GD7.1 | Anisotropy from crust to core: Observations, models and implications
EDI
Anisotropy from crust to core: Observations, models and implications
Co-organized by EMRP1/SM6
Convener: Manuele Faccenda | Co-conveners: Tuna Eken, Judith Confal
Orals
| Thu, 18 Apr, 10:45–12:30 (CEST), 14:00–15:45 (CEST)
 
Room -2.91
Posters on site
| Attendance Wed, 17 Apr, 10:45–12:30 (CEST) | Display Wed, 17 Apr, 08:30–12:30
 
Hall X1
Orals |
Thu, 10:45
Wed, 10:45
Many regions of the Earth, from crust to core, exhibit anisotropic fabrics which can reveal much about geodynamic processes in the subsurface. These fabrics can exist at a variety of scales, from crystallographic orientations to regional structure alignments. In the past few decades, a tremendous body of multidisciplinary research has been dedicated to characterizing anisotropy in the solid Earth and understanding its geodynamical implications. This has included work in fields such as: (1) geophysics, to make in situ observations and construct models of anisotropic properties at a range of depths; (2) mineral physics, to explain the cause of some of these observations; and (3) numerical modelling, to relate the inferred fabrics to regional stress and flow regimes and, thus, geodynamic processes in the Earth. The study of anisotropy in the Solid Earth encompasses topics so diverse that it often appears fragmented according to regions of interest, e.g., the upper or lower crust, oceanic lithosphere, continental lithosphere, cratons, subduction zones, D'', or the inner core. The aim of this session is to bring together scientists working on different aspects of anisotropy to provide a comprehensive overview of the field. We encourage contributions from all disciplines of the earth sciences (including mineral physics, seismology, magnetotellurics, geodynamic modelling) focused on anisotropy at all scales and depths within the Earth.

Orals: Thu, 18 Apr | Room -2.91

Chairpersons: Manuele Faccenda, Tuna Eken, Judith Confal
10:45–10:50
10:50–11:00
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EGU24-18148
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ECS
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On-site presentation
Gianmarco Del Piccolo, Rosalia Lo Bue, Brandon Paul VanderBeek, Manuele Faccenda, Ornella Cocina, Marco Firetto Carlino, Elisabetta Giampiccolo, Andrea Morelli, and Joseph Byrnes

Trans-dimensional inference identifies a class of methods for inverse problems where the number of free parameters is not fixed. In seismic imaging these methods are applied to let the data, and any prior information, decide the complexity of the models and how the inferred fields partition the inversion domains. Monte Carlo trans-dimensional inference is performed implementing the reversible-jump Markov chain Monte Carlo (rjMcMC) algorithm; the nature of Monte Carlo exploration allows the algorithm to be completely non-linear, to explore multiple possibilities among models with different dimensions and meshes and to extensively investigate the under-determined nature of the tomographic problems, showing quantitative evidence for the limitations in the data-sets used. Implementations of this method overcome the main limitations of traditional linearized solvers: the arbitrariness in the selection of the regularization parameters, the linearized iterative approach and in general the collapse of the information behind the solution into a unique inferred model.

We present applications of the rjMcMC algorithm to anisotropic seismic imaging of Mt. Etna with P-waves. Mt. Etna is one of the most active and monitored volcanoes in the world, typically investigated under the assumption of isotropic seismic speeds. However, since body waves manifest strong sensitivity to seismic anisotropy, we parametrize a multi-fields inversion to account for the directional dependence in the seismic velocities. Anisotropy increases the ill-condition of the tomographic problem and the consequences of the under-determination become more relevant. When multiple seismic fields are investigated, such as seismic speeds and anisotropy, the data-sets used may not be able to independently resolve them, resulting in non-independent estimates and corresponding trade-offs. Monte Carlo exploration allows for the evaluation of the robustness of seismic anomalies and anisotropic patterns, as well as the trade-offs between isotropic and anisotropic perturbations, key features for the interpretation of tomographic models in volcanic environments. The approach is completely non-linear, free of any explicit regularization and it keeps the computational time feasible, even for large data-sets.

How to cite: Del Piccolo, G., Lo Bue, R., VanderBeek, B. P., Faccenda, M., Cocina, O., Firetto Carlino, M., Giampiccolo, E., Morelli, A., and Byrnes, J.: Trans-dimensional Mt. Etna P-wave anisotropic seismic imaging, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18148, https://doi.org/10.5194/egusphere-egu24-18148, 2024.

11:00–11:10
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EGU24-4528
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On-site presentation
Ying Li, Yuan Gao, and Jianhui Tian

Complex tectonics and significant crustal anisotropy are observed at the intersection of the Red River Fault (RRF) and the Xiaojiang Fault (XJF) in the southern Sichuan-Yunnan block, in the SE Tibetan Plateau. The fast S polarization of upper crustal anisotropy varies from NW-SE in the west to NE-SW in the east near the Yimen region. However, small-scale anisotropic structures remain challenging due to limited measurements. Using two years of seismic data from the temporary linear HX Array and permanent stations, this study employed machine learning to construct a high-precision earthquake catalog for S-wave splitting, revealing the upper crustal anisotropy. The new catalog has nearly twice as many earthquakes as the China Earthquake Networks Center. The seismicity is concentrated in the Yimen region with various strike-slip faults, which has a strong correlation with high- and low-velocity boundaries, especially near the edge of the low-velocity zone. Spatial variations in upper crustal anisotropy along the HX Array correspond to geological structures and regional stress. Despite a dominant NE-SW PFS (i.e., fast S-wave polarization) in the Yimen region, stations show dual dominant directions with high values of DTS (i.e., delay times between split S-waves), indicating intricate tectonic and stress interactions. The middle segment of the RRF shows significantly lower DTS values than either side, along with a vertically distributed earthquake swarm, possibly indicating locked structures with high seismic hazard. A comparison between the upper and whole crustal anisotropy reveals consistent deformation within blocks and nearly orthogonal deformation near the RRF and the XJF. The boundary faults likely play a crucial role in influencing the crustal anisotropy both horizontally and vertically. The faults like the Shiping-Jianshui and the Puduhe, running parallel to the RRF and the XJF, are believed to affect the crustal structure. This study highlights that microseismic detection enhances earthquake catalog completeness, providing insights into detailed structures [supported by NSFC Projects 42074065 & 41730212].

How to cite: Li, Y., Gao, Y., and Tian, J.: Microseismic records based on machine-learning reveal the crustal anisotropy beneath the southern Sichuan-Yunnan block in the SE Tibetan Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4528, https://doi.org/10.5194/egusphere-egu24-4528, 2024.

11:10–11:20
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EGU24-3697
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On-site presentation
Hitoshi Kawakatsu

The seismic moment tensor, which represents the equivalent body-force system of the seismic source (Backus and Mulcahy, 1973), may exhibit non-double couple components (NDCs) when the earthquake occurs on a planer fault if the source medium is anisotropic (Aki and Richards, 1981; Kawasaki and Tanimoto, 1981). Kawakatsu (1991, GRL) reported that the NDCs of the moment tensors for shallow earthquakes from the Harvard CMT catalog (Dziewonski et al.,1981; predecessor of GCMT) exhibit a systematic characteristic dependent on faulting types. Specifically, the sign of NDC on average systematically switches between normal-faulting and reverse-faulting. The average NDC parameter ε (Giardini, 1983) is negative for thrust faulting and positive for normal faulting. This behavior can be explained if the source region is transversely isotropic with a vertical symmetry axis (radially anisotropic). In fact, the transverse isotropy model of PREM at a depth of 24.4 km predicts the observed systematic NDC pattern, although the magnitude is slightly underestimated, indicating the potential to enhance our understanding of the lithospheric transverse isotropy using the NDC of the moment tensors.

