The tectonic history of the Korean Peninsula includes the Permo-Triassic collision between the North and South China blocks and the subsequently opening of the Yellow and East seas during the Late Oligocene and Miocene. Due to the lack of evidence and based on different geological and geophysical data, several models and mechanisms have been proposed to explain how the collision and openings happened. We studied seismic anisotropy from core-refracted shear-wave splitting to place constraints on lithospheric-scale and upper mantle structures and dynamics and provide insight into the tectonic evolution of the Korean Peninsula. We implemented the eigenvalue-based method to measure the splitting parameters and used the transverse energy minimization and cross-correlation techniques to validate our results. We found delay times ~1.4 s which is consistent with anisotropy residing in the asthenospheric and/or lithospheric mantle. Our results strongly suggest that the anisotropy signature of past tectonic events have been preserved and that the upper asthenosphere and lithosphere have undergone coherent deformation. Based on our model, we interpret that the Hongseong-Imjingang belt is part of the collision boundary, since we observed a lateral variation of the splitting parameters coinciding with it. We suggest two possible scenarios for the continuation of this collision suture: (1) one offshore with the boundary coinciding with the West Marginal Fault Zone, and (2) another one onshore along the southern limit of the Gyeonggi massif, going from the Hongseong to the Odesan belt. Our observations along the east and west coasts support a fan-shaped opening mechanism for the East Sea and an eastward post-collisional extension for the Yellow Sea, respectively. The fan-shaped opening mechanism, which implies a clockwise rotation of the Japanese Islands away from the Korean Peninsula, appears to have occurred in two stages: an approximately E-W rifting followed by a N-S spreading. Lastly, our splitting observations beneath the western Gyeonggi and Yeongnam Precambrian massifs appear to be in good agreement with a possible fossil anisotropy. The fast axes observed for the former might reveal the true direction of motion of the Nort China Block, while the ones observed for the latter appear to have been affected by post-collisional tectonic episodes since they are not parallel to the infer direction of motion of either the North or South China blocks.
How to cite: Celis, S., Hong, T.-K., Lee, J., Park, S., Liu, Y., Kim, B., Lee, J., and Kim, D. G.: Core-Refracted Shear-Wave Anisotropy beneath the Korean Peninsula: Insights into its Tectonic Evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5291, https://doi.org/10.5194/egusphere-egu25-5291, 2025.
Located on the western edge of the North American plate, Alaska is formed over time through the accretion of various terranes. The subductions of the Pacific and Yakutat plates have significantly influenced the intense tectonic activity in this region, making Alaska an attractive area for geophysical study.
Seismic anisotropy provides critical insights into the deformation mechanisms beneath this tectonically active region and serves as a key factor for regional seismotectonic analysis. In this study, we invert the SKS wave splitting intensities for the 3D variations of shear-wave anisotropy. Using broadband seismograms from 344 regional seismic stations with unprecedented spatial density, we measure the splitting intensities of SKS waves from teleseismic events with magnitudes greater than 5.5 recorded between 2000 and 2023. A total of 9,604 SKS splitting intensity measurements are obtained and incorporated into a multi-scale inversion framework, utilizing sensitivity kernels calculated by normal-mode summation.
The resulting 3D anisotropy model reveals detailed deformation patterns which is interpreted in the context of Alaska’s complex seismotectonic environment. This work enhances our understanding of mantle flow and tectonic processes in a region of significant geophysical and geological interest.
How to cite: Lin, Y., Faccenda, M., and Zhao, L.: Full-wave anisotropy tomography for the upper mantle of Alaska, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12095, https://doi.org/10.5194/egusphere-egu25-12095, 2025.
Understanding observations of the subsurface and its behavior over time requires quantifying inherited geologic structures, the intrinsic material properties, and the in-situ conditions. In Oklahoma and Kansas, a surge in seismic activity occurred between 2010 and 2019 with the vast majority of hypocenters located in the Precambrian crystalline basement. This surge in seismicity drove significant interest in characterizing the structures, material and state of stress in the region. Velocity anisotropy can be a powerful tool for determining the in-situ stress orientations in the subsurface. Interpretation of apparent anisotropy from regional-scale seismic measurements can be hampered due to assumptions regarding the physical mechanism for the observed velocities. For the crystalline basement, rocks are often assumed as isotropic and thus observed anisotropy is attributed solely to the stress orientations. However, factors other than the stress field are capable of generating velocity anisotropy, including the orientation of structural features, fracture orientations, and mineral alignment. In this work we investigated the intrinsic velocity anisotropy of crystalline basement rocks through a field experiment and a series of direct laboratory velocity measurements. In the field, we measured the variation of P-wave velocity with respect to azimuthal direction in a basement rock outcrop located near Mill Creek, Oklahoma. Observed velocity variations correlated with the local fracture pattern and two locally mapped faults. We then performed experiments on samples, from both Oklahoma and Kansas, taken from both outcrops and recovered core. Two sets of tests were conducted to measure the horizontal and vertical velocities of each rock sample. Stereologic techniques were then used to quantify the microstructural variation and relate it to both the laboratory and field observations. Our experimental results were then compared with well log and seismically measured anisotropy. Overall, our results document velocity anisotropy at a variety of scales in the midcontinent crystalline basement. Observed anisotropy was dependent on local structures, the presence of oriented microfractures, and the scale at which velocity anisotropy was measured. Our analyses indicate a clear intrinsic anisotropy in the crystalline basement rocks of the midcontinent and show that such characterization must be conducted prior to interpreting velocity polarization data at regional scales.
