the number of data is sufficient, and 2) the receivers are placed far away from heterogeneities. In the case that receivers are placed near heterogeneities, we find that due to the effect of these heterogeneities, an incorrect model is recovered from inversion. In this case therefore, the coupling tensor J needs to be taken into account for each receiver to get rid of the effect.

How to cite: Singh, S., Capdeville, Y., Igel, H., Hedjazian, N., and Bodin, T.: Full-waveform inversion with wavefield gradients, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15143, https://doi.org/10.5194/egusphere-egu21-15143, 2021.

While there are seismic techniques which make use of surface waves in imaging the subsurface, there are also those where these types of waves are considered coherent noise. Important examples where the surface waves may significantly degrade the obtained images include different types of reflection seismic surveys (e.g., shallow surveys for engineering, environmental and groundwater investigations, and deep surveys for imaging hydrocarbon reservoirs). In a strongly heterogeneous medium (encountered typically in onshore surveys), the conventional methods for attenuating these waves (such as f-k "velocity" filtering) often do not give satisfactory results.

Seismic interferometry is a data-driven approach that offers a viable alternative for removal of surface waves from active seismic records. In this approach, the reflection data of several sources is considered and for each source, the seismic signals at a pair of receivers are cross-correlated to produce the surface wavefield between the receivers. The cross-correlated waveforms are then summed over all the sources to obtain the "interferometric" signal for the considered receiver pair. During this summation, the reflection and non-physical events cancel out due to the variable differences in the travel times to the considered receiver pair from different sources. The "interferometric" signal consequently contains predominantly the surface waves and this makes it conducive for adaptive subtraction (or filtering) from the original records. This study investigates the efficacy of the commonly used filtering techniques in interferometry to remove the surface waves from active seismic records. For this, the reflection data of a complex 2-D elastic medium is simulated and the filtering techniques are applied to this data. The limitations of these techniques inferred from the quality of the filtered data are discussed and possible remedies to overcome them are suggested.

How to cite: Singla, V. K. and Lokmer, I.: Efficacy of Seismic Interferometry in Removing Surface Waves from Active Seismic Records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-279, https://doi.org/10.5194/egusphere-egu21-279, 2021.

The Earth’s oldest oceanic lithosphere preserved in-situ is in the eastern Mediterranean Sea. It can offer essential information on the oceanic plate evolution. Yet, its thickness and other properties have been difficult to determine by means of seismic imaging due to the high heterogeneity of the region. Here, we combine a large, new surface wave dataset with published wide-angle data in order to map the properties and lateral variability of the oceanic lithosphere, as well as the ocean-continent transition in the easternmost Mediterranean beneath the Levant Basin. We use stochastic joint inversion of broad band, phase-velocity dispersion measurements and seismic refraction P-wave velocity models to obtain 1-D, shear wave velocity models down to 300 km depth and compare the structure beneath the Ionian Sea and the Levant Basin. The thickness of the crust is about 16.4 ± 3 km and 22.3 ± 2 km beneath the chosen locations within the Ionian Sea and the Levant Basin, respectively. The Poisson’s ratio of about 0.32 and Vp/Vs of about 1.93 in the crystalline crust, yielded by the inversion, confirm the presence of oceanic crust beneath the Ionian Sea. The thickness of the Ionian oceanic lithosphere is around 180 km, whereas the continental lithosphere beneath the eastern Levant Basin is ~70 km thick, with low crustal Vp/Vs (~1.7) and Poisson’s (~0.24) ratios. According to 3-D shear wave velocity tomography using the surface wave data, the thickness of the oceanic lithosphere increases from the Triassic Ionian Sea towards the Permian-Carboniferous Libyan Sea and Herodotus Basin. Thicknesses of the Permo-Triassic oceanic lithosphere considerably larger than 100 km indicate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. The transition from oceanic to continental lithosphere occurs at about 31°E in the crust, as indicated by magnetic and gravity measurements. The continental mantle lithosphere further to the east of this boundary is ~150 km thick beneath the westernmost Levant Basin, as indicated by shear wave velocity tomography and long wavelength gravity anomalies, and strongly thins eastward towards the area of the Levantine Coast and the Dead Sea Fault. The localization of the lithospheric deformation and crustal seismicity along the Dead Sea Fault correlates spatially with the thinning of the underlying continental lithosphere.