To investigate the lithospheric transverse isotropy structure utilizing the NDCs of the moment tensors, we propose a novel inversion scheme, building upon the approaches employed by Vavrycuk (2004) and Li, Zheng, et al. (2018) for deep and intermediate-depth earthquakes, but with necessary modifications to address shallow sources (Kawakatsu, 1996, GJI). Synthetic tests conducted under conditions of random faulting indicate the potential to constrain the S-wave anisotropy (ξ) and the fifth parameter (ηκ; Kawakatsu, 2016, GJI). However, in realistic scenarios where a predominant stress regime influences earthquake occurrence to limit the diversity of faulting types, a significant correlation between these two parameters is anticipated, especially in regional-scale cases. Preliminary application of this method to real data sourced from the GCMT catalog suggests that the lithospheric transverse isotropy of PREM serves as a suitable initial model. However, some adjustments may be necessary, particularly regarding the fifth parameter, to enhance the model's fidelity in representing observed NDCs of the moment tensors.

How to cite: Kawakatsu, H.: Characterizing Lithospheric Transverse Isotropy via Non-double Couple Components of Moment Tensors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3697, https://doi.org/10.5194/egusphere-egu24-3697, 2024.

11:20–11:30
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EGU24-17055
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ECS
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Virtual presentation
Arijit Chakraborty, Monumoy Ghosh, Siddharth Dey, Shubham Sharma, Sankar N. Bhattacharya, and Supriyo Mitra

The Indian Lithosphere has been shaped by multiple tectonic processes, which include break-up from the Gondwana Supercontinent, traversing over the Reunion and Kerguelen hotspots, collision with Eurasia, and underthrusting beneath the Himalaya and Tibetan Plateau. Seismic velocity structure and radial anisotropy of the lithosphere preserves imprints of these  tectonic processes and related deformation. We perform joint-modeling of fundamental-mode Rayleigh (LR) and Love (LQ) wave group-velocity dispersion, for periods between 10 and 120s, to obtain radially anisotropic shear-wave velocity structure across India, Himalaya and Tibet. 1D path-average dispersion curves, computed for ~14700 regional earthquake-receiver raypaths, has been passed through systematic quality control of signal-to-noise ratio (>3), elimination of multipathed energy using polarization analysis, and removal of overtone interference, by synthetic tests. These 1D dispersion data are combined through a tomographic formulation to obtain 2D maps. The tomographic parametrization is done using  4906 nodes as apex of triangular elements of side 1°. LR and LQ fundamental-mode group-velocity dispersion data at these nodes are the observation input to the joint inversion. The inversion is done in 2-steps, first by parameterizing the model as isotropic layers and using an isotropic inversion scheme to obtain the best fitting Vs model; second using this output Vs model into an anisotropic inversion scheme, implemented using Genetic Algorithms (GA). GA exhaustively searches the model-space composed of Vsh, Vph and Xi[Vsh^2/Vsv^2] as free parameters. The fit to both LR and LQ datasets significantly improve in the anisotropic inversion. 

 

Results are presented as 2D depth-slice maps and cross-sections constructed using bilinear interpolation. The main findings from our models are lateral variation in the voigt-average Vs beneath the Tibetan Plateau at depth between 80-140 km. Western Tibet has high Vs and positive Xi, while Central-Eastern TIbet has Low Vs and negative Xi. From cross-sections across both regions, we infer that the dip and underthrusting of the Indian Plate beneath Tibet has lateral variation. The high Vs and positive anisotropy in Western Tibet indicates a shallow underthrusting of the Indian lithosphere up to the Tarim Basin, with simple-shear deformation. Where as, the lower Vs and negative anisotropy in Central-Eastern Tibet is a result of partial-underthrusting of India at a steeper-angle up to the Bangong-Nujiang Suture, and pure-shear deformation of thickened Tibet Lithosphere beneath North-Central Tibet. A negative anisotropy signature along the Reunion volcanic track is observed between 100 and 160 km depth. We infer this to be the signature of  Reunion hotspot volcanism in the Indian lithosphere caused by the vertical ascent of a huge volume of melt arising from the plume-head. Similar observations are also made beneath the track of the Kergulean hotspot. 

How to cite: Chakraborty, A., Ghosh, M., Dey, S., Sharma, S., Bhattacharya, S. N., and Mitra, S.: Deformation of the Indian Lithosphere from radial anisotropy: Signatures of laterally varying plate geometry beneath Tibet and hotspot volcanism beneath the Deccan Plateau. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17055, https://doi.org/10.5194/egusphere-egu24-17055, 2024.

11:30–11:40
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EGU24-3339
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On-site presentation
Ban-Yuan Kuo, Cheng-Chien Peng, and Po-Chen Wang

How continental plate boundary faults develop with depth has been under debate. We inverted SKS shear wave splitting data along the San Andreas Fault (SAF) into two layers of anisotropy using a Bayesian inversion. While the two layers are statistically required, the fast polarization directions of the upper layer do not match the strike of the SAF as previously reported. To capture the lithospheric shear zone, we progressively decrease the upper limit of the delay time of the upper layer. In northern California where SAF strikes 140-150, the upper layer fast directions get close to these azimuths when the delay time is reduced to ~0.5 s. In southern California where the SAF strikes 120-130, the upper layer fast directions capture the SAF with delay times of a similar magnitude. For olivine LPO with vertical shear plane, these delay times translate to a anisotropy layer of 40 km thickness, or a depth of 70 km from the surface, assuming the seismogenic zone in the crust is too localized to influence the SKS splitting. This depth coincides with the depth of lithosphere-asthenosphere boundary independently estimated. The lower-layer fast directions are in between the absolute plate motion directions of the American and the Pacific plates, or at least agree with that predicted from surface wave study in northern California. We picture a vertical continental shear zone widening to at least 200 km at the bottom of the lithosphere, transitioning to a horizontal shear regime in the asthenosphere driven by plate motions. This architecture of continental shear zone is consistent with our understanding of the rheology of crust and mantle.

How to cite: Kuo, B.-Y., Peng, C.-C., and Wang, P.-C.: Structures of the continental shear zone beneath the San Andreas Fault inferred from two-layer modeling of SKS splitting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3339, https://doi.org/10.5194/egusphere-egu24-3339, 2024.

11:40–11:50
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EGU24-3769
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ECS
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Virtual presentation
Ramin Movaghari and Yingjie Yang

The continental lithosphere of the Iranian plateau is complicated by a multitude of tectonic processes resulting from the convergence between the Arabian and Eurasian plates. In order to investigate the deformation mechanisms of the uppermost mantle, this study presents a radial anisotropy model beneath the Iranian Plateau constructed by long period (10-100 s) Rayleigh and Love waves from ambient noise data. The broadband Rayleigh and Love signals are extracted from continuous data recorded by 88 seismic stations using a double-beamforming algorithm. Due to utilizing long period surface waves, we apply finite-frequency ambient noise tomography to generate two-dimensional dispersion maps. These phase velocity maps are consistent with those obtained from conventional methods. Finally, we invert Rayleigh and Love local phase velocity dispersion curves using a Bayesian Markov chain Monte Carlo inversion method. The obtained radial anisotropy model shows negative values in Central Iran suggesting a horizontal character of the minerals likely due to a channelized asthenospheric flow in the upper mantle.

