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GD6.2

Anisotropy from crust to core: Observations, models and implications

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.

Co-organized by EMRP1/SM5
Convener: Manuele Faccenda | Co-convener: Tuna Eken
Presentations
| Wed, 25 May, 14:05–16:40 (CEST)
 
Room -2.91

Wed, 25 May, 13:20–14:50

Chairperson: Manuele Faccenda

14:05–14:10
Introduction

14:10–14:15
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EGU22-5497
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ECS
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On-site presentation
Brandon VanderBeek

Teleseismic travel-time tomography remains one of the most popular methods for obtaining images of Earth's upper mantle. However, despite extensive evidence for a seismically anisotropic mantle, assuming an isotropic Earth remains commonplace in such imaging studies. This assumption can result in significant imaging artefacts which in turn may yield misguided inferences regarding mantle dynamics. Using realistic synthetic seismic datasets produced from waveform simulations through elastically anisotropic geodynamic models of subduction, I show how such artefacts manifest in teleseismic P- and S-wave tomography. The anisotropy-induced apparent anomalies are equally problematic in both shear and compressional body wave inversions and the nature of the shear velocity artefacts are dependent on the coordinate system in which the delay times are measured. In general, the isotropic assumption produces distortions in slab geometry and the appearance of large sub- and supra-slab low-velocity zones. I summarise new methods for inverting P- and S-delay times for both isotropic and anisotropic heterogeneity through the introduction of three anisotropic parameters that approximate P and S propagation velocities in arbitrarily orientated hexagonally symmetric elastic media. Through a series of synthetic tomographic inversions, I demonstrate that both teleseismic P- and S-wave delay time data can resolve complex anisotropic heterogeneity likely present in subduction environments. Moreover, including anisotropic parameters into the inversions improves the reconstruction of true isotropic anomalies. Particularly important to the removal of erroneous velocity structure is accounting for dipping fabrics as many imaging artefacts remain when simpler azimuthal anisotropy is assumed. I conclude by highlighting results from recent applications of the anisotropic imaging method to P-wave datasets in the Western US and Mediterranean.

How to cite: VanderBeek, B.: New imaging strategies for constraining upper mantle anisotropy with teleseismic P- and S-wave delay times, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5497, https://doi.org/10.5194/egusphere-egu22-5497, 2022.

14:15–14:20
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EGU22-10088
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On-site presentation
Thomas Bodin et al.

Global tomographic models depict long-wavelength azimuthal anisotropy in the oceanic upper mantle, with a fast axis direction orthogonal to divergent plate boundaries. This anisotropy is usually attributed to the Lattice Preferred Orientation (LPO) of olivine due to asthenospheric mantle flow away from the ridge axis. In this work, we want to test an alternative hypothesis, whether this observed anisotropic signal could be partially explained by the presence of transform faults and associated fracture zones in the lithosphere. The transform plate boundaries represent sharp structures perpendicular to the ridge-axis with the wavelength (˜10 km), which is much smaller than the wavelength of seismic surface waves used to image the mantle (˜100 km). Therefore, transform faults could potentially result in an effective anisotropy in tomographic images through their Shape Preferred Orientation (SPO). We base our calculations on several thermo-chemical models that follow the observed ridge-transform geometry at different spreading rates. To produce the effective medium as seen by long-period waves, we use a non-periodic homogenization algorithm. The resulting seismic velocity field can be interpreted as the tomographic image that would be obtained after inverting long-period seismic data; it is smooth, fully anisotropic, and comparable to actual tomographic models.

How to cite: Bodin, T., Janin, A., Marjanovic, M., Prigent, C., Capdeville, Y., Chevrot, S., and Durand, S.: Quantifying the effective seismic anisotropy produced by a ridge-transform model , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10088, https://doi.org/10.5194/egusphere-egu22-10088, 2022.

14:20–14:25
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EGU22-9662
High-resolution imaging of subduction zones through SKS splitting intensity tomography: first results from test on numerically modelled complex settings
(withdrawn)
Paola Baccheschi et al.
14:25–14:30
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EGU22-7807
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ECS
Rosalia Lo Bue et al.

Earth’s crust and upper mantle (above 400 km) exhibit strong anisotropic fabrics which reflect the strain history of the rocks and can provide important constraints on mantle dynamics and tectonics. Although the well-established anisotropic structure of Earth’s upper mantle, the influence of elastic anisotropy on the seismic tomography remains largely ignored. It is in fact commonplace to neglect the effects of seismic anisotropy in the construction of tomographic models assuming an isotropic Earth. This approximation certainly simplifies the computational approach but can introduce notable imaging artefacts hence errors in the interpretation of the tomographic results.

Here, we want to bring new insights into the 3D upper mantle structure and dynamics by combining geodynamic modelling and seismological methods taking into account seismic anisotropy.

An ideal environment for studying seismic anisotropy and related geodynamic processes is the Central-Western Mediterranean, that, in the last 20-30 million years, has experienced a complex tectonic activity characterized by back-arc extension related to slab retreat in the Liguro-Provençal, Alborean, Algerian and Tyrrhenian basins and episodes of slab break-off, lateral tearing and interactions between slabs.