Key words: surface wave tomography, wide angle seismic imaging, joint inversion, Vp/Vs and Poisson’s ratios, eastern Mediterranean, Oceanic Lithosphere, Continental Lithosphere, Dead Sea Fault.

How to cite: El-Sharkawy, A., Meier, T., Hübscher, C., Lebedev, S., Dannowski, A., Kopp, H., Behrmann, J. H., McGrandle, A., and Hamada, M.: Permo-Triassic Oceanic and Adjacent Continental Lithosphere in the Eastern Mediterranean, from surface-wave and wide-angle imaging, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7231, https://doi.org/10.5194/egusphere-egu21-7231, 2021.

We present a new shear-wave velocity model of the upper mantle beneath the East Asia, ASIA2021, derived using the Automatic Multimode Inversion technique. We use waveform fits of over 1.3 million seismograms, comprising waveforms of surface waves, S and multiple S waves.  In total, data from 9351 stations and 23344 events constrain ASIA2021, which maps in detail the structure of the lithosphere and underlying mantle beneath the region. Our model reveals deep structure beneath the tectonic units that make up East Asia. It shows agreement with previous models at larger scales and, also, sharper and stronger velocity anomalies at smaller regional scales. High-velocity continent roots are mapped in detail beneath the Sichuan Basin, Tarim Basin, Ordos Block, and Siberian Craton, extending to over 200 km depths. The lack of a high-velocity continental root beneath the Eastern North China Craton (ENCC), underlain, instead, by a low-velocity anomaly, is consistent with the destruction of this Archean nucleus. Strong low-velocity anomalies are mapped within the top 100 km beneath Tibet, Pamir, Altay-Sayan area, and back-arc basins. At greater depths, ASIA2021 shows high-velocity anomalies related to the subducted and underthrusted lithosphere of India beneath Tibet and the subduction of the Pacific and other plates in the upper mantle. In the mantle transition zone (MTZ), we find high-velocity anomalies probably related to deflected subducted slabs or detached portions of ancient continent cratons. In particularly, ASIA2021 reveals separate bodies, probably originating from the Indian Plate lithosphere beneath central Tibet, with one at 100-200 km beneath Songpan-Ganzi Block (SGFB) and the other in the MTZ. A strong low-velocity anomaly extending from the surface to the lower mantle beneath Hainan volcano and South China Sea is consistent with the hypothesis of the Hainan mantle plume. The high-velocity anomaly beneath ENCC in MTZ can be interpreted as a detached Archean continent root. The Pacific Plate subducts beneath the eastern margin of Asia into the MTZ and appears to deflect and extend horizontally as far west as the Songliao Basin. The absence of major gaps in the stagnant slab is consistent with the origin of Changbaishan volcano above being related to the Big Mantle Wedge, proposed previously. The low-velocity anomalies down to ~ 700 km depth beneath the Lake Baikal area suggest a hot upwelling (mantle plume) feeding the widely distributed Cenozoic volcanoes in central and western Mongolia.

How to cite: Dou, H., Lebedev, S., Chagas de Melo, B., Wang, B., and Wang, W.: Imaging the East Asia using waveform tomography with massive datasets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13665, https://doi.org/10.5194/egusphere-egu21-13665, 2021.

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. 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.

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, provides data from over 300 stations which promises a significant improvement in data coverage around the Banda Arc, Borneo and Sulawesi. We employ a geographical weighting scheme to minimise the effect of dense regional arrays and compile a catalogue of 118 well-constrained earthquakes, optimising for coverage, signal-to-noise ratio and data availability. An optimised window selection algorithm allows us to balance amplitude differences and include as much signal as possible while avoiding noisy data.

Here, we present a seismic waveform tomography for upper mantle structure in Southeast Asia, imaging radially anisotropic S velocity, P velocity and density. We use a gradient-based optimisation scheme (L-BFGS) and adjoint methods to obtain sensitivity kernels as the corresponding gradients. In the first part of the inversion, periods down to 50 s are used to update a 1D initial model, adapting a multi-scale approach in which long periods are inverted for first to avoid cycle skipping. In our long-period results, we observe a strong regional low S-velocity structure with an underlying high-velocity anomaly. The results are consistent with the global S40RTS model. 

How to cite: Wehner, D., Blom, N., Rawlinson, N., Miller, M., Widiyantoro, S., and Daryono, M.: Towards 3D Multiscale Adjoint Waveform Tomography of the Lithosphere and Underlying Mantle beneath Southeast Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1586, https://doi.org/10.5194/egusphere-egu21-1586, 2021.