How to cite: Movaghari, R. and Yang, Y.: Uppermost mantle radial anisotropy based on double-beamforming of ambient noise cross correlation beneath Iranian plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3769, https://doi.org/10.5194/egusphere-egu24-3769, 2024.

11:50–12:00
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EGU24-14416
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On-site presentation
Ratna Mani Gupta, Hsin-Hua Huang, Po-Fei Chen, Cheng-Horng Lin, and Cheng-Chien Peng

The Taiwan orogenic belt originates from the collision between the Philippine Sea Plate (PSP) and the Eurasian Plate (EU) with a subduction polarity reversal. The reversal around northern Taiwan creates a complex geodynamic process from subduction waning to post-collision extension. We study the deformation fabric with the shear wave splitting (SWS) method to unravel this tectonic complexity using multiple core phases (PKS, SKS, and SKKS, hereafter XKS). Prevailing SWS research acknowledged the presence of orogen-parallel anisotropy. However, recent studies with numerical modeling and coherency analysis suggested that the anisotropy source is in the asthenosphere. A recent dense seismic array (Formosa Array) of 148 seismic stations in northern Taiwan enables us to revisit this debate with improved spatial and back azimuthal coverage of the SWS measurements. The results show distinct variations in fast direction (Φ) from different back-azimuths and a much larger average delay time (dt) of ~2 sec compared to that derived from local subduction events (at 100-250 km depth). Application of the Fresnel zone and spatial coherency analysis also support an asthenospheric source for the observed anisotropy. The findings emphasize the need for depth-source analysis of anisotropy to better elucidate the responsible mechanisms of complex tectonic settings.

How to cite: Gupta, R. M., Huang, H.-H., Chen, P.-F., Lin, C.-H., and Peng, C.-C.: Examining Depth Origin of Anisotropy in an Active Orogenic Belt of Taiwan Using Shear Wave Splitting Results from the Formosa Array., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14416, https://doi.org/10.5194/egusphere-egu24-14416, 2024.

12:00–12:10
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EGU24-6845
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On-site presentation
Distinct mantle flow patterns in the Sunda subduction zone revealed by a layered anisotropic structure
(withdrawn)
Fansheng Kong, Stephen S. Gao, Kelly H. Liu, Sijia Li, Jie Zhang, and Jiabiao Li
12:10–12:30
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EGU24-13005
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solicited
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On-site presentation
Ebru Bozdag, Ridvan Orsvuran, Lijun Liu, and Daniel Peter

Earth’s upper mantle and lithosphere show significant evidence for anisotropy related to deformation and composition. First-generation GLAD (GLobal ADjoint) models (GLAD-M15 (Bozdag et al. 2016), GLAD-M25 (Lei et al. 2020), GLAD-M35 (Cui et al. submitted)) are radially anisotropic in the upper mantle. Starting from GLAD-M25, we performed 25 conjugate gradient iterations and constructed model GLAD-M50-AZI by including azimuthal anisotropy in the parameterization of the inverse problem. We inverted azimuthally anisotropic normalized parameters Gc’ and Gs’ simultaneously with vertically and horizontally polarized shear waves beta_v and beta_h, respectively. Due to our parameterization, our data set consists of only minor- and major-arc Rayleigh and Love waves from 300 globally distributed earthquakes. GLAD-M50-AZI captures plate motions globally well, which are also supported by the transverse isotropy, specifically at the subducted slabs and mid-ocean ridges. Furthermore, it approaches continental-scale resolution in regions with good data coverage depicting smaller-scale tectonic and flow patterns, giving us a chance to have a more detailed and unified view of the anisotropy globally. In the next step, we explore how anisotropy derived from seismic tomography compares to geodynamical modeling observations to have better insight into mantle dynamics. We perform numerical simulations to compute synthetic seismograms and full-waveform inversion on Texas Advanced Computing Center’s Frontera system. 

How to cite: Bozdag, E., Orsvuran, R., Liu, L., and Peter, D.: Azimuthal and radial anisotropy in the upper mantle from global adjoint tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13005, https://doi.org/10.5194/egusphere-egu24-13005, 2024.

Lunch break
Chairpersons: Tuna Eken, Judith Confal, Manuele Faccenda
14:00–14:10
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EGU24-8694
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On-site presentation
Luděk Vecsey, Jaroslava Plomerová, and AlpArray, AlpArray-EASI, PACASE Working Groups and AdriaArray Seismology Group

Splitting of shear waves proves their propagation within an anisotropic medium. Frequently used methods of evaluating of upper mantle anisotropy search for two parameters - the delay time of the slow split shear wave and the polarization direction of the fast split shear wave. The parameters retrieved by the standard methods such as energy minimization on the transverse component of the shear waveforms or eigenvalue of cross-correlation matrix suffer from e.g., ubiquitous noise, errors in sensor orientation and numerous so-called ‘null splits’ or unrealistically large values. However, well-resolved splitting parameters from core-mantle refracted shear SK(K)S phases are limited to relatively narrow fans of back azimuths. Such incomplete back-azimuth coverage prevents modelling anisotropic structures with symmetry axes oriented generally in 3D, i.e., with tilted axes, to be compatible with 3D anisotropic models from independent observables.  Generally used averages of time delays and polarization pairs lead to simplified models of the upper mantle, which concentrate on modelling the present-day flow in the sub-lithospheric mantle.

Therefore, we propose a new method directly exploiting variations in width and orientation of particle motion (PM) of split shear waves, which allows measuring anisotropic characteristics for a larger amount of waveforms and improves azimuthal coverage in a region. We characterize the PM by two parameters, the PM width and the PM orientation. At each station, we plot the normalized width of the PM as a ratio of lengths of the minor to major axes in dependence on back-azimuths. Variations of the PM width with back-azimuth exhibit oscillations with several extremes of different amplitudes. Such behaviour results from wave propagation through the anisotropic upper mantle. One of the advantages of the method is that the width of the PM is invariant of potential mis-orientation of sensors.

We test the PM method on a set of SKS waveforms recorded at a subset of stations included in several recent or running passive seismic experiments (EASI, AlpArray, PACASE, AdriaArray). The stations form a band of about 200km broad running from the western Bohemian Massif through the Eastern Alps to the Adriatic Sea. Stations characterized by similar variations of the PM parameters group into sub-regions, which are compatible with the main tectonic features of the whole region. The formation of such lithospheric blocks of similar anisotropic signals is in agreement with 3D self-compatible anisotropic models of the mantle lithosphere domains derived from independent observables. We present complementary studies of the anisotropic structure of the mantle lithosphere in contributions by Zlebcikova et al. (GD7.1, EGU 2024), which shows anisotropic model of the upper mantle derived from 3D coupled anisotropic-isotropic teleseismic tomography (code anitomo), and in contribution by Kvapil et al. (GD7.1, EGU 2024), in which anisotropic structure of the lower crust is modelled from ambient noise.

How to cite: Vecsey, L., Plomerová, J., and Working Groups and AdriaArray Seismology Group, A. A.-E. P.: A new method to measure seismic anisotropy of the upper mantle directly from the width and orientation of the particle motion of shear waves , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8694, https://doi.org/10.5194/egusphere-egu24-8694, 2024.