How to cite: Carpenter, B.: Velocity Anisotropy in Crystalline Basement Rocks of the US Midcontinent, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12967, https://doi.org/10.5194/egusphere-egu25-12967, 2025.
Fluids hosted in fractures, or low aspect ratio inclusions, exist in many different settings within the Earth. In the near surface, understanding systems of fluid-filled fractures is important to various industrial applications such as geothermal energy production, monitoring CO2 storage sites and exploring for metalliferous sub-volcanic brines (e.g., Blundy et al., 2021). In the mantle, melting is an important geodynamic process, exerting control over its composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting (e.g., Kendall et al., 2005) and other surface expressions of tectonic processes. In the lowermost mantle, it has been suggested that ultra-low velocity zones could contain partial melt. A challenge, however, in all these settings is finding a geophysical observation which is sensitive to the presence of fluids and the host fracture networks.
The presence of fluids has a significant effect on the overall elasticity of the medium. It is well known that aligned fluid-filled fractures, or inclusions with small aspect ratios, produce seismic velocity anisotropy, even for very low volume fractions (e.g., Hudson, 1982, Chapman 2003). This mechanism is often used by shear-wave splitting studies to interpret the orientation of maximum horizontal stress within the crust (e.g., Crampin 1999, Asplet et al., 2024). The same rock physics models, however, also predict attenuation anisotropy that is frequency-dependent and sensitive to important fracture properties, such as fracture length and orientation. Therefore, if attenuation anisotropy can be measured, it offers an exciting new avenue to seismically detect fluids in the subsurface.
Here we show that attenuation anisotropy can be measured in conjunction with shear-wave splitting analysis. Using an instantaneous frequency matching method (after Mathenay and Nowack, 1995) the differential attenuation between fast and slow shear-waves can be measured. We explore the potential of this technique using synthetic data and SKS data collected at FURI, Ethiopia. We also demonstrate the potential systematic error, in both fast polarisation and delay times, that attenuation anisotropy can have on shear-wave splitting measurements and outline an approach for correcting measurements. For SKS data recorded at FURI shear-wave splitting and attenuation anisotropy is measured that requires poroelastic squirt flow of aligned melt inclusions oriented perpendicular to the Main Ethiopia Rift. This is a result which would not be interpreted by only considering SKS shear-wave splitting. These intriguing results highlight the potential for attenuation anisotropy as a tool to detect and characterise fluids in the subsurface.
How to cite: Asplet, J., Wookey, J., Kendall, M., and Chapman, M.: Shear-wave attenuation anisotropy: a new constraint on mantle melt near the Main Ethiopian Rift, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11699, https://doi.org/10.5194/egusphere-egu25-11699, 2025.
Seismic anisotropy may reveal the state of stress in the crust, and its temporal changes have been attributed to deformation, seismicity, magmatic activity and geothermal extraction. We review crustal anisotropy in volcanic and geothermal regions. We compile the results to test hypotheses about the origin of anisotropy and about its utility for monitoring magmatic unrest or geothermal production. The majority of the articles that were published through 2019 (~100) examined shear-wave splitting.
Of the 88 studies examining the effects of stress vs. structure, the results were about evenly divided between causes related entirely to regional stress (16), local stress (10) or structure (11) alone or combinations of these possibilities. Delay times (a measure of anisotropy strength) increased with period and with depth in the two sets, but with much scatter. Because geothermal areas tended to be studied at shallower depths (median 2.5 km), they yielded lower delay times (0.1 s) at shorter periods (0.1 s) than volcanoes (median 12 km depth, 0.25 s period, 0.19 s time delay and 6% anisotropy).
Surface wave studies of anisotropy have also become more common, and they are often interpreted in terms of radial anisotropy, i.e., the difference between horizontally polarised waves (SH) and vertical polarisations (SV). In volcanic areas, they can distinguish between magmatic storage in dykes, in which SV >SH , or sills, with SH >SV. Because the lower crust in non-volcanic areas often has SH >SV, the presence of low absolute velocity should be used to confirm that magma is involved.
Time variations in shear wave splitting were examined in 29 studies, but few of these presented statistical tests. Studies were divided between those that reported changes in delay times (12) or fast azimuths (8) alone, or both (8). Time variations were mostly reported to vary with the occurrence of eruptions or intrusions (19 volcanoes), seismicity or tremor rate changes (9), or deformation changes such as GNSS, tilt or strain measurements (10). Focal mechanisms, b-value, isotropic velocity, Vp/Vs ratio, gas flux, coda Q, unrest level, geothermal activity, and fluid injection were also correlated with splitting in some studies. There is a clear need for studies that examine statistical relationships between anisotropy and other parameters to test monitoring capabilities.
How to cite: Savage, M., Arnold, R., Aoki, Y., Johnson, J., Illsley-Kemp, F., and Zal, H.: Spatial and Temporal Patterns of Seismic Anisotropy on Volcanoes and Geothermal Areas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7533, https://doi.org/10.5194/egusphere-egu25-7533, 2025.