We present a new S-wave velocity tomographic model of the Australian Plate, Aus21.  It is constrained by waveforms of 0.9 million seismograms with both the corresponding sources and stations located within the half-hemisphere centred at the Australian continent. Waveform inversion extracts structural information from surface, S- and multiple S-waves on the seismograms in the form of a set of linear equations. These equations are then combined in a large linear system and inverted jointly to obtain a tomographic model of S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle. The model has been validated by resolution tests and, for particular locations in Australia with notable differences with previous models, by independent inter-station measurements of surface-wave phase velocities, which we performed using available array data. 

Aus21 offers new insights into the structure and evolution of the Australian Plate and its boundaries. The Australian cratonic lithosphere occupies nearly all of the western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it is colliding with island arcs, without subducting. The rugged eastern boundary of the cratonic lithosphere provides a lithospheric definition of the Tasman Line. The thin, warm lithosphere below the eastern part of the continent, east of the Tasman Line, underlies the Cenozoic volcanism locations in the area. The lithosphere is also thin and warm below much of the Tasman Sea, underlying the Lord Howe hotspot and the submerged part of western Zealandia. A low velocity anomaly that may indicate the single source of the Lord Howe and Tasmanid hotspots is observed in the transition zone offshore the Australian continent, possibly also sourcing the East Australia hotspot. Another potential hotspot source is identified below the Kermadec Trench, causing an apparent slab gap in the overlying slab and possibly related to the Samoa Hotspot to the north. Below a portion of the South East Indian Ridge (the southern boundary of the Australian Plate) a pronounced high velocity anomaly is present in the 200-400 km depth range just east of the Australian-Antarctic Discordance (AAD), probably linked to the evolution of this chaotic ridge system.

How to cite: de Laat, J., Lebedev, S., Chagas de Melo, B., Celli, N., and Bonadio, R.: The Australian Plate and underlying mantle from waveform tomography with massive datasets , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5272, https://doi.org/10.5194/egusphere-egu21-5272, 2021.

Classical surface wave tomography based on waveform scattering through seismic data has an important role in studying the structure of the Earth‘s crust and upper mantle on regional and global scale. The shallow crustal velocity structure is studied using earthquake waveforms in Khorasan/E-NE Iran. For this purpose, 522 local recorded waveforms with M≥4, which occurred between 53°-63°E and 30°-42°N, were selected. Therefore, all available vertical components of waveforms recorded at the stations in the Iranian Seismological Center (IrSC), the International Institute of Earthquake Engineering and Seismology (IIEES), and the IRIS global network collected in the period between January 2006 to October 2020. Then, some data selection criteria were applied for each waveform, including (i)SNR>4, (ii) the gap time less than 2 s within the expected signal window (1.5-4.5 km/s), (iii) epicentral distance greater than 20 km. The multiple-filter analysis technique was then applied by the computer program in seismology Hermann and Ammon (2013) to measure Rayleigh wave dispersion curves in the period range of 3-50 s. Finally, Rayleigh wave 2D horizontal group velocity maps are calculated by the fast marching surface wave tomography method. Our tomographic results indicate some local low velocity anomalies appeared in the W-NW of the study area where it connected with the East-Alborz tectonic structure. Also, the Doruneh Fault System clearly separates Kopeh-Dagh tectonic zone and Central Iran micro-plateau. However, a high velocity anomaly appears in Kopeh-Dagh tectonic zone at periods larger than 16 s and 30 s.

How to cite: Rezaei, M. and Shirzad, T.: Crustal structure of Khorasan (E-NE Iran) using the Rayleigh wave tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1186, https://doi.org/10.5194/egusphere-egu21-1186, 2021.

Deep geothermal resources in low- to medium-temperature settings remain poorly understood and untapped in Ireland and much of Europe. Our new project DIG (De-risking Ireland’s Geothermal Potential) integrates multi-disciplinary, multi-scale datasets in order to investigate Ireland’s low-enthalpy geothermal energy potential. Seismic measurements constrain the distributions of seismic velocities and, through them, the composition and temperature within the lithosphere and underlying mantle. Recent deployments of broadband seismic stations and the surface-wave measurements using the new data yield an unprecedentedly dense data sampling of the crust and upper mantle beneath Ireland and neighbouring Britain. These data form a foundation for the region-scale, multi-parameter modelling of the thermal state of the lithosphere.