14:10–14:20
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EGU24-15225
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ECS
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On-site presentation
Brandon VanderBeek, Gianmarco Del Piccolo, and Manuele Faccenda

The Cascadia subduction system is an ideal location to investigate the nature of mantle flow and associated driving forces at a convergent margin owing to the dense network of on- and off-shore seismic instrumentation. While numerous shear wave splitting and tomography studies have been performed with these data, they have produced conflicting views of mantle dynamics collectively referred to as the Cascadia Paradox. On the overriding plate, splitting observations are consistent with large-scale 3D toroidal flow while off-shore splitting patterns are more easily explained by 2D plate-driven flow. Either geometry is difficult to reconcile with seismic tomographic models that image a fragmented Juan de Fuca slab descending beneath the Western USA. However, these observations offer only an incomplete image of Cascadia mantle structure. Shear wave splitting provides a depth integrated view of anisotropic fabrics making inferences regarding the 3D nature of mantle deformation difficult. Prior high-resolution body wave tomography typically neglects anisotropic effects which can in turn yield significant isotropic imaging artefacts that complicate model interpretation. To overcome these limitations, we invert P-wave delay times for a 3D hexagonally anisotropic model with arbitrarily oriented symmetry axes using the reversible jump Markov chain Monte Carlo algorithm. This stochastic imaging approach is particularly well-suited to the highly non-linear and under-determined nature of the anisotropic seismic tomography problem. The resulting ensemble of solutions allows us to rigorously assess model parameter uncertainties and trade-off between isotropic and anisotropic heterogeneity. We investigate whether the fragmented nature of the subducted Juan de Fuca slab is a well-resolved feature and to what extent its geometry trades off with anisotropic parameters. In light of our new 3D anisotropic model, we re-evaluate the Cascadia Paradox and attempt to reconcile disparate views of Western USA mantle dynamics.

How to cite: VanderBeek, B., Del Piccolo, G., and Faccenda, M.: Exploring Mantle Dynamics of the Cascadia Subduction System through Anisotropic Tomography with Transdimensional Inference Methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15225, https://doi.org/10.5194/egusphere-egu24-15225, 2024.

14:20–14:30
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EGU24-2395
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On-site presentation
Joshua Muir

The faceting behaviour of olivine controls many properties on the grain surface such as diffusion and storage.  This behaviour and its effects are poorly understood due to the difficulty of examining them and the many controls that are on this paper.  In this talk I shall present a thermodynamic model of olivine faceting and how it is controlled by temperature, pressure, iron, grain size and water content.  In turn I shall discuss how this faceting then controls other important properties such as storage of water on the grain boundaries and how these are perhaps an overlooked sink in the Earth’s mantle.

How to cite: Muir, J.: Olivine faceting and water storage: A complex dynamic anisotropic sink, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2395, https://doi.org/10.5194/egusphere-egu24-2395, 2024.

14:30–14:40
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EGU24-17743
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On-site presentation
Andrea Tommasi, Nestor Cerpa, Fernando Carazo, and Javier Signorell

Both elastic and viscoplastic behaviors of the Earth’s upper mantle are highly anisotropic, because olivine, which composes 60-80% of the mantle, has a strong intrinsic anisotropy and develops strong crystal preferred orientations (CPO). Predicting the evolution of anisotropy with strain is essential to: (1) probe indirectly the deformation in the mantle based on seismic measurements and (2) accounting for the deformation history when simulating the long-term dynamics of the Earth. However, traditional micro-mechanical approaches to model the evolution of CPO-induced elastic and viscous anisotropies are too memory-costly and time-consuming for coupling into geodynamical simulations. To speed up the prediction of seismic anisotropy in the mantle, we developed deep-learning (DL) surrogates trained on a synthetic database built with viscoplastic self-consistent simulations of texture evolution of olivine polycrystals in typical 2D geodynamical flows. A first challenge was the choice of memory-saving representations of the CPO. Training the DL models on the evolution of the elastic tensor components avoided the need of storing the CPOs. However, the major challenge has been to prevent error compounding in a recursive-prediction scheme – where a model prediction at a given time step becomes the input for the next one - to evaluate the anisotropy evolution along a flow line. We implemented multilayer feed-forward (FFNN), ensemble, and transformer neural networks, obtaining the best efficiency/accuracy ratio for the FFNN. The results highlight the importance of (1) the standardization of the outputs in the training stage to avoid overfitting in predictions, (2) the statistical characteristics of the strain histories in the training database, and (3) the influence of non-monotonic strain histories on error propagation. Predictions for complex unseen strain histories are accurate, much more time-efficient and memory-costly than the traditional micro-mechanical models. Our work opens thus new avenues for modeling the strain-controlled evolution of mechanical anisotropy in the Earth’s mantle. This work was supported by the European Research Council (ERC) under the European Union Horizon 2020 Research and Innovation programme [grant agreement No 882450 – ERC RhEoVOLUTION.

How to cite: Tommasi, A., Cerpa, N., Carazo, F., and Signorell, J.: Predicting seismic anisotropy in the upper mantle using supervised deep-learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17743, https://doi.org/10.5194/egusphere-egu24-17743, 2024.

14:40–14:50
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EGU24-13585
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ECS
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On-site presentation
Poulami Roy, Bernhard Steinberger, Manuele Faccenda, and Juliane Dannberg

Seismic anisotropy, which involves directionally dependent wave propagation, is likely to occur in the lowermost few hundreds km of the mantle, especially at the edges of Large Low Shear Velocity Provinces (LLSVPs). This anisotropy may be indicative of significant deformation, potentially due to mantle flow interacting with the sides of these provinces or the generation of mantle plumes. In this study, we investigate subducted slab induced plume generation from an LLSVP boundary and the flow behaviour of the lower mantle using compressible 2-D and 3-D mantle convection models in the geodynamic modeling software ASPECT combined with mantle fabric simulation in ECOMAN. In our geodynamic simulation, we assume that the LLSVPs are chemically distinct piles with intrinsically high viscosity. We use the Clapeyron slope of the phase transition from Bridgmanite to post-Perovskite from the previous mineralogical study by Oganov & Ono (2004) in the mantle fabric calculation. Modeling lattice preferred orientation of Bridgmanite and post-Perovskite in the lower mantle reveals that the lower mantle is overall isotropic except the regions of plume conduits and the surroundings of the subducted slab where vertically polarized shear wave (Vsv ) is faster. The generation of anisotropy are caused by the accumulation of high finite strain in these regions. The bottom 300 km of the lower mantle is characterized by fast horizontally polarized shear wave (Vsh ) beneath the subducted slab which deflects to fast Vsv at the margins of the LLSVPs due to the rheological contrast between the highly viscous LLSVP and less viscous ambient mantle. Our result shows that six possible slip systems [100](010), [100](001), [010](100), [001](100), [110](-110) and [-110](110) of Bridgmanite and the slip system [100](001) of post-Perovskite can produce a fast Vsv in the plume generation zones where post-Perovskite transforms to Bridgmanite and fast Vsh at the base of the subducted slab where post-Perovskite is preserved in the D”. However, our models do not show anisotropy inside of the LLSVPs and the subducted slab, possibly because of their high viscosity. Our findings are comparable with the previous seismic observations beneath the Iceland plume where Vsv > Vsh and the slab-driven flow at the base of the mantle beneath the northeastern Pacific Ocean where Vsh > Vsv .