When an olivine polycrystal (dunite) is deformed in simple shear, the expectation value of the [100] axis orientation follows the long axis of the finite strain ellipsoid (FSE) for small shear strains γ < 50%, but for larger strains rotates toward the shear plane more rapidly than does the FSE (Zhang & Karato, Nature, 1995; ZK95). This observation implies that texture in dunites is not a unique function of the finite strain when dynamic recrystallization (DRX) is active. We propose a simple kinematic model for DRX that explains the experimental observations. We model DRX by adding an inhomogenous term f J (where J has zero mean over all orientations) to the right-hand side of the standard evolution equation for the orientation distribution function (ODF) f. We then posit J = λ F(Δ), where λ is a dimensionless recrystallization rate, Δ = (e - E)2, e is the strain rate tensor within a crystal, and E is the macroscopic strain rate tensor imposed on the polycrystal. We choose the function F(Δ) such that crystals poorly oriented for slip on the dominant slip system (i.e., crystals with larger Δ) gradually disappear by DRX in favor of well-oriented crystals. We solve the resulting ODF evolution equation analytically (for small strains) and numerically (for large strains). We find that for λ = 3 the predictions of our model agree remarkably well with a simple shear texture at γ = 140% obtained by Lee et al. (Tectonophys., 2002). An important advantage of our new model is that it has only a single free parameter, as opposed to e.g. the three-parameter model implemented in D-Rex (Kaminski et al., Geophys. J. Int., 2004).
How to cite: Ribe, N. and Faccenda, M.: Dynamic recrystallization and texture development in deformed olivine polycrystals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1642, https://doi.org/10.5194/egusphere-egu25-1642, 2025.
Understanding the internal dynamics, structure, and composition of our planet is a fundamental goal in Earth science. Geodynamic modelling has played a key role in this task, offering a theoretical window into the Earth’s convective mantle at present-day and in the past. Seismological studies provide robust evidence of mantle structure and dynamics. Furthermore, the detection of anisotropy of mantle minerals, such as olivine, which tend to align with the asthenosphere flow allows to map global anisotropy. This offers a seismic window into convective flow patterns beneath the lithosphere. In this endeavor, the asthenosphere plays a crucial role in connecting mantle dynamics to surface observations. Its channelized nature allows it to be modeled analytically within the framework of Couette and Poiseuille flow regimes. Thus, this methodology enables an efficient and comprehensive evaluation of a range of plausible models by systematically comparing them against global azimuthal anisotropy models.
Here, I will introduce a fundamental analytical flow model designed to identify datasets that are sentive to the mantle flow, such as seismic anisotropy. The model predicts present-day asthenosphere flow and its azimuthal anisotropy, offering a clear expectation to where the model aligns well with seismic observations and where discrepancies occur.
How to cite: Stotz, I. L., Bunge, H.-P., Vílacis, B., and Hayek, J. N.: Tracking mantle flow through seismic anisotropy and its link to geological observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17990, https://doi.org/10.5194/egusphere-egu25-17990, 2025.
The processes of partial melting and subsequent melt transport are fundamental to Earth's differentiation, from the separation of the core and mantle to magma generation, evolution, segregation and ascent within mantle and crustal domains. Active volcanoes have as their source the partially molten areas of the upper mantle and crust. The possibility of a melt to ascend depends on its connectivity, where the melt percentage, dihedral angle between melt and solid, as well as recrystallisation processes, play a fundamental role (Llorens et al., 2016). The deformation of the upper mantle is primarily governed by the mechanical behavior of olivine (Karato et al., 1989). During mantle flow, olivine undergoes crystal-plastic deformation and dynamic recrystallisation, leading to the development of Crystallographic Preferred Orientations (CPOs) and associated mechanical and seismic anisotropy. While the influence of plastic deformation is well understood, the role of the presence of melt in the rheology and anisotropy of mantle rocks during dynamic recrystallization remains unclear.
This contribution presents microdynamic numerical simulations of olivine polycrystalline aggregates during dynamic recrystallisation (Yu et al., 2024), varying the melt content to predict the CPO and associated mechanical and seismic anisotropy. We combine the VPFFT approach (Lebensohn and Rollett, 2020) within the ELLE numerical simulation platform (http://www.elle.ws; Piazolo et al., 2019) to reproduce partially molten olivine under simple shear deformation. The numerical results allow us to understand how the percentage of melt and intensity of recrystallisation affects the connectivity of melt, and how they influence the evolving anisotropy, which have implications for different upper-mantle and crustal basaltic mush zones.
How to cite: Llorens, M.-G., González-Esvertit, E., Griera, A., Qi, C., Prieto-Torrell, C., Gómez-Rivas, E., Yu, Y., and Lebensohn, R. A.: Anisotropy development during dynamic recrystallisation of partially molten olivine, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8414, https://doi.org/10.5194/egusphere-egu25-8414, 2025.