Firstly, we apply the modelling methodology of Lo Bue et al., 2021 to reproduce the geodynamic evolution of the study region over the last ∼20-30 Myr. We validate this geodynamic model by comparing seismological synthetics (e.g., SKS splitting) and major tectonic features (i.e., slab and trench geometry) with observations. Next, we use the elastic tensors of the present-day modelled Mediterranean set-up to performed 3D P-wave anisotropic tomography by inverting synthetics delay times as in VanderBeek and Faccenda, 2021 validated through comparison with the geodynamic reference model.

In this work, we attempt to answer some fundamental questions. Compared to Lo Bue et al., 2021 how does using a more complex initial geometry affect the geodynamic modelling result? How well does P-wave anisotropic tomography recover the isotropic and anisotropic features? By performing purely isotropic inversions, which are the main artefacts introduced in the tomographic image by neglecting seismic anisotropy? How much the vertical smearing effect bias P-wave tomographic models?

 

References

Lo Bue, R., Faccenda, M., & Yang, J. (2021). The role of adria plate lithospheric structures on the recent dynamics of the central mediterranean region. Journal of Geophysical Research: Solid Earth, 126(10), e2021JB022377.

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.

How to cite: Lo Bue, R., Rappisi, F., Vanderbeek, B. P., and Faccenda, M.: New insights into tomographic image interpretation and upper mantle dynamics by combining geodynamic modelling and seismological methods, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7807, https://doi.org/10.5194/egusphere-egu22-7807, 2022.

14:30–14:35
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EGU22-12169
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ECS
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On-site presentation
John Keith Magali et al.

Large-scale anisotropy inferred from long-period seismic tomography mainly results from the crystallographic preferred orientation (CPO) of olivine aggregates due to mantle deformation. In the 410-km transition zone, the inclusion of wadsleyite CPO diminishes the overall anisotropy. This may predispose the latter below the seismic detection limit.  In this study, we attempt to assess the detectability of the anisotropy in the 410-km transition zone using surface wave dispersion measurements. Proceeding as a purely-forward approach, we consider non-Newtonian mantle flows reminiscent to the deformation by dislocation creep of olivine. A wadsleyite layer is imposed underneath the discontinuity down to a depth of 520 km. We model the CPO development in olivine and in wadsleyite using a visco-plastic self-consistent (VPSC) approach. Finally, we compute local surface wave dispersion curves and its azimuthal variations to study the surface imprint of transition zone anisotropy.  We anticipate the sensitivity kernels to as well provide key insights in evaluating its detectability.

How to cite: Magali, J. K., Merkel, S., and Ledoux, E.: Surface wave detectability of transition zone anisotropy induced by non-Newtonian mantle flow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12169, https://doi.org/10.5194/egusphere-egu22-12169, 2022.

14:35–14:40
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EGU22-7201
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ECS
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Virtual presentation
Yijun Wang et al.

The development of olivine texture, or lattice preferred orientation (LPO), has been implemented in many numerical modelling tools to predict seismic anisotropy, which places constraints on mantle dynamics. However, a few recent studies have linked olivine texture development to its mechanical anisotropy, which in turn can affect deformation rates and also the resulting texture. To study the effect of anisotropic viscosity (AV) and LPO evolution in geodynamics processes, it is important to know the role of AV and LPO and the differences between the numerical methods that calculate them.

The modified director method parameterizes the olivine LPO formation as relative rotation rates along the slip systems that participate in the rotation of olivine grains due to finite deformation. When it is coupled with a micromechanical model for olivine AV, it allows the anisotropic texture to modify the viscosity. We compare the olivine textures predicted by the modified director method both with and without a coupled micromechanical model and textures predicted by the D-Rex LPO evolution model. To do this, we recalculate the texture observed in simple 3D models such as a shear box model and two other well-understood models: a corner flow model and a subduction model. 

In general, we observed that the D-Rex models predict a stronger anisotropic texture compared to the texture predicted by the modified director method both with and without the micromechanical model, in agreement with previous studies. When including the micromechanical model, the anisotropic texture changes the observed strain rates, which allows for a slightly faster texture evolution that is more similar to the D-Rex predictions than it is to those produced by the modified director method alone. We found that even for the simplest settings there is an increase of 10~15% in strain rate during deformation until a strain of 2.5. When shearing the asthenosphere over ~10 Myr, such anisotropy could modify the effective viscosity of the mantle,causing an up to 40% increase in plate velocity for the same applied stress. The anisotropy can also induce deformation in planes other than the initial shear plane, which can change the direction of the primary deformation.

Our ultimate goal is to understand the role of AV and LPO evolution in geodynamic processes by looking at deformation paths predicted by geodynamic models in ASPECTWith this initial test, we will gain a basic understanding of olivine AV behavior and LPO evolution under different deformation settings calculated with different numerical methods, which we will carry onto our next step of implementing anisotropic viscosity of olivine in 3D into ASPECT.