How to cite: Roy, P., Steinberger, B., Faccenda, M., and Dannberg, J.: Exploring the Development of Shear Wave Radial Anisotropy in the Lower Mantle due to Slab-induced Plume Generation from LLSVPs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13585, https://doi.org/10.5194/egusphere-egu24-13585, 2024.

14:50–15:00
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EGU24-12689
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On-site presentation
Dorian Soergel, Utpal Kumar, Nicolas Valencia, and Barbara Romanowicz

The presence of ponding slabs at the base of the mantle transition zone (600-700 km) has been well known for a long time and can be explained by the changes in material properties related to phase changes around this depth. However, recent tomographic studies have shown the presence of slabs stagnating at larger depths of around 1000 km. While geodynamic simulations and experiments provide different insights, seismic tomography is crucial to constrain these geodynamic models. More specifically, seismic anisotropy is of particular interest to understand the dynamics of the mantle because of its sensitivity to the flow of mantle material.

The south-west pacific zone is an area with a very complex tectonic setting, with several subduction zones in a relatively small area, illuminated by a very high level of seismicity. It is thus of particular interest to understand the dynamics of the extended transition zone. As such, it has been the object of numerous tomographic studies, including high-resolution full-waveform tomography. In most cases, these studies only invert for radial anisotropy, as azimuthal anisotropy is generally more difficult to measure. However, azimuthal anisotropy is equally important as radial anisotropy and a proper interpretation in terms of mantle flow requires both. We present updated results of a full-waveform inversion of the region including azimuthal anisotropy recovered from body and surface waveforms and XKS-splitting data.

How to cite: Soergel, D., Kumar, U., Valencia, N., and Romanowicz, B.: Full waveform anisotropic tomography of the transition zone beneath the south west Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12689, https://doi.org/10.5194/egusphere-egu24-12689, 2024.

15:00–15:20
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EGU24-8851
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solicited
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On-site presentation
Christine Thomas and Angelo Pisconti

Detection of D" anisotropy is usually carried-out with shear wave splitting analysis. To constrain azimuthal anisotropy and infer mineralogy and deformation style, a number of crossing paths is necessary. Here we use an approach that utilises the polarity of P- and S- wave reflections from the D" discontinuity, compared with the main phases P and S, and combines these measurements with ScS splitting results. Using deformation scenarios for a number of lower(most) mantle candidate materials, we calculate the reflection coefficient for P and S-wave reflections and ScS splitting predictions. From our modelling, a clear distinction between different anisotropic media is possible by using both types of observations together. Furthermore, the approach allows to use only a single direction to distinguish between different scenarios. We apply the method to the Central/South Atlantic and South Africa, across the border of the large-low seismic velocity province (LLSVP). Shear wave splitting observations suggest that anisotropy is present in this region of the mantle, in agreement with previous studies that partially sampled this region. Modelling the observations with lattice preferred orientation and shape preferred orientation of materials expected in the D" region, we find two domains of mineralogy and deformation: sub-horizontally aligned post-perovskite outside the LLSVP, beneath the South and Central Atlantic, which is replaced by up-tilted aligned bridgmanite within the LLSVP beneath South Africa.

How to cite: Thomas, C. and Pisconti, A.: Constraining D" seismic anisotropy with reflections and splitting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8851, https://doi.org/10.5194/egusphere-egu24-8851, 2024.

15:20–15:45

Posters on site: Wed, 17 Apr, 10:45–12:30 | Hall X1

Display time: Wed, 17 Apr 08:30–Wed, 17 Apr 12:30
Chairpersons: Judith Confal, Manuele Faccenda, Tuna Eken
X1.155
|
EGU24-5468
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ECS
Synthetic Seismic Waveform Analysis for Constraining Anisotropy in the D'' Layer Using the PcSdiff Phase
(withdrawn)
Foivos Karakostas, Teh-Ru Alex Song, and Kai Deng
X1.156
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EGU24-21240
Jan Philipp Kruse, Georg Rümpker, Frederik Link, Thibault Duretz, and Harro Schmeling

We utilize three-dimensional geodynamic models to predict XKS-splitting in double subduction scenarios characterized by two outward-dipping slabs. These models are highly applicable in various realistic settings, such as the central Mediterranean. Our primary focus is on the analysis of XKS-splitting, a key geophysical observable used for inferring seismic anisotropy and mantle flow patterns.Our models simulate the concurrent subduction of two identical oceanic plates separated by a continental plate. The variation in the strength of the separating plate causes a transition from a retreating to a stationary trench. The models offer detailed insights into the temporal evolution of mantle flow patterns, particularly the amount of trench-parallel flow induced by this specific type of subduction.In the subsequent step, we employ the well-known D-Rex model to estimate Crystallographic Preferred Orientation (CPO) development in response to plastic deformation resulting from mantle flow. Based on the D-Rex model results, which incorporate the full elastic tensor of a deformed multiphase polycrystalline mantle aggregate, we derive synthetic apparent splitting parameters and splitting intensities at virtual receivers placed at the surface using multiple-layer anisotropic waveform modeling. To identify regions with pronounced depth-dependent variations of anisotropic properties, particularly the fast polarization directions, we define a complex anisotropy factor dependent on the apparent splitting parameters and splitting intensities.Finally, using the apparent splitting parameters, we conduct two-layer model inversions at selected locations characterized by a large complex anisotropy factor. The two-layer model provides apparent splitting parameters as a result of analytical waveform modeling for two anisotropic layers. We observe that while several models can effectively explain the apparent splitting parameters, only a subset can accurately reproduce the depth-dependent anisotropic properties. Our findings unequivocally demonstrate that a classical XKS-splitting analysis can effectively identify areas characterized by complex anisotropy and provide accurate approximations of the depth-dependent variations of anisotropic properties within these regions. However, caution is warranted when interpreting results obtained through inversion based on a two-layer analysis.

How to cite: Kruse, J. P., Rümpker, G., Link, F., Duretz, T., and Schmeling, H.: Anisotropy and XKS-splitting from geodynamic models of double subduction: Testing the limits of interpretation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21240, https://doi.org/10.5194/egusphere-egu24-21240, 2024.

X1.157
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EGU24-8905
Jiří Kvapil, Jaroslava Plomerová, and AlpArray, AlpArray-EASI, PACASE Working Groups and AdriaArray Seismology Group

Previous research of the Bohemian Massif (BM) crust with the use of ambient noise tomography (ANT) indicates a transversely isotropic structure of the lower crust (Kvapil et al. 2021). In this study, we have developed a new approach for evaluations of localised seismic anisotropy by travel time integration method.

The method calculates synthetic Rayleigh (vertical ZZ correlation) and Love (transverse TT correlation) velocities and derives the vSH/vSV from the initial 3D isotropic vSV model. The higher ratio of measured vSH/vSV to synthetic vSH/vSV indicates the existence of velocity anisotropy in the lower crust of the BM in the reference ANT model (Kvapil et al., 2021). The new method evaluates azimuthal variations of the synthetic parameters due to heterogeneities reflecting local geology effects and corrects the observed velocity ratios. Then the 1D stochastic joint (ZZ, TT) inversion is applied to retrieve the depth dependence of the velocity ratio. We use cross-correlation of ambient noise and earthquake data from seismic stations included in the AlpArray, PACASE, and Adria Array passive seismic experiments and data from the PASSEQ experiment, which complement the sparse coverage in the northern part of the BM.