The formation mechanism of curved subduction zones remains poorly understood. To address this issue, we conduct a joint inversion of P-wave travel-time data from local earthquakes and teleseismic events for 3-D isotropic and anisotropic velocity tomography of the curved Calabrian subduction zone. Our results show that in the central and northern Apennines, the Adriatic Sea plate subducts on both the eastern and western sides. The westward-dipping slab retreats eastward, compressing the mantle below the double-side subduction zone. This compression pushes the mantle material to flow through a slab window below Mt. Vesuvius toward the Tyrrhenian Sea, resulting in nearly east-west oriented seismic anisotropy. As the distance between the double-side slabs decreases, the slab retreat slows down, leading to a differential retreat rate along the Apennines-Sicily. This difference, combined with mantle flow around the southwestern edge of the Calabrian slab, contributes to the observed curvature of the Calabrian subduction zone. Our findings provide new insights into dynamics of the curved subduction zone, highlighting the complex interaction between the slab retreat and mantle flow.
How to cite: Hua, Y., Zhao, D., Yu, Y., Xu, Y.-G., and Huang, X.-L.: Mantle flow dynamics and formation of the curved Calabrian subduction zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1963, https://doi.org/10.5194/egusphere-egu25-1963, 2025.
The southwest region of Western Australia comprises the Archean Yilgarn Craton, which is bounded by the Proterozoic Albany-Fraser and Pinjarra orogens. This ancient region has undergone significant deformation and reworking since its formation. We calculate shear wave splitting of the PKS and SKS teleseismic phases to investigate seismic anisotropy across the region. The temporary broadband seismic arrays that we use, including the new WA Array Phase 1 data, provide unprecedented seismic station density within the Western Australian continental interior. We find evidence for significant seismic anisotropy, with the regional average delay time of 1.13 s comparable to the global average of δt = 1 s. Although fast polarisation orientations show variation, they are not aligned with current, sub-lithospheric mantle flow associated with absolute plate motions. Instead, seismic anisotropy parallels dyke orientations across the cratonic interior. Fast polarisation directions in the Youanmi Terrane are oriented approximately parallel to the E–W trending Widgiemooltha dyke suite. This correlation is likely due to pre-existing mantle fabric that both formed a locus for the subsequent emplacement of the dykes during a period of ancient Archean lithospheric extension, as well as controlling the orientation of seismic anisotropy. Further evidence for this fabric comes from new isotope geochemistry analysis of primary ENE-trending architecture within the Yilgarn Craton. In the Southwest Terrane, fast polarisation orientations match both structural faults and dykes, suggesting crust-mantle coupling. The Youanmi Terrane shows less coherence between surface and mantle deformation, with structural faults oriented at an angle compared to the E–W and NE–SW trends in the anisotropy. Our results are evidence that large-scale, fossilised lithospheric fabric within the Yilgarn Craton is the dominant mechanism for seismic anisotropy in the region.
How to cite: Gauntlett, M., Eakin, C., Bishoyi, N., O'Donnell, J. P., Murdie, R., Miller, M., Pickle, R., and Ebrahimi, R.: Significant anisotropic fabric across South Western Australia and the Yilgarn Craton revealed by the new WA Array, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2956, https://doi.org/10.5194/egusphere-egu25-2956, 2025.
The curvatures of the slowness surface for anisotropic media
The Gaussian curvature of the slowness surface plays very important role in wave propagation in anisotropic media. It controls the wave amplitudes via the geometrical spreading factor (Gajewski, 1993; Cerveny, 2001; Stovas, 2018; Stovas et al, 2022).
We define the Gaussian and mean curvatures in vicinity of arbitrary point of the slowness surface are convenient to describe in cylindrical coordinate system. If the point on the slowness surface is regular, there is no azimuthal dependence for series coefficients. In case of non-degenerated singularity point (double or triple), all the coefficients in series are azimuthally dependent, and Gaussian and mean curvatures are not defined. For degenerated singularity points, we have only zero-order term which is azimuthally dependent.
We show that one of the principal curvatures can turn to zero at some azimuth angles. In this case, we have the parabolic line (zero Gaussian curvature) associated with singularity point and resulting the caustic in the group velocity domain.
We show examples of parabolic line computed for S1 and S2 waves in vicinity of double (S1&S2) singularity point on the vertical axis with conical and wedge degeneracies (Stovas et al. 2024).
References
Cerveny, V., 2001, Seismic ray theory, Cambridge Univ. Press.
Gajewski, D., 1993, Radiation from point sources in general anisotropic media, Geophysical Journal International, 113(2), 299-317.
Stovas, A., 2018, Geometrical spreading in orthorhombic media, Geophysics, 83(1), C61-C73.
Stovas, A., Roganov, Yu., & V. Roganov, 2022, The S waves geometrical spreading in elliptic orthorhombic media, Geophysical Prospecting 70(7), 1085-1092.
Stovas, A., Roganov, Yu., & V. Roganov, 2024, Singularity points and their degeneracies in anisotropic media, Geophysical Journal International 238 (2), 881-901.
How to cite: Stovas, A.: The curvatures of the slowness surface for anisotropic media, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3244, https://doi.org/10.5194/egusphere-egu25-3244, 2025.