How to cite: Wang, Y., Király, Á., Conrad, C. P., Hansen, L., and Fraters, M.: Olivine texture evolution under simple deformation: Comparing different numerical methods for calculating LPO and anisotropic viscosity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7201, https://doi.org/10.5194/egusphere-egu22-7201, 2022.

14:40–14:45
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EGU22-11438
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ECS
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On-site presentation
William Halter et al.

Strain localization and associated softening mechanisms in a deforming lithosphere are important for subduction initiation or the generation of tectonic nappes during orogeny. Many strain localization and softening mechanisms have been proposed as being important during the viscous, creeping, deformation of the lithosphere, such as thermal softening, grain size reduction, reaction-induced softening or anisotropy development. However, which localization mechanism is the controlling one and under which deformation conditions is still contentious. In this contribution, we focus on strain localization in viscous material due to the generation of anisotropy, for example due to the development of a foliation. We numerically model the generation and evolution of anisotropy during two-dimensional viscous simple shear in order to quantify the impact of anisotropy development on strain localization and on the effective softening. We calculate the finite strain ellipse during viscous deformation. The aspect ratio of the finite strain ellipse serves as proxy for the magnitude and evolution of anisotropy, which determines the ratio of normal to tangential viscosity. To track the orientation of the anisotropy during deformation we apply a director method. We benchmark our implementation of anisotropy by comparing results of two independently developed numerical algorithms based on the finite difference method: one algorithm employs a direct solver and the other a pseudo-transient iterative solver. We will present results of our numerical simulations and discuss their application to natural shear zones.

How to cite: Halter, W., Macherel, E., Duretz, T., and Schmalholz, S. M.: Numerical modelling of strain localization by anisotropy evolution during 2D viscous simple shearing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11438, https://doi.org/10.5194/egusphere-egu22-11438, 2022.

14:45–14:50
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EGU22-1166
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On-site presentation
Alexey Stovas

The singularity points are very important for elastic waves propagation in low-symmetry anisotropic media (Stovas et al., 2021a). Being converted into the group velocity domain, they result in internal refraction cone with anomalous amplitudes and very complicated polarization fields. I analyze the conditional singularity point in acoustic orthorhombic (ORT) model which is very popular in processing and analysis of 3D seismic data. The elliptic ORT model has one singularity point in one of the symmetry planes (Stovas et al., 2021b). The elastic ORT model has 1 to 6 singularity points. It is shown that for acoustic ORT model the only one S1-S2 wave singularity point (per quadrant) can conditionally be defined in-between the symmetry planes. The required conditions and position of singularity point are computed. The projection of the slowness vector    for singularity point are given by

where are the elements of the stiffness coefficients matrix. I show that the singularity point for this model has the stable conical type of degeneracy (Shuvalov, 1998), which means that the internal refraction cone is always represented by ellipse in 3D space. The slowness surface for acoustic orthorhombic model that consists of three sheets corresponding to P (the inner one) and S1-S2 waves. The image of singularity point in the group domain and its three projections on the symmetry planes can be computed analytically.

 

References

Shuvalov, A.L., 1998, Topological features of the polarization fields of plane acoustic waves in anisotropic media, Proc. R. Soc. Lond., A., 454, 2911–2947.

Stovas, A., Roganov, Yu., and V. Roganov, 2021a, Geometrical characteristics of P and S wave phase and group velocity surfaces in anisotropic media, Geophysical Prospecting, 68(1), 53-69.

Stovas, A., Roganov, Yu., and V. Roganov, 2021b, Wave characteristics in elliptical orthorhombic medium, Geophysics, 86(3), C89-C99.

How to cite: Stovas, A.: On singularity point for acoustic orthorhombic model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1166, https://doi.org/10.5194/egusphere-egu22-1166, 2022.

Wed, 25 May, 15:10–16:40

Chairperson: Tuna Eken

15:10–15:11
Introduction

15:11–15:16
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EGU22-5322
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Highlight
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Virtual presentation
Caroline Beghein et al.

Seismic anisotropy is now commonly studied on Earth and has been detected at various depths, from the crust to the top of the lower mantle, in the lowermost mantle, and in the inner core. In the mantle, observations of seismic anisotropy are often taken as an indication of past or present deformation resulting in the preferential orientation of anisotropic minerals. In the crust, it can come from stress-induced oriented cracks, compositional layering, or crystallographic preferred orientation of minerals. 

While many questions remain regarding the presence and interpretation of seismic anisotropy on Earth, scientists are now faced with new, exciting challenges in trying to constrain the structure of other planetary bodies. One of the goals of NASA’s InSight mission, which landed on Mars in November 2018 and includes a very broadband seismometer, is to constrain Mars interior structure. Compared to seismic studies of Earth, which benefit from the availability of a wealth of high quality data recorded on many seismic stations, difficulties with InSight stem from having only one seismic instrument and only a few high quality events. 