Seismic anisotropy records the stress/strain conditions of each originally independent tectonic microplates during the formation of the BM crust. We demonstrate that synthetic modelling over the reference isotropic velocity model is an efficient tool for extracting radial and azimuthal shear velocity anisotropy in the lower crust directly from Rayleigh and Love wave dispersion curves. Regions with consistent parameters of seismic anisotropy correlate well with the major tectonic units of the BM. We show that variations in azimuthal and radial anisotropy of the lower crust on a regional scale can provide constraints for the reconstruction of geodynamic processes during the formation of the BM. We present complementary studies of the anisotropic structure of the mantle lithosphere in contributions by Zlebcikova et al. (GD7.1, EGU 2024), which shows an anisotropic model of the upper mantle derived from a 3D coupled anisotropic-isotropic teleseismic tomography (code anitomo), and in contributions by Vecsey et al. (GD7.1, EGU 2024), suggesting a new method for evaluation of anisotropy from shear waves.

How to cite: Kvapil, J., Plomerová, J., and Working Groups and AdriaArray Seismology Group, A. A.-E. P.: Anisotropic structure of the central European lower crust from ambient noise inversion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8905, https://doi.org/10.5194/egusphere-egu24-8905, 2024.

X1.158
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EGU24-8306
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ECS
Lin Liu, Stephen Gao, Kelly Liu, Sanzhong Li, and Youqiang Yu

In contrast to many prior studies that relied on station-averaged shear-wave splitting (SWS) measurements and sparsely distributed stations, this study takes advantage of the recent deployment of broadband seismic stations at intervals of less than 50 km to explore the intricate seismic azimuthal anisotropy between the dynamic Tibetan Plateau and the stable North China Craton. Our analysis encompasses 6,409 high-quality individual SWS measurements from 465 closely spaced stations located along the boundary of the northeastern Tibetan Plateau and the western North China Craton. Notably, twenty of these stations show splitting parameters with a π/2 periodicity based on azimuthal variations, indicating a complex double-layer horizontal anisotropy structure. The anisotropy of the upper layer is linked to ductile flow in the middle-to-lower crust originating from the Tibetan Plateau. This flow encounters the rigid lithosphere of the Alxa Block, leading to a bifurcation into northeastward and southeastward directions. The anisotropy in the lower layer, exhibiting fast orientations in an NW-SE direction, is consistent with the observed one-layered anisotropy and aligns with the absolute plate motion (APM) of the Eurasian plate. The coherence in the spatial distribution of splitting parameters indicates that the predominant source of observed anisotropy is asthenospheric flow. This mantle flow exhibits a southeastward orientation beneath the Alxa block, transitioning to an almost eastward direction along the thinner lithospheric passage between the Ordos and Sichuan cratonic keels. This pattern unveils the influence of cratonic edges in modulating localized mantle flow systems.

How to cite: Liu, L., Gao, S., Liu, K., Li, S., and Yu, Y.: Modulation of crustal and mantle flow systems by a heterogeneous cratonic root: Evidence from seismic azimuthal anisotropy analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8306, https://doi.org/10.5194/egusphere-egu24-8306, 2024.

X1.159
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EGU24-7126
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ECS
Changhui Ju, Junmeng Zhao, Qiang Xu, and Guohui Li

The East Pamir seismic experiment (8H) was conducted in the eastern Pamir and the adjacent Tarim Basin from August 2015 to May 2017. Utilizing seismograms from the 8H network and nine permanent seismic stations operated by the China Earthquake Administration, we computed shear wave splitting parameters through cluster analysis of the minimum energy method. A total of 452 high-quality individual SKS-splitting measurements were obtained at 39 seismic stations. Given the predominant availability of events with a back-azimuth (BAZ) around ~110° and the absence of a broad range of BAZ values, we opted for a single-layer anisotropic model to interpret the measurements.

The upper mantle seismic anisotropy structure in the Western Himalayan Syntaxis (WHS) exhibits distinctive regional characteristics in various regions. Group A comprises 15 stations situated near the Alai Valley and the Tien Shan with a northeast-oriented Fast Polarization Direction (FPD), aligned with the strike of the orogen and the Absolute Plate Motion (APM) azimuthal direction (~80°) of the Eurasian plate. This group exhibits relatively larger Delay Time (DT) and may be originated from the oriented arrangement of olivine crystals in the mantle lithosphere of the Eurasian continent during the northward subduction of the Indian continent. Group B consists of 21 stations located in the eastern Pamir and adjoining Tarim Basin, demonstrating a curved orientation. While this orientation contradicts the APM direction, it approximately parallels the trend of large-scale surface structures. Combining these observations with previous imaging results, we propose that during the northward advancement of the Indian continent, mantle material flow (escape) in the Pamir-Hindu Kush region formed seismic anisotropy structures similar to those in the Western Himalayan Syntaxis (WHS).

How to cite: Ju, C., Zhao, J., Xu, Q., and Li, G.: Seismic anisotropy of the crust and upper mantle beneath eastern Pamir, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7126, https://doi.org/10.5194/egusphere-egu24-7126, 2024.

X1.160
|
EGU24-8928
Helena Žlebčíková, Jaroslava Plomerová, Luděk Vecsey, and AlpArray Working Groups

Teleseismic body waves recorded during passive seismic experiments allow us to investigate isotropic velocities of the Earth’s upper mantle in a great detail, on scales of tens of kilometres. However, most of the tomography studies neglect the body-wave anisotropy completely or limit it either to azimuthal or radial anisotropy. We have developed a code called AniTomo for coupled anisotropic-isotropic travel-time tomography of the upper mantle (Munzarová et al., Geophys. J. Int. 2018) which allows for inversion of relative travel-time residuals of teleseismic P waves simultaneously for 3D distribution of P-wave isotropic-velocity perturbations and anisotropy of the upper mantle. We assume weak anisotropy of hexagonal symmetry with either ‘high-velocity’ axis a (lineation) and low velocity (b,c) plane or ‘low-velocity’ axis b and high velocity plane (a,c) (foliation) that is oriented generally in 3D. Such an approach of searching for orientation of the symmetry axes freely in any direction is unique and more general in comparison with the published methods that usually assume only horizontal or vertical orientation of the high-velocity symmetry axis. The code represents a step further from modelling homogeneously anisotropic blocks of the mantle lithosphere (e.g., Vecsey et al., Tectonophysics 2007; Plomerová et al., Solid Earth 2011) towards modelling anisotropy arbitrarily varying in 3D. We present complementary studies of anisotropic structure of the mantle lithosphere in contributions by Vecsey et al. (GD7.1, EGU 2024), suggesting a new method for evaluation of anisotropy from shear waves and in contribution by Kvapil et al. (GD7.1, EGU 2024), in which anisotropic structure of the lower crust is modelled from ambient noise. 

We have applied the AniTomo code on P-wave travel time deviations recorded during passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019) to image the upper mantle large-scale anisotropy beneath the western part of the Bohemian Massif and the Eastern Alps. We interpret the P-wave tomography results along with results of splitting parameters from core-mantle refracted shear waves at 240 broad-band stations in about 200 km broad and 540 km long band along 13.3° E longitude. The code allows to control the depth variations and an extent of the fabric. The joint inversion/interpretation allows for distinguishing which type of the models (a-axes model or b-axis model) approximates better the anisotropic structure.