We use the recently assembled dataset of over 11,000 Rayleigh-wave, phase-velocity curves, measured for pairs of stations across Ireland and Britain (Bonadio et al. 2021) and complement it with new interstation measurements of Love-wave phase velocities. The measurements were performed using two methods with complementary period ranges, the teleseismic cross-correlation method and waveform inversion. Spanning a very broad period range (from as short as 4 s to as long as 500 s), the phase velocities provide resolution from the upper-middle crust to the asthenosphere. The joint analysis of Rayleigh and Love measurements constrains the isotropic-average shear-wave velocity, relatable to temperature and composition. The optimal-resolution, phase-velocity maps of Bonadio et al. (2021) for Rayleigh waves and the new maps for Love waves computed in this study provide essential constraints on the thermal structure of the region’s lithosphere. We demonstrate this by inverting the data using an integrated geophysical-petrological thermodynamically self-consistent approach (Fullea et al., 2021). The multi-parameter models produced by the integrated inversions fit the surface-wave and surface-elevation data and reveal the temperatures and geothermal gradients within the crust. 

Bonadio, R., Lebedev, S., Meier, T., Arroucau, P., Schaeffer, A. J., Licciardi, A., Agius, M. R., Horan, C., Collins, L., O'Reilly, B. M., Readman, P. and the Ireland Array Working Group (2021). Optimal resolution tomography with error tracking: imaging the upper mantle beneath Ireland and Britain. Geophys. J. Int., in revision.

Fullea, J., Lebedev, S., Martinec, Z., & Celli, N. L. (2021). WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical-petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophys. J. Int., revised version under evaluation.

How to cite: Chambers, E. L., Bonadio, R., Lebedev, S., Fullea, J., Kiyan, D., Bean, C., O'Reilly, B., and Meere, P.: Imaging the temperature beneath Ireland and Britain using massive, broadband surface-wave datasets and petrological inversion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5381, https://doi.org/10.5194/egusphere-egu21-5381, 2021.

We present new thermochemical models of the lithosphere and upper mantle beneath the Superior craton and surrounding regions. The study area is dominated by the Archean Superior Province, surrounded by Proterozoic orogenic belts such as the Trans-Hudson Orogen (THO) to the north and the Grenville Orogen to the southeast. Portions of the Rae and Hearne cratons north of the THO are also studied, as is the Mid-continent Rift to the south. Over a period of ∼3 Ga, the region has seen assembly and modification by accretionary and orogenic events, periods of rifting, and the influence of a number of mantle hotspots. Here, we use a probabilistic inverse method to jointly invert Rayleigh wave dispersion data, Vp data, geoid anomalies, surface heat flow, and absolute elevation. The output is a 3D model of the seismic, temperature, bulk density, and compositional structure of the whole lithosphere beneath the Superior craton.

The resulting model will provide new opportunities for joint studies of the structure of the upper mantle and will shed light on the thermal and compositional variations beneath the region. In this presentation, we will discuss the results from our model and several robust features that carry important geological and geodynamical implications for this region.

How to cite: Dave, R., Darbyshire, F. A., Afonso, J. C., and Ali, K.: Thermochemical structure of the lithosphere and upper mantle beneath Superior craton: Results from multi-observable probabilistic inversion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6456, https://doi.org/10.5194/egusphere-egu21-6456, 2021.

Crustal deformation in volcanic areas relates ground motions, measured by geodetic techniques, to physical (e.g. pressure or volumetric) changes of magmatic sources below the surface. These measurements contribute to studies of ongoing processes at the source of possible unrest, and are thus key for hazard assessment in active volcanic areas around the globe. However, such assessments often rely on geodetic-based models with quite simplistic assumptions of the physical structure of the volcanic complex. Particularly, constant values of elastic parameters (e.g. Poisson’s ratio and shear moduli) are commonly used for entire active volcanic areas, thus overlooking the spatial effects of lithological properties, depth-dependant compression and temperature variations on those parameters. These simplifications may lead to inaccurate interpretation of the location, shape, and volume change of deformation sources.