AdriaArray is a very important opportunity to improve the availability of measurements of seismic anisotropy in the region from the Adriatic Sea toward the east. Shear wave splitting data measured on teleseismic core phases are already available for most of the regions interested in the AdriaArray project. In particular, the Italian peninsula, the entire Alpine region up to the Pannonia basin, and the Carpathian belt and the Vrancea zone, but also toward its southeastern border, including Greece and the Aegean Sea, all these regions have a dense amount of shear wave splitting data already published. A database of this data will be made available and enriched using the results of the studies that are going on within the AdriaArray project. On the other hand, new types of analyses, such as for instance the splitting intensity of the anisotropy, have already been measured for the Alps and for the Italian peninsula as a whole, but are lacking toward the east, in the rest of the AdriaArray study region. In this work, the improvement in the application of splitting intensity measures on the AdriaArray data is described, starting from regions 1) where the possibility to compare the results with already available core phases seismic anisotropy measurements exists, and 2) where stations are enough dense to allow in the future the use of splitting intensity measurements to produce an anisotropy tomography as performed elsewhere (e.g. Italy).
How to cite: Pondrelli, S., Salimbeni, S., and Confal, J. M.: Splitting Intensity Measurements on AdriaArray data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4481, https://doi.org/10.5194/egusphere-egu25-4481, 2025.
We investigated lithospheric deformation beneath the Sikkim Himalaya using data from core-refracted shear wave phases (SKS/SKKS) collected at 27 broadband seismic stations. This study focuses on the Dhubri-Chungthang Fault Zone (DCFZ), a significant mid crustal fault traversing Sikkim, which potentially segments the underthrusting Indian plate. Although seismic activity suggests the presence of the DCFZ, its impact on deep lithospheric structures remains unclear. Through analysis of shear wave splitting (SWS) parameters - specifically the fast polarization direction (Φ) and delay time (δt), we assessed deformation patterns across the region. Our results reveal varied deformation across Sikkim, marked by a pronounced change in δt (0.3-2.5s) and Φ across the DCFZ. In the Himalayan foreland basin, mantle flow related to absolute plate motion (APM) is predominant, with the fast-axis direction closely aligning with the APM of the Indian plate. Notably, a NE orientation of Φ is prevalent, though deviations occur, possibly due to varying driving forces associated with the plate's position. In southern Sikkim, EW pattern in Φ and lower δt values (0.3 s) suggest dominant compressional tectonics and potential multi-layer anisotropy. A sharp transition in deformation patterns across the DCFZ highlights its significant role in segmenting the Indian plate's lithosphere, with a distinct NNW pattern in Φ observed in the Higher Himalayas, indicating lithospheric segmentation facilitated by the DCFZ.
How to cite: Siddiqui, A. S., Singh, A., Singh, C., Mohanty, D. D., Kumar, G., and Jana, N.: Probing lithospheric deformation beneath the Sikkim Himalaya using shear wave splitting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9218, https://doi.org/10.5194/egusphere-egu25-9218, 2025.
Between July 2018 and September 2019, a natural swarm of shallow seismicity, with event depths between 1.5 and 3.6 km, was recorded primarily along the Newdigate Fault in Southeastern England (Hicks et al., 2019). After the first nine events, a monitoring network of five stations was installed. This network recorded approximately 280 earthquakes, with a maximum magnitude of 3.2. This wealth of data, in a seismically quiet region of the UK, gives an opportunity to use shear-wave splitting analysis to improve constraints on the state of stress in the Weald Basin — a region with limited data on the 2022 Stress Map of Great Britain and Ireland — and to study the change in local stress during the sequence. We acquire new stress data from across the Weald Basin using borehole breakout analysis of dual calliper logs for six wells across the basin. This analysis gives a mean regional SHmax orientation of 142° with a circular standard deviation of 15°.
We present shear-wave splitting measurements for 108 earthquakes in the sequence, which produce two intriguing features. Firstly, there is a significant (near 90°) rotation in fast polarisation directions for shear-wave splitting measured at stations north of the Newdigate Fault, which are aligned with the regional SHmax,and measurements made at stations south of the fault. This stark, but consistent change in fast polarisation directions over a 3–4 km region demonstrates the potential of shear-wave splitting to resolve local variations in stress around the Newdigate Fault. Secondly, we observe temporal variations in the measured anisotropy, with percentage anisotropy increasing and then decay after the larger events in the earthquake sequence. Combining these observations, we unravel the evolution of the state of stress during the Newdigate earthquake sequence and highlight the power of shear-wave splitting to constrain crustal stress.
How to cite: Asplet, J., Fellgett, M., Kettlety, T., and Kendall, M.: Spatiotemporal variations in shear-wave splitting during the 2018-9 Surrey, UK earthquake sequence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11908, https://doi.org/10.5194/egusphere-egu25-11908, 2025.