The derived anisotropic-velocity models of the mantle lithosphere cluster into domains with boundaries coinciding with boundaries of the main tectonic sub-regions. These domains are compatible with domains inferred from a joint interpretation of directional variations of P-wave travel-time residuals and SKS-wave splitting parameters. The coincidence of boundaries of the anisotropic models of the mantle lithosphere domains with main tectonic features, correlation of the anisotropy depth extent with the LAB models as well as a decrease of anisotropy strength in the sub-lithospheric mantle support fossil origin of the directionally varying component of the detected anisotropic fabrics of the continental mantle lithosphere.

How to cite: Žlebčíková, H., Plomerová, J., Vecsey, L., and Working Groups, A.: Anisotropic tomography of the upper mantle beneath the Eastern Alps and the Bohemian Massif, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8928, https://doi.org/10.5194/egusphere-egu24-8928, 2024.

X1.161
|
EGU24-9012
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ECS
|
Rupak Banerjee, Frederik Tilmann, Supriyo Mitra, Tuna Eken, Keith Priestley, and Sunil Wanchoo

Recent enhancement in instrumentation in the northwest (NW) Himalaya provides an unprecedented dataset to study the deformation due to the Indo-Eurasia convergence. The NW Himalaya is unique in terms of hosting a seismic gap and a flat-ramp geometry at its decollement. We determine azimuthal  seismic anisotropy using core refracted shear waves (SKS) and interpret the results to develop insight about the prevailing geodynamics. Here, 459 raypaths with moment magnitude (Mw) >= 5.5, recorded at 15 seismographs of the J&K Seismological NETwork (JAKSNET), operational between 2013 and 2022, are used. To avoid contamination from direct S and SKiKS phases, we analyze the data within the epicentral distance range of 90°-125°, filtered at 0.04-0.2 Hz. We perform a 2-D grid search over the splitting parameters (delay time and fast axis azimuth) and compute their optimum values, for which the energy of the transverse component is minimum (MTE) after correcting for the inferred splitting. Simultaneously, the Rotation Correlation (RC) method is employed to calculate the delay time and fast axis azimuth corresponding to the maximum correlation coefficient between the splitting-corrected horizontal components. We use selection criteria based on the quality factor and the signal-to-noise ratio (SNR) to determine the measurements to be used for station averaging. The quality factor depends on the similarity of results obtained from the RC and MTE methods, hence helps in avoiding subjective interpretation about the quality of the measurement. The non-null splitting measurements passing these selection criteria are then used for station averaging applying the circular mean method and the energy map stacking method. We observe mostly N-S to NE-SW trending fast axes azimuths (13 of 15 stations); this direction corresponds to the absolute plate motion of India in a no-net rotation frame. The two remaining stations show average NW-SE fast directions, which are parallel to the mountain front, but also these stations show somewhat contradictory single splitting measurements, and one of those two anomalous stations is located very close to stations with NE-SW fast measurements, so we will not interpret these. The mean delay times range from 1.5-3.3 s, with the majority of the stations exhibiting > 2s split time being situated on the foreland basin deposits of the Sub-Himalaya. The high absolute Indian plate motion of 51 mm/yr appears to align the upper mantle olivine beneath the orogeny and NE oriented fast axes track the mantle flow manifested by the basal shear of the plate motion. To complement the SKS data, which are dominated by results with eastern backazimuths and corresponding initial polarisation, we further measured splitting for direct S-wave with the reference station method. Here, the correlation between the horizontal traces of the target and reference stations is maximised after correcting the target trace with trial splitting parameters and differences in gain; the reference station trace has previously been corrected for SKS splitting. We will present direct S splitting measurements from 370 events with Mw>=5.5 and distance within 40°-80° with an interstation spacing of <120 km.

How to cite: Banerjee, R., Tilmann, F., Mitra, S., Eken, T., Priestley, K., and Wanchoo, S.: Seismic Anisotropy in Northwest Himalaya from core refracted shear (SKS) and direct S waves , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9012, https://doi.org/10.5194/egusphere-egu24-9012, 2024.

X1.162
|
EGU24-16887
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ECS
Dustin Lang, Rebecca Kühn, Rüdiger Kilian, Hannah Pomella, and Michael Stipp

The interpretation of seismic data in orogens is usually difficult to decipher as structural information is limited to surface and borehole data. Seismic interpretations very much depend on the elastic wave velocity model, which in the simplest case is a function of rock composition. Seismic velocities can also be anisotropic, i.e. depend on the wave propagation direction inside the rock. Seismic anisotropy can be subdivided into intrinsic (crystallographic preferred orientation (CPO)) and extrinsic (shape preferred orientation, compositional layering or fractures) anisotropy. Microstructures in thin section scale have an impact not only on millimeter-scale but also on larger anisotropies in the field such as meter- to kilometer-scale folds. Here we explore the effect of microstructure (mainly folding and crenulation) on the homogenization of seismic anisotropy from samples of millimeter to thin section scale.

The investigated samples are phyllosilicate- and graphite-rich samples (Innsbruck quartzphyllite and Bündner schist) from the N-S running Brenner Base Tunnel Project (NW-Tauern Window). Phyllosilicate-rich sections with layers of different composition and structure were selected from drill core samples of the exploration tunnel. The CPO of phyllosilicates and graphite from 1.5 – 3.5 mm thick cylinders was measured using high energy X-ray diffraction at DESY (Hamburg, Germany) and the ESRF (Grenoble, France). Pole figure data was directly extracted using single peak fitting. The CPO of quartz was determined by using EBSD. Seismic velocities for each sample were computed using µXRF-based modal composition and single crystal stiffness tensors. We measured the smallest representative volume element which we consider to be undisturbed by microstructural effects. Therefore, we estimate an upper bound of expected intrinsic velocity anisotropies. Thin section-scale anisotropies were modeled from the upper bound anisotropy and the observed microstructure, i.e., small-scale folding. Computed velocities were compared to Vp-anisotropy measurements on the drill cores.

The velocity anisotropy is primarily governed by the content and distribution of phyllosilicates and graphite. Given the crystal symmetry and the low single crystal elastic anisotropy, phases such as feldspar, quartz or calcite can be considered as irrelevant with respect to seismic anisotropies. The simulation of a crenulation cleavage has a stronger impact than centimeter-size folding: The crenulation cleavage reduces the anisotropy for example from 14 % to 12 %. Centimeter-size folding with observed interlimb angles of 140° in contrast is negligible.

The effect of microstructures like centimeter-scale folds and crenulation has only a limited impact on anisotropies of foliated rocks during homogenization from millimeter to thin section-scale. We assume that during homogenization to a larger scale, the effect of folding with small interlimb angles or different fold axes within the homogenized volume will have a stronger influence on seismic anisotropy.

How to cite: Lang, D., Kühn, R., Kilian, R., Pomella, H., and Stipp, M.: The effect of centimeter-scale folding and crenulation on anisotropy homogenization of schists and phyllites from the NW-Tauern Window (Eastern Alps, Austria), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16887, https://doi.org/10.5194/egusphere-egu24-16887, 2024.