In this study we ask how the 3-D heterogeneities of the elastic crustal structure beneath the Hengill volcanic system, SW Iceland, affects models of deformation sources in the area. The Hengill area hosts two active volcanic systems (Hengill and Hrómundartindur), and two high-enthalpy geothermal power plants (Nesjavellir and Hellisheiði), which provide thermal and electrical power to Reykjavík, the capital of Iceland, only 30 km away. To retrieve information on the spatial heterogeneities in the shear moduli and Poisson’s ratio beneath the Hengill area, we first estimate the 3-D shallow density structure of the area using results from regional and local gravimetric surveys. We implement this structure, along with seismic tomographic studies of the SW Iceland, in a Finite Element Model to solve, using forward models, for the 3-D heterogeneities in the shear moduli and Poisson’s ratio beneath the Hengill area. Furthermore, we discuss the difference between static and kinematic elastic moduli, which is important when building deformation models from seismic tomography. The new 3-D inferred elastic model is then used to re-estimate parameters for different sources of deformation causing uplift and subsidence in the area in the past decades. This study shows the importance of accounting for heterogeneities in the crustal elastic structure to estimate with higher accuracy the sources of deformation in volcanic areas around the world.

How to cite: Ducrocq, C., Geirsson, H., Hobé, A., Hersir, G. P., Árnadóttir, T., Sigmundsson, F., Tryggvason, A., and Drouin, V.: 3-D Heterogeneous Elastic Crustal Structure for Deformation Models in the Hengill Area, SW Iceland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7553, https://doi.org/10.5194/egusphere-egu21-7553, 2021.

We present a thermal model for the lithosphere in Tibet and adjacent regions based on the new thermal isostasy method and our compilation of the Moho depth based on published seismic models. The predicted surface heat flow is in agreement with the few available, reliable borehole measurements. Cratonic-type cold and thick lithosphere (200-240 km) with a surface heat flow of 40-50 mW/m² typifies the Tarim craton, the north-western Yangtze craton, and most of the Lhasa Block that is possibly refrigerated by underthrusting Indian lithosphere. The thick lithosphere of the Lhasa block extends further north in its western and eastern segments than in its central section. We identify a North Tibet anomaly with a thin (<80 km) lithosphere and high surface heat flow (>80-100 mW/m²), possibly associated with the removal of lithospheric mantle and asthenospheric upwelling. Other parts of Tibet have an intermediate lithosphere thickness of 120-160 km and a surface heat flow of 45-60 mW/m², with a patchy style in eastern Tibet. In the Qaidam deep sedimentary basin the lithosphere is about 100-120 km thick. The heterogeneous thermal lithosphere beneath Tibet suggests an interplay of several mechanisms as the driver of the observed uplift.

How to cite: Xia, B., Artemieva, I., and Thybo, H.: Extreme variability of Tibetan thermal lithosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14135, https://doi.org/10.5194/egusphere-egu21-14135, 2021.

Shear wave splitting analysis is a frequently used tool to study elastic anisotropy from the lower mantle to the crust. Several methods have been developed to evaluate the splitting parameters, Φ (fast axis) and δt (delay time), including the correlation of wave components, minimization of covariance matrix eigenvalues, and minimizing energy on the transverse component. Despite massive progress in introducing sophisticated methods, still fundamental problems, related mainly to noisy data, interfering phases, length of the analyzed waveform, and stability and reliability of results, remain. This study presents a sparsity-based adaptive filtering method to magnify the SKS waveforms and suppress the unwanted noise and interfering phases. The study is an extension of Jurkevics (1988), computing the semi-minor and semi-minor axis of the polarized motion in the time-frequency domain using a regularized inversion-based approach imposing a sparsity constraint. Afterward, the elliptical particle motion caused by the split shear waves and correspond to high semi-minor amplitude is derived in the time-frequency domain. The information is used to design an adaptive filter in the time domain to amplify the SKS phase and suppress the noise and other phases having non-elliptical polarization. The regularized inversion-based approach enables obtaining a sparse time-frequency semi-minor map while handling noise problems in the time-frequency decomposition. Conducting synthetic simulations, we show that the proposed method increases the signal-to-noise ratio of the SKS phase in radial and transverse components, giving a better estimation of anisotropy parameters in the presence of noise and other interfering phases. Future work involves implementing the processing algorithm on real data recorded in São Tomé and Prı́ncipe, Madeira, and Canary islands. This research contributes to the FCT-funded SHAZAM (Ref. PTDC/CTA-GEO/31475/2017) and SIGHT (Ref. PTDC/CTA-GEF/30264/2017) projects.

How to cite: Mohammadigheymasi, H., Ebrahimi, M. R., Silveira, G., and schlaphorst, D.: A sparsity-based adaptive filtering approach to Shear Wave Splitting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13988, https://doi.org/10.5194/egusphere-egu21-13988, 2021.