The devastating earthquakes of Mw 7.8 and Mw 7.7 with left-lateral strike-slip mechanisms occurred on the 6th February 2023 along the East Anatolian Fault (EAF) and the Sürgü-Çardak Fault (SÇF) in southeastern Türkiye. Observed intense aftershock activity triggered by the 2023 doublet provides a valuable opportunity to study the upper crustal anisotropy along the EAF, the SÇF, and surrounding rock volumes. In this study, we measured the shear-wave splitting parameters of several local earthquakes that occurred between 1st July 2022 and 31st August 2023 - approximately seven months before and after the 2023 mainshocks. To improve the accuracy of the event locations, we initially relocated 10.823 earthquakes (M > 2) using the HypoDD code, building up a catalogue of high-precision earthquake locations. Subsequently, the splitting parameters, including the fast polarization direction (FPD) and the delay time (DT), were estimated for ~1.615 events recorded at 34 broadband seismic stations operated by AFAD (Turkish National Seismic Network). Only event-station pairs with an incidence angle of less than 45° and an event-station distance of smaller than 1° (~111 km) were considered to be suitable for detailed analysis. The spatial variations in both FPD and DT imply a complex anisotropic structure beneath the study region, likely caused by structure-induced mechanisms. At each station, the fast polarization directions are closely aligned with geometry of mapped faults and active faulting mechanisms which vary along the structurally intricate deforming zones in SE Türkiye. The overall observation suggests that the crustal anisotropy is predominantly controlled by the fault-related structures within the region of study. Besides, the delay times (~0.2s) are significantly larger at stations in close proximity to the active fault-lines.
How to cite: Baccheschi, P., Erman, C., Yolsal-Çevikbilen, S., Eken, T., and Taymaz, T.: Pattern of Crustal Anisotropy Along the East Anatolian Fault, the Sürgü-Çardak Fault and Surroundings Associated with the 2023 Kahramanmaraş Earthquakes, SE Türkiye, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12441, https://doi.org/10.5194/egusphere-egu25-12441, 2025.
On the 6th February 2023 two devastating earthquakes of Mw 7.8 and Mw 7.7 with left-lateral strike-slip mechanisms occurred along the East Anatolian Fault (EAF) and the Sürgü-Çardak Fault (SÇF) in southeastern Türkiye. The doublet nucleated and instantaneously ruptured for ~350 km and ~160 km during the complex network of multi-fault segments reaching a maximum slip of ~8 m and >10 m, respectively. The consecutive large earthquakes are likely to have caused permanent changes in the shallow crustal properties, especially in the vicinity of the fault zone. The variations in crustal velocity and anisotropy during the pre-, co-, and post-seismic periods could be efficiently monitored using the ambient noise data. The primary objective of this work is to monitor isotropic velocity changes for the pre-, co-, and post-seismic periods, as well as rapid changes in seismic anisotropy potentially caused by coseismic stress field rotation beneath the EAFZ. To achieve this, we analyze continuous three-component digital recordings from 52 broadband seismic stations located along the EAF, the SÇF and surroundings that are operated by AFAD (Turkish National Seismic Network) and KOERI (Kandilli Observatory and Earthquake Research Institute). First, we analyze the daily correlation functions of all rotated components (ZZ, TT, RR, ZT, TZ, TR, RT, RZ, and ZR) in order to obtain the isotropic seismic velocity change. Second, we rotate the nine-component cross-correlation tensors (CCTs) to minimize tangential components (TZ, ZT, TR, RT), as expected to be zero for an isotropic medium with randomly distributed noise. This approach enables us to monitor the temporal variations of crustal anisotropy before, after, and during these two devastating earthquakes, effectively. Here we present our preliminary results on the spatiotemporal variations of crustal anisotropy derived from ambient seismic noise cross-correlations between station pairs during the 2023 Kahramanmaraş doublet.
How to cite: Erman, C., Baccheschi, P., Yolsal-Çevikbilen, S., Eken, T., Çubuk-Sabuncu, Y., and Taymaz, T.: Monitoring the Variations in Crustal Seismic Velocity and Anisotropy Associated with the 2023 Kahramanmaraş Earthquakes, Türkiye, using Ambient Noise Cross Correlation , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16174, https://doi.org/10.5194/egusphere-egu25-16174, 2025.
In the Earth's upper-mantle, the isotropic (i.e., directional-invariant) symmetry of elastic wave velocities is typically broken by crystal-scale mechanisms, such as crystallographic-preferred orientation of anisotropic minerals (e.g., olivine) in regions subject to significant strain (e.g., subduction zones, mantle plumes, ridges...). The resulting anisotropic (i.e., directional-dependent) elastic properties are manifested in the seismic observations at the surface (e.g., travel-times), making them primary carriers of information related to the geodynamic processes occurring in the Earth’s mantle. However, the seismic tomography problem is notoriously under-determined (i.e., infinite solutions), due to limitations in the distribution of data at the Earth’s surface, and this condition is even exacerbated when simplifying imaging assumptions, such as isotropy, are replaced by more realistic anisotropic approximations that increase the degrees of freedom of the inverse problem.
Reconstructing seismic anisotropy is a challenging inference problem, where uncertainty estimation plays a crucial role in the separation of robustly inferred features and anomalies resulting from misinterpreted trade-offs with isotropic structure. In this context, the high non-linearity of the problem hampers uncertainty assessment when regularized iterative linearized methods (e.g., LSQR) are used.
In this study we show how to setup a joint inversion of multiple observables, such as body-wave delay times and Rayleigh-wave station-station differential phase travel-times, to constrain upper-mantle structure. Rayleigh and body waves illuminate - respectively - the shallower and the deeper sections of the imaging domain, leading to a cross-constrain for mantle anisotropy and isotropic structure. We implement a trans-dimensional probabilistic sampling algorithm to populate an ensemble of likely hexagonal anisotropic mantle models describing the observations within the uncertainties. Probabilistic sampling allows a greater exploration of the model space, with the possibility to evaluate uncertainty and trade-off metrics. To test the inference method, we make use of synthetic seismograms simulated with SPECFEM through geodynamic models of the Earth's mantle.