X1.163
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EGU24-14883
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ECS
John Keith Magali, Christine Thomas, Jeffrey Gay, Angelo Pisconti, and Sebastien Merkel

There is growing evidence, both from a modelling perspective and seismic observations, that seismic anisotropy in the lower mantle is localized around penetrating slabs where large straining is anticipated. It is believed that the high stresses experienced near the slab activate dislocation creep mechanisms that drive the crystallographic preferred orientation (CPO) of bridgmanite aggregates. Still, deformation mechanisms in bridgmanite remain enigmatic. In recent years, deformation experiments in bridgmanite subjected to mantle temperatures and pressures suggest that its microstructures evolve with pressure, providing another perspective on the debated structure and deformation in the lower mantle. Using this information, we develop a numerical technique that calculates pressure-dependent large-scale seismic anisotropy in a pyrolitic mantle with variable velocity gradients. As a first test, we use the method to predict seismic anisotropy by calculating anisotropic reflection coefficients of underside reflections off a depth corresponding to 50 GPa where pressure-induced slip transitions in bridgmanite are expected. For this, we consider two simple deformation styles: (1) uni-axial compression, akin to vertically penetrating slabs, and (2) simple shear associated with corner-type flows. Finally, we demonstrate a multiscale approach that calculates large-scale seismic anisotropy from a fully time-dependent thermo-chemical model of free subduction with latent heating and phase transitions. The result is a long-wavelength equivalent azimuthal and radial anisotropy maps that are actually comparable to a seismic tomography model. We demonstrate how such an approach can create discontinuities in anisotropy at ~1000 km and provide insights as to how it relates to the heterogeneous distribution of the 1000-km discontinuity.

How to cite: Magali, J. K., Thomas, C., Gay, J., Pisconti, A., and Merkel, S.: Modeling pressure-dependent seismic anisotropy in the lower mantle reveals anisotropic discontinuity at 1000 km, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14883, https://doi.org/10.5194/egusphere-egu24-14883, 2024.

X1.164
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EGU24-12788
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ECS
Rosalia Lo Bue, Francesco Rappisi, Marco Firetto Carlino, Elisabetta Giampiccolo, Ornella Cocina, Brandon Vanderbeek, and Manuele Faccenda

Mount Etna (Italy), renowned for its persistent eruptive activity, is a hazardous volcano shaped by the intricate interplay between magma uprising and a complex tectonic and geodynamic context. Despite extensive monitoring, seismic tomography encounters challenges in accurately depicting the shallow-intermediate P-wave velocity structures, primarily due to the common assumption of isotropy. This study discards such simplification, employing a novel methodology (Vanderbeek and Faccenda, 2021) to simultaneously invert for perturbations to P-wave isotropic velocity and three additional anisotropic parameters (i.e., magnitude of hexagonal anisotropy, azimuth, and dip of the symmetry axis).

By analysing the seismicity recorded in the Mt. Etna area from 2006 to 2016, we constructed 3D anisotropic P-wave tomography models to better constrain the crustal structure of Etna volcano within the framework of its local tectonic setting. The revealed anisotropy patterns are consistent with the structural trends of Etna, unveiling the depth extent of fault segments. We identify a high-velocity volume, deepening towards northwest, recognized as the collision-related subducting foreland units (i.e. Hyblean foreland carbonate slab; Firetto Carlino et al., 2022) that appear to confine a low velocity anomaly, hypothesized to be the expression of a deep magmatic reservoir. A likely tectonic-origin discontinuity affects the subducting units, facilitating the transfer of magma from depth to the surface. This geological setting may explain the presence of such a very active basaltic strato-volcano within an atypical collisional geodynamic context. 

This research improves our understanding of the dynamics governing magma and fluid ascent beneath the volcanic edifice and emphasises the importance of considering anisotropy in seismic investigations. It contributes to our framework for understanding volcanic processes and mitigating associated risks.

 

VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.

Firetto Carlino, M., Scarfì, L., Cannavò, F., Barberi, G., Patanè, D., & Coltelli, M. (2022). Frequency-magnitude distribution of earthquakes at Etna volcano unravels critical stress changes along magma pathways. Communications Earth & Environment, 3(1), 68.

How to cite: Lo Bue, R., Rappisi, F., Firetto Carlino, M., Giampiccolo, E., Cocina, O., Vanderbeek, B., and Faccenda, M.: P-wave anisotropic tomography unveils the crustal structure of Etna volcano (Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12788, https://doi.org/10.5194/egusphere-egu24-12788, 2024.

X1.165
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EGU24-2468
Alexey Stovas

A connected set of singularity points is called the singularity line. Along this line, the slowness surfaces (or phase velocity surfaces) of different wave modes coincide. For anisotropic models, the singularity lines are mostly known in transversely isotropic media (Crampin and Yedlin, 1981). Recently, it was shown that they also can be defined in the special types of orthorhombic media: degenerate (Stovas et al, 2023b) and pathological (Stovas et al, 2023a). Singularity line can also exist in the low symmetry anisotropic models, monoclinic and triclinic (Khatkevich, 1963; Vavrycuk, 2005; Roganov et al, 2019).

In this paper, we focus on singularity lines in monoclinic media with a horizontal symmetry plane. We define the singularity lines in all coordinate planes, and in vertical planes of arbitrary azimuthal orientation. Since the monoclinic anisotropic model can be considered as the transversely isotropic medium with a vertical symmetry axis being perturbed with the multiple azimuthally non-invariant fracture sets, identification of singularity lines can give additional constraints in inversion of seismic data for fracture prediction. The singularity lines being converted into the group velocity domain results in continuous bands in the group velocity surface (traveltime surface) shaping the lacunas for S1 wave and internal refraction cones for S2 wave associated with strong anomalies in wave amplitudes.

The singularity directions satisfy the following polynomial equations (Alshits, 2004; Roganov et al., 2019), , where  are the third-order polynomials given by the elements of the Christoffel matrix. Resolving this system of equations, we define the conditions (in terms of stiffness coefficients) for existence of singularity lines in vertical planes. The Sylvester criterion is applied to control the physical realizable model. Mostly, the obtained models have singularity lines formed by S1 and S2 waves, while one model has singularity line composed of S1S2 and PS1 legs connected by the triple PS1S2 singularity point.

 

 

References

Alshits, V.I., 2004, On the role of anisotropy in crystalloacoustics, In: Goldstein R.V., Maugin G.A. (eds) Surface Waves in Anisotropic and Laminated Bodies and Defects Detection. NATO Science Series II: Mathematics, Physics and Chemistry, vol 163. Springer, Dordrecht.

Crampin, S., and M. Yedlin, 1981, Shear-wave singularities of wave propagation in anisotropic media, J. Geophys., 49, 43–46.

Khatkevich, A.G., 1963 Acoustic axes in crystals, Sov. Phys. Crystallogr. 7, 601–604.

Stovas, A., Roganov, Yu., and V. Roganov, 2023a, On pathological orthorhombic models, Geophysical Prospecting, 71(8), 1523- 1539.                                                                    

Stovas, A., Roganov, Yu., and V. Roganov, 2023b, Degenerate orthorhombic models, Geophysical Journal International, accepted for publication.                  Roganov, Yu., Stovas, A., and V. Roganov, 2019, Properties of acoustic axes in triclinic media, Geophysical Journal, 41(3), 3-17.                                                    Vavrycuk, V., 2005, Acoustic axes in triclinic anisotropy, The Journal of the Acoustical Society of America 118, 647-653.

How to cite: Stovas, A.: Singularity lines for monoclinic media with a horizontal symmetry plane, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2468, https://doi.org/10.5194/egusphere-egu24-2468, 2024.