In this study, we analyzed the horizontally polarized (SH)-wave reflections generated from the shallowest crustal layer (layer 1) detected at 8 ± 2 km beneath the InSight lander site by a previous receiver function (RF) study. From Sol 105, when the first low-frequency marsquake was recorded, to Sol 1094, a total of 83 broadband and low-frequency events were detected, but only nine are rated as quality-A with constraints on both their epicentral distance and back azimuth. Of those nine events, we selected four that did not show any interference with mantle triplications generated by the olivine to the wadsleyite phase transition and that had a clear signal after the direct SH phase. A model space search approach enabled us to obtain a range of acceptable SH-wave velocities and layer thicknesses, which we then compared with the RF models of Knapmeyer-Endrun et al. (2021). We found that the acceptable SH-wave speeds are systematically lower than those from the RF study. Since this RF analysis is sensitive to vertically polarized (SV)-waves, we interpret this difference as the signature of radial anisotropy with an anisotropy coefficient 𝜉=(𝑉𝑆𝐻/𝑉𝑆𝑉)2 between 0.7 and 0.9. Modeling of preferred alignment of inclusions shows that dry or fluid-filled cracks/fractures, and igneous inclusions can reproduce the observed radial anisotropy amplitude with VSV>VSH. 

How to cite: Beghein, C., Li, J., Wookey, J., Davis, P., Lognonné, P., Schimmel, M., Stutzmann, E., Golombek, M., Montagner, J.-P., and Banerdt, W.: Constraining Seismic Anisotropy on Mars: New Challenges and First Detection , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5322, https://doi.org/10.5194/egusphere-egu22-5322, 2022.

15:16–15:21
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EGU22-370
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ECS
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On-site presentation
Conor Bacon et al.

Existing evidence points towards the evolution of magmatic intrusions being a complex function of both existing structures and the stress state within the crust. Consequently, developing means to make in-situ measurements and effective models of these two factors would provide crucial insight into the dynamics of volcanic systems, feeding forward to volcanic monitoring and crisis response agencies. Seismic anisotropy—the directional dependence of seismic wavespeeds—has been shown to be a direct proxy for the in-situ stress state of the crust, as well as the existing fabric, but its potential for further developing our general understanding of magmatic intrusions has yet to be realised. The wealth of geophysical data recorded during eruptions in the last decade presents a unique opportunity to explore these important natural phenomena in exceptional detail.

We first establish a general model for the bulk properties and structure of upper crust in the central highlands of Iceland by analysing shear-wave splitting (SWS), a common and near-unambiguous indicator of seismic anisotropy. Using this model as a starting point, we subsequently explore the evolution of seismic anisotropy before, during, and after the 2014–15 Bárðarbunga-Holuhraun dyke intrusion and eruption. Seismicity associated with this magmatic intrusion was used to capture the spatial evolution through time of this event in unprecedented detail. Persistent seismicity at “knot points” along the path of the dyke intrusion allow us to negate the effect of changes to source-receiver path on the measured variations in seismic anisotropic properties.

Our preliminary work suggests the far-field response of seismic anisotropy to the intrusion can be explained by existing models relating the stress field to the orientation of the fast direction. It is apparent, however, that this simple model fails to explain sufficiently our observations in the near field. Whether this is due to shortfalls in the stress modelling, the influence of the presence of melt along the raypath, or potentially a breakdown in the established relationship between stress and seismic anisotropy remains unclear.

How to cite: Bacon, C., Baltas, E., Johnson, J., White, R., and Rawlinson, N.: Investigating the response of seismic anisotropy in the crust to the 2014–15 Bárðarbunga-Holuhraun dyke intrusion and eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-370, https://doi.org/10.5194/egusphere-egu22-370, 2022.

15:21–15:26
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EGU22-284
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ECS
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Virtual presentation
Derya Keleş et al.

Central Anatolia is a seismically active region with complex tectonic provinces and represents one of the significant regions experiencing active deformation in Turkey. It involves the Anatolide-Tauride Block settled in southern Anatolia that is separated from the Pontides by the İzmir-Ankara-Erzincan Suture Zone (IAESZ). In central Anatolia, the Kırşehir Massif mainly comprises complex crystalline metamorphic and plutonic rocks with obducted ophiolitic fragments. It is detached from the Anatolide-Tauride Block by the Intra-Tauride Suture (ITS). The ITS is thought to represent the footprint of subducted Neo-Tethyan ocean. This region further includes a number of active tectonic features, i.e., the Central Anatolian Fault Zone (CAFZ), the Tuz Gölü Fault (TGZ), the East Anatolian Fault zone (EAFZ), the Dead Sea Fault (DSF), and the Bitlis-Zagros Suture. In order to investigate the style of deformation of the region and its influence on the crustal and lithospheric structure and to better understand the relationship between tectonic features and regional deformation at different depth and tectonic features, we quantify the strength and orientation of seismic anisotropy. To achieve this, we focus on the directional dependence of P-to-S converted teleseismic waves (i.e., receiver functions) through the harmonic decomposition analysis. Our findings indicate that seismic anisotropy is mostly localized in the mid-crust (15-25 km) with an overall NE-SW and NNW-SSE orientation in the west and east portions of the study area which is present in the mid-crust (15-25 km). In the uppermost mantle, we observed NE-SW oriented and relatively strong anisotropy. This is compatible with fast shear wave azimuths inferred from SKS splitting measurements reported in previous studies and likely be associated with a sub-lithospheric origin. Anisotropic orientations found at crustal and upper mantle depths are consistent with a model of the ITS reaching to great depths suggest anisotropic fabrics in frozen related to past deformation events. We further perform a joint inversion of receiver functions with apparent S wave velocities to better constrain crustal thickness estimates derived from the harmonic decomposition analysis. The resulting crustal thicknesses vary from about 25-28 km nearby the EAFZ and DSF, and to ~35 and 40 km beneath the Kırşehir block and the Eastern Tauride Mountains.

How to cite: Keleş, D., Eken, T., Licciardi, A., Schiffer, C., and Taymaz, T.: Imprints of Crust- and Mantle-Scale Deformation in Central Anatolide-Tauride Region: Exploiting Receiver Functions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-284, https://doi.org/10.5194/egusphere-egu22-284, 2022.

15:26–15:36
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EGU22-7184
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solicited
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Virtual presentation
Jaroslava Plomerová et al.

Seismic anisotropy, modelled from propagation of teleseismic longitudinal (P) and shear (S/SKS) waves, provides unique constraints on tectonic fabrics and character of past and present-day deformations of the continental lithosphere in different tectonic environments (e.g., Babuška and Plomerová, Solid Earth Sci. 2020). We evaluate body-wave anisotropic parameters (directional variations of velocities or shear-wave splitting) in 3D and invert for three-dimensional structure of the upper mantle (Munzarová et al., GJI 2018) with no limitation imposed on the symmetry axis orientation into the horizontal or vertical directions. Resulting models of the continental lithosphere are based on data from several passive seismic experiments in Archean, Proterozoic and a variety of Phanerozoic provinces of Europe. We emphasize the importance of the three-dimensional approach of modelling anisotropy to be able to detect tilts of symmetry axes in individual domains of the mantle lithosphere. The extent of the domains is delimited by changes in orientation and strength of anisotropy. Assuming only azimuthal anisotropy, similarly to only isotropy, may create artefacts and lead to spurious interpretations (e.g., VanderBeek and Faccenda, GJI 2021). Prevailingly sub-horizontal preferred orientation of olivine, the most abundant mantle mineral, arises from mantle convection in newly formed oceanic lithosphere on both sides of the mid-oceanic ridges. Systematically oriented dipping fabrics in domains of the continental mantle lithosphere reflect series of successive subductions of ancient oceanic plates and their accretions enlarging primordial continent cores. Consequent continental break-ups and assemblages of wandering micro-plates preserve “frozen” anisotropic fabrics and create patchwork structures of the present-day continents.

How to cite: Plomerová, J., Žlebčíková, H., and Vecsey, L.: Patchwork structure of continental lithosphere captured in 3D body-wave images of its anisotropic fabrics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7184, https://doi.org/10.5194/egusphere-egu22-7184, 2022.

15:36–15:41
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EGU22-13364
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Virtual presentation
Nóra Liptai et al.

Information on mantle anisotropy can be obtained from methods such as
studying the lattice-preferred orientation (LPO) in mantle peridotites,
or conducting shear-wave splitting (SKS) analyses which allow to
determine whether it is a single or multi-layered anisotropy and the
delay time of the fast and slow polarized wave can indicate the
thickness. In this study we provide a detailed SKS mapping on the
western part of the Carpathian-Pannonian region (CPR) using an increased
amount of splitting data, and compare the results with seismic
properties reported from mantle xenoliths to characterize the depth,
thickness, and regional differences of the anisotropic layer in the
mantle.
According to the combined SKS and xenolith data, mantle anisotropy is
different in the northern and the central/southern part of the western
CPR. In the northern part, the lack of azimuthal dependence of the fast
split S-wave indicates a single anisotropic layer, which agrees with
xenolith data from the Nógrád-Gömör volcanic field. In the central
areas, multiple anisotropic layers are suggested by systematic azimuthal
variations in several stations, which may be explained by two,
petrographically and LPO-wise different xenolith subgroups described in
the Bakony-Balaton Highland. The shallower layer is suggested to have a
‘fossilized’ lithospheric structure, which could account for the
occasionally detected E-W fast S-orientations, whereas the deeper one
reflects structures responsible for the regional NW-SE orientations
attributed to the present-day convergent tectonics. In the Styrian
Basin, results are ambiguous as SKS splitting data hints at the presence
of multiple anisotropic layers, however, it is not supported clearly by
xenolith data.
Spatial coherency analysis of the splitting parameters put the center of
the anisotropic layer at ~140-150 km depth under the Western
Carpathians, which implies a total thickness of ~220-240 km. Thickness
calculated from seismic properties of the xenoliths resulted in lower
values on average, which may be explained by heterogeneous sampling by
xenoliths, or the different orientation of the mineral deformation
structures (foliation and lineation).

How to cite: Liptai, N., Gráczer, Z., Szanyi, G., Süle, B., Aradi, L., Falus, G., Bokelmann, G., Timkó, M., Timár, G., Cloetingh, S., Szabó, C., and Kovács, I. and the AlpArray Working Group: Seismic anisotropy beneath the western part of the Carpathian-Pannonianregion inferred from combined SKS splitting and mantle xenolith studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13364, https://doi.org/10.5194/egusphere-egu22-13364, 2022.

15:41–15:46
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EGU22-2576
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ECS
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On-site presentation
Abolfazl komeazi et al.

Presence of the Etendeka continental flood basalts in northwestern Namibia, at the eastern extension of Walvis Ridge toward the African coast, is taken as evidence for the assumption that this region was affected by the Tristan da Cunha mantle plume during the rifting/break-up process between Africa and South America. Investigation of seismic anisotropy can provide further evidence for the cause-and-effect relationship between mantle flow, lithospheric deformation and surface structures. We investigate seismic anisotropy beneath NW Namibia by splitting analysis of core-refracted teleseismic shear waves (SKS family). The waveform data was obtained from two different GEOFON seismic networks in the region. The XC network with 5 stations, which has been operating for two years since 1998 and 6A network with 40 stations including both land and off-shore (OBS) stations, operated for longer than two years in 2010-2012.

The data was analyzed using the SplitRacer software and the results of joint splitting analysis assuming a one-layer of anisotropy are presented here. The less-noisy waveform data from the land stations provide reliable and consistent measurements. We obtained few reliable measurements from the OBS stations due to higher noise level and ambiguity about the sensor orientation. The majority of our fast directions exhibit an NE-SW direction consistent with the regional trend of seismic anisotropy in western Africa compatible with a model of large-scale mantle flow due to the NE-ward motion of the African plate. In the northern part of the study area, we observe an anti-clockwise rotation of the splitting polarization directions that seems to be caused by the Kaoko belt and the Puros shear zone. Based on the short-scale variation of the splitting parameters in this region, we believe that the cause of the lateral variation in SKS-splitting observation is the shallow lithospheric structure rather than a variation of deep mantle flow. Our results does not show any direct plume related observations in the study region.

How to cite: komeazi, A., Rümpker, G., and Kaviani, A.: Investigation of mantle anisotropy in NW Namibia by shear-wave splitting analysis: evidence for large-scale mantle flow and fossil-anisotropy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2576, https://doi.org/10.5194/egusphere-egu22-2576, 2022.

15:46–15:51
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EGU22-3042
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ECS
Siddharth Dey et al.

We use regional Rayleigh and Love wave data, from 4750 earthquakes (M >= 4.0) recorded at 726 stations across India and Tibet, to compute fundamental mode group velocity dispersion between 10 s and 120 s, using the Multiple Filter Technique (MFA). These result in the dense coverage of 14,706 and 14,898 ray-paths for Rayleigh and Love waves, respectively. The dispersion data at discrete periods have been combined through a ray-theory based tomographic formulation to obtain 2D maps of lateral variation in group velocities, where the best resolution is upto 2.5° and 4° for Rayleigh and Love waves tomographic maps, respectively. The Peninsular Shield, the Himalayan foreland basin, the Himalayan collision-zone and the Tibetan Plateau, have been sampled at unprecedented detail. Rayleigh and Love wave dispersion curves, at each node point of the tomography, have been inverted for 1D isotropic shear-wave velocity structure of Vsv and Vsh, respectively, which are combined to obtain 3D Vsv and Vsh structures across India and Tibet. We jointly invert the two datasets at each node to obtain an isotropic 1D velocity structure. The isotropic inversion fits the two datasets reasonably well, however, the misfit in the dispersion dataset both at high and low periods is high. For this, we incorporate radial anisotropy in the velocity structure and parameterize the crust with three layers and upper mantle with two layers. Assuming this radially anisotropic earth structure, we use Genetic Algorithms (GA) to explore the model space extensively. The synthetic dispersion curves are computed using Thomson-Haskell method with reduced delta matrix. The free parameters used in the inversion are VPH and VSH, layer thickness (h) and Vs anisotropy represented by Xi (ξ=VSH/VSV)2. The non-linear inversion technique converges to a best-fitting model by iteratively minimising the misfit between the observed and the data. The 2D group velocity dispersion heterogeneities, the 3D structures of Vsv and Vsh (both isotropic and transversely isotropic) will be presented with a focus to characterize a) the structure of the Indian plate and it’s extent of underthrusting beneath Tibet, and b) to quantify the low-velocity zone at the base of the Himalayan wedge, across the basal decollement, which ruptures in megathrust earthquakes.

How to cite: Dey, S., Ghosh, M., Banerjee, R., Sharma, S., Mitra, S., and Bhattacharya, S.: 3D transversely isotropic shear-wave velocity structure of India and Tibet from joint modeling of Rayleigh and Love waves group velocity dispersion., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3042, https://doi.org/10.5194/egusphere-egu22-3042, 2022.

15:51–15:56
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EGU22-9402
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ECS
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Virtual presentation
Satyapriya Biswal et al.

For the understanding of deformational mechanism and geodynamics of a tectonic set up, the source localization and central depth of anisotropy plays a vital role. Though mantle dynamics and deformation patterns can be understood from studying the shear wave splitting mechanism, the true interpretation of under earth mechanism governing the geodynamics remains little biased and unrealistic without the  proper justification and identification of the source localization and depth of anisotropy. Our present study is focused on the possible central depth determination and source localization of anisotropy beneath the Sikkim Himalayan region based upon the well-established spatial coherency method of Splitting parameters, an improved and dynamic principle of grid search analysis based on the Fresnel zone concept. The principle is based upon the maximum coherency relation between the splitting parameters suggested by a minimization in the variation factor as a function of true depth of the anisotropy. Sikkim Himalaya, sandwiched between the central Nepal Himalaya and the eastern Bhutan Himalaya, demarcates the distinct change in the width of the Himalayan foreland basin and the Main Himalayan Thrust (MHT), which is a part of the active deforming eastern Himalayan fold axis and thrust belt. The Spatial coherency analysis of splitting parameters suggests the central depth of heterogeneity at around 130 km beneath this Sikkim Himalayan region as a consequence of the deformation patterns governed by the complex lithospheric mass at this particular depth.

 

KEYWORDS

Spatial coherency, Shear wave splitting, Sikkim Himalaya, lithosphere.

How to cite: Biswal, S., Dey, G., and Mohanty, D. D.: Implications on source localization and central depth of anisotropy beneath the Sikkim Himalaya: an appraisal on lithospheric deformation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9402, https://doi.org/10.5194/egusphere-egu22-9402, 2022.

15:56–16:01
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EGU22-5526
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ECS
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On-site presentation
Kuan-Yu Ke et al.

Radial seismic anisotropy (RA) designates the difference between the speeds of vertically and horizontally polarized shear waves. RA in the crust can provide information on past tectonic events. Since the amplitude and impact of anisotropic are smaller than the variation of velocity, it is more difficult to distinguish whether radially anisotropic anomalies are driven by the structure or uncertainty. Hence, a lack of considering uncertainty and trade-off here may underestimate radial anisotropy and lead to divergent geodynamical interpretations. The hierarchical transdimensional Bayesian approach is able to provide uncertainty estimates taking fully into account the nonlinearity of the forward problem. Under the Bayesian framework, the mean and the variance of the ensemble containing a large set of models are interpreted as the reference solution and a measure of the model error respectively. 

In our study, we applied a two-step RA inversion of surface wave dispersion and receiver function based on a hierarchical transdimensional Bayesian Monte Carlo search with coupled uncertainty propagation to a temporary broadband array covering all of Sri Lanka. First, we constructed Rayleigh and Love wave phase velocity and errors maps at periods ranging from 0s to 20s. To remove outliers, data uncertainty distribution was expressed as a mixture of a Gaussian and uniform distribution. Next, we inverted local dispersion curves and receiver functions jointly to obtain 1D shear velocity and RA models. The method effectively quantifies the uncertainty of the final crustal shear wave velocity and RA model and shows robust results. The negative RA (Vsv > Vsh) anomalous with low uncertainty found in the mid-lower crust of Central Sri Lanka may show evidence that the charnockite inclusion is associated with the shear zones confined to the cores of some doubly-plunging synforms. In the east Highland Complex, the positive radial anisotropy (Vsh > Vsv) anomalous with low uncertainty may reveal the evidence for sub-horizontal shear zones along the thrust boundary.

How to cite: Ke, K.-Y., Tilmann, F., Ryberg, T., and Dreiling, J.: Radial anisotropy models and their uncertainties beneath Sri Lanka derived from joint inversion of surface wave dispersion and receiver functions using a Bayesian approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5526, https://doi.org/10.5194/egusphere-egu22-5526, 2022.

16:01–16:06
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EGU22-6498
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ECS
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On-site presentation
Foivos Karakostas et al.

Seismic anisotropy exists in various depths on Earth. However, computational complexities and limited data coverage often lead many seismic tomographic efforts to neglect it. This isotropic assumption can lead to various misinterpretations, which become more important when the spatial resolution is increased. 

In our project, we aim at constructing, through full-waveform inversion, a 3D seismic model of upper mantle anisotropic structure (approximately 500 km depth) below the Tyrrhenian Sea -- a region of great geodynamic interest mainly because of the Calabro-Ionian subduction zone. 

Here we present the framework and the forward modelling, based on the joint use of SPECFEM3D and AxiSEM software, for the implementation of the so-called "box tomography" [1]. By this, a 3D, anisotropic, model spans the region that we aim to resolve, whereas the rest of the globe is represented by a 1D model with lower resolution. This methodology allows the inclusion of teleseisms -- thus a much larger dataset than allowed by closed-domain modelling, as we can also use numerous seismic events out of the region of interest recorded by the dense network of stations within it. We show that this approach in fact highly improves the coverage of data, that can be used for inversion. 

We use SPECFEM3D for the region of interest and AxiSEM for the global simulation. We process the topography, seismic velocities and anisotropy, in order to construct a realistic 3D input model for the area of interest, that honours the Earth's curvature and transforms the geometry of an a priori model from geographical to Cartesian coordinates, with respect to a point of reference, situated in the middle of the top layer of the constructed mesh. We then process the waveforms, resulting from such forward simulation, with the application of a rotation from the Cartesian coordinates to the geographical ones, in order to perform the inversion with the use of real data of seismic recordings. The forward modelling is then to be used for computation of anisotropic Fréchet kernels and inversion. 

[1] Yder Masson, Barbara Romanowicz, Box tomography: localized imaging of remote targets buried in an unknown medium, a step forward for understanding key structures in the deep Earth, Geophysical Journal International, Volume 211, Issue 1, October 2017, Pages 141–163, https://doi.org/10.1093/gji/ggx141

How to cite: Karakostas, F., Morelli, A., Molinari, I., VanderBeek, B., and Faccenda, M.: A hybrid computational Framework for 3D anisotropic full-Waveform inversion at a regional scale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6498, https://doi.org/10.5194/egusphere-egu22-6498, 2022.

16:06–16:11
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EGU22-11315
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Virtual presentation
Jonathan Wolf et al.

Differential SKS-SKKS splitting is often interpreted as evidence for lowermost mantle anisotropy, because while SKS and SKKS raypaths are very similar in the upper mantle, they diverge substantially in the lowermost mantle. While discrepant SKS-SKKS splitting is a valuable tool to probe D'' anisotropy, individual measurements are typically noisy and have large scatter, making interpretation challenging. Array techniques are commonly used in observational seismology to enhance signal-to-noise ratios and extract seismic phases that would not be reliably detectable in single seismograms. Such techniques, however, have rarely been applied to resolve seismic anisotropy via shear wave splitting. In this study, we apply stacking and beamforming for different subarrays across the USArray to analyze SKS-SKKS splitting discrepancies measured across the North American continent. A benchmarking exercise demonstrates that the effect of upper mantle anisotropy on the beamformed phases can be understood as a relatively simple average of splitting over different upper mantle volumes, and that discrepant measurements reflect a contribution from the lowermost mantle. We obtain robust differential splitting intensity measurements for beamformed data from a selection of events that occurred in the western Pacific and Scotia subduction zones. This approach yields a robust set of splitting intensity discrepancy values for phases that sample the lowermost mantle beneath North America and the surrounding region, with much less scatter than comparable datasets based on individual seismograms. We find evidence for several distinct regions with strong anisotropy at the base of the mantle beneath our study region, plausibly due to subduction-related lowermost mantle flow and deformation. 

How to cite: Wolf, J., Long, M. D., Frost, D. A., Aderoju, A. O., Creasy, N., Garnero, E., and Bozdag, E.: Differential SKS-SKKS splitting due to lowermost mantle anisotropy beneath North America measured from beamformed SmKS phases, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11315, https://doi.org/10.5194/egusphere-egu22-11315, 2022.

16:11–16:16
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EGU22-2366
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ECS
Imaging inner core anisotropy using normal mode and body wave data
(withdrawn)
Henry Brett et al.
16:16–16:21
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EGU22-3339
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ECS
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Virtual presentation
Thuany Costa de Lima et al.

Progress on seismic imaging of the Earth’s inner core (IC) is fairly limited by the uneven distribution of sources and receivers; large earthquakes are primarily confined to plate margins, and seismic stations are unevenly deployed on the Earth’s surface. Advances in data processing techniques and new methods are required to bridge new opportunities to probe the centre of our planet and provide us with valuable information on the IC seismic structure and its surrounding dynamics. In this study, we present a newly-developed method based on the global earthquake coda-correlation wavefield to investigate the anisotropic structure of the IC. Anisotropy in seismic velocity is the directional dependence of seismic waves. Under IC pressure and temperature conditions, different phases of iron – the core’s main mineral constituent can stabilize and form elastic anisotropy. Thus, improved constraints on its strength and distribution are required to understand the crystallographic structure of iron in the IC, which is linked to the evolution of its solidification and deformation processes. Here, we stack the cross-correlation functions of the late-coda seismic wavefield (the correlation wavefield) that reverberates within the Earth up to 10 hours after large earthquakes. We analyse the travel times of the I2* correlation feature, a mathematical manifestation of similarity among IC seismic phases with the same slowness detected in global correlograms at small interstation distances (<10°). The I2* spatial sampling offers an unprecedented data coverage of the IC’s central portion, also known as the innermost IC (IMIC), which overcomes the shortage of the traditional approach using PKIKP ray paths sampling. By comparing the time residuals of different paths of I2* propagating through the IC, we confirm the presence of a deep IC structure with anisotropy fundamentally different from the IC’s outer layers. Our observations support an IMIC cylindrical anisotropy model with a slow direction oriented 55° from the Earth’s spin axis. This new evidence reinforces previous inferences on the existence of the IMIC, with implications for our understanding of the core’s geodynamical evolution. In the future, a similar approach could be applied to advance our understanding of anisotropy in the Earth’s mantle.

How to cite: Costa de Lima, T., Tkalčić, H., and Waszek, L.: Evidence for Anisotropy in the Innermost Inner Core from the Earthquake Coda-correlation Wavefield, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3339, https://doi.org/10.5194/egusphere-egu22-3339, 2022.

16:21–16:40
Final discussion