How to cite: Del Piccolo, G., Byrnes, J. S., Gaherty, J. B., VanderBeek, B. P., Faccenda, M., and Morelli, A.: Imaging upper-mantle anisotropy with joint body-surface wave trans-dimensional inference, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16409, https://doi.org/10.5194/egusphere-egu25-16409, 2025.
The Indo-Burmese Wedge (IBW) is a complex geological environment comprised of active deformation, subduction, and accretion processes. It is situated near the tectonic intersection of the Indian and Burmese plates. We examine the lower mantle anisotropy beneath IBW using the shear wave splitting (SWS) analysis of teleseismic phases to decipher the impact of lower mantle contribution and mantle dynamics beneath the IBW, even though limited research has been carried out to understand upper mantle dynamics. Here, we examine the role of lower mantle anisotropy beneath the IBW using differential phase combinations like SKS-SKKS and PKS-PKKS. Using the robust shear wave splitting approach, we analyzed data from 17 broadband seismic stations spread across the IBW and obtained around 57 pairs of discrepant results. Our findings show significant and comparable anisotropy in the lower mantle beneath the IBW, which may be caused by differential flow alignment consequence and anisotropic intensity in the deep mantle and subducted slab compared to the lithospheric deformation. These results demonstrate the way the lower mantle contributes towards the geodynamic environment and its wider ramifications for mantle dynamics in subduction zones around the IBW region.
Key Words: Seismic anisotropy, Lower mantle, Indo-Burmese Wedge, Subduction Zone, Shear Wave Splitting
How to cite: Biswal, S. and Mohanty, D. D.: Lower mantle anisotropy beneath the Indo-Burmese Wedge through Shear Wave Splitting of Core-Refracted Phases, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19035, https://doi.org/10.5194/egusphere-egu25-19035, 2025.
Understanding the structure and dynamics of oceanic lithosphere is essential for unraveling the processes of plate formation and mantle evolution. Heat-flow and bathymetry observations over aging oceanic lithosphere, suggest that oceanic plates conductively cool and thicken up to a given age (Hasterok, 2013, Lucazeau 2019). More direct observations of the lithosphere-asthenosphere boundary (LAB) come from seismological observations of the LAB across different oceanic basins. Surface wave tomography studies of shear wave velocity and azimuthal anisotropy interpretations reveal that the oceanic lithosphere thickness is strongly age-dependent, primarily controlled by its thermal structure (e.g., Burgos et al., 2014, Beghein et al., 2014). In contrast, radial anisotropy observations representative of the lattice preferred orientation of olivine indicate that, for ages > 50Ma, the interpreted anisotropy gradient is at nearly constant depth of ~70-80 km (Burgos et al., 2014). This apparent age-independence of radial anisotropy diverges from the age-dependent patterns observed in azimuthal anisotropy and isotropic velocities and can be an artifact of tomography inversion techniques (Beghein et al., 2019, Kendall et al., 2022) or representative of distinct processes shaping the oceanic lithosphere during its evolution (Hansen et al., 2016). This discrepancy along with observations of scattered wave imaging of LAB-related discontinuities (e.g., Tharimena et al., 2017) and active source seismic observations of oceanic lithosphere (e.g., Adhukasi and Singh, 2022) raises important questions about the thermo-mechanical definition of the lithosphere, how it differs from the weaker asthenosphere below, and what constitutes the LAB. To address these questions, we use 2D geodynamic models to investigate the thermal and viscosity evolution of the oceanic lithosphere, from continental breakup to oceanic plate formation. Our goal is to reconcile these contrasting seismological observations with geodynamic model results to enhance our understanding of the processes that influence the structure of the oceanic lithosphere.
How to cite: Gudipati, R. R., Pérez-Gussinyé, M., and García-Pintado, J.: Understanding the Structure and Evolution of Oceanic mantle lithosphere using 2D geodynamic models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18162, https://doi.org/10.5194/egusphere-egu25-18162, 2025.
The tectonic evolution of the Tibetan Plateau is still debated. Two predominant hypotheses have been put forth: one posits a northward subduction of the Indian plate, coupled with a concurrent southward subduction of the Eurasian plate; the other suggests a unidirectional northward subduction of the Indian plate alone.
In this study, we introduce new data derived from peridotite mantle xenoliths, which were exhumed by Eocene volcanoes in the Qiangtang terrane. The systematic lateral and radial variations in the petrological, geochemical, and microstructural characteristics of these xenoliths reveal a heterogeneous structure within the lithospheric mantle beneath central Tibet. The uppermost portion of the lithospheric mantle is refractory and displays an AG-type olivine fabric, characterized by a point maximum of the [010] axes perpendicular to the foliation plane, and a girdle distribution of the [100] and [001] axes within the foliation plane. In contrast, the lower segment has been re-fertilized and exhibits a distinct fabric, marked by the dominant activation of the 001 slip system. We infer that the fabric of the lower part of the lithospheric mantle was transformed from an AG-type to a B-type fabric during melt-related deformation associated with re-fertilization triggered by asthenosphere upwelling. The most plausible scenario driving this re-fertilization in the lower sections of the lithospheric mantle is the convective removal of the lowermost lithosphere. Concurrently, the refractory ‘ghost lithosphere’ residing in shallower regions beneath the Qiangtang terrane has preserved the earlier AG-type fabric, potentially representing a residual subcontinental lithospheric mantle that remains within the current lithospheric mantle. This vertical dichotomy of the mantle generates multiple seismic interferences, which align well with deep seismic observations and substantiate the model of a single northward subduction of the Indian plate.
How to cite: Yang, Y.: Dynamic Implication and Constraint of seismic anisotropy in Central Tibetan Lithosphere: insights from the Mantle Xenoliths, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16760, https://doi.org/10.5194/egusphere-egu25-16760, 2025.
The Northwest Pacific subduction zone, characterized by complex plate interactions and active tectonics, is a key area for geodynamics research. These tectonic movements generate seismic anisotropy, causing shear waves to split into orthogonal fast and slow components. Analyzing shear-wave splitting at surface stations allows for inferring fast directions and splitting times, offering insights into slab deformation, mantle flow, and stress field during subduction. The S-net seafloor observation network provides an ideal setup for studying anisotropy within the subducting slab. Using high-SNR S-net stations near trenches, we focus on anisotropy within subducting slabs and sub-slab mantle, excluding influences from overriding slabs and mantle wedges.
This study focuses on the anisotropy of two major subduction zones: the Japan Trench and the Izu-Bonin subduction zone. For the Japan Trench subduction zone, 11 S-net stations located east of the trench and 8 seismic events in the Japan Sea were selected. The events, with magnitudes of 3.6 < MJMA< 4.4, and focal depths of 373.8–444.78 km, had ray path lengths of approximately 853.19–1138.50 km, with only a small portion propagating through the sub-slab mantle. Using the minimum eigenvalue minimization and waveform rotation cross-correlation methods, 20 reliable shear-wave splitting measurements were obtained with a predominant fast direction of NNW-SSE, splitting times ranging from 0.1–0.86 s (average 0.363 s, median 0.32 s), and anisotropy intensities of 0.002%–0.017% (average 0.008%).
For the Izu-Bonin subduction zone, 16 S-net stations at its northern end and 5 seismic events from its central and southern segments were analyzed. The events have magnitudes of 4.1 < MJMA < 5.6 and focal depths of 399–464 km. The ray path lengths are within 712–1101 km. The splitting measurements on different rays are classified into two types based on the length of sub-slab paths: 1) for those smaller than 222 km, 17 reliable measurements are obtained with the predominant fast direction of NNW-SSE, splitting times of 0.08–0.6 s (with an average of 0.226 s and a median of 0.18 s), and anisotropic intensities of 0.001%–0.02% (with an average of 0.006%); 2) for those greater than 222 km, 9 reliable measurements are obtained with the predominant fast direction of NNW-SSE, splitting times of 0.12–0.86 s (with an average of 0.34 s and a median of 0.34 s), and anisotropic intensities of 0.003%–0.02% (average 0.008%).
According to paleomagnetic studies, the paleo-spreading direction of the western Pacific Plate was NNW-SSE, consistent with the fast directions obtained from the three types of results in this study. This alignment suggests that the anisotropy within the subducting slab primarily originates from "fossil" anisotropy retained during the slab's formation and subduction. Since these rays sample more sub-slab mantle paths, they carry more sub-slab mantle anisotropy characteristics, indicating that the anisotropy intensity in the sub-slab mantle is greater than the "fossil" anisotropy preserved within the subducting Pacific Plate. The wide range of splitting times across the three types of results reflects the heterogeneous nature of anisotropy in the region.
How to cite: Li, X. and Xue, M.: Anisotropy in the Northwest Pacific Subduction Zone from Shear-Wave Splitting Analysis Based on S-net Seismic Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9534, https://doi.org/10.5194/egusphere-egu25-9534, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 1
EGU25-964 | ECS | Posters virtual | VPS22
Mantle Deformation Pattern Beneath Central Indian Tectonic Zone: A Seismic Anisotropy Study in Satpura Gondwana Basin and Surrounding AreasTue, 29 Apr, 14:00–15:45 (CEST) | vP1.19
This study analyses shear wave splitting measurements for core-refracted SKS and SKKS phases using data from nine strategically positioned seismic stations operated between 2023 to 2024 in the Central Indian Tectonic Zone (CITZ). The CITZ was formed during the mesoproterozoic orogeny in central India, resulting from the collision of the northern Bundelkhand Craton with a jumble of South Indian cratons (Dharwar, Bastar and Singhbhum Cratons). Understanding seismic anisotropy in this region is essential for elucidating mantle deformation patterns, which provides vital insights into geodynamic processes, lithospheric interactions, and ongoing tectonic activities shaping the CITZ. We employed both rotation-correlation and transverse energy minimisation techniques to determine the shear wave splitting parameters, namely the fast polarization directions (FPDs) and splitting delay times (δt). A total of 104 high-quality splitting measurements and 37 null measurements were obtained from 85 earthquakes (M ≥ 5.5) within epicentral distances of 84°-145° for SKS phases and 84°-180° for SKKS phases. The averaged δts at each seