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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.

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.

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=(𝑉_𝑆𝐻/𝑉_𝑆𝑉)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 V_SV>V_SH. 

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.

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.

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.

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.

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.

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.

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.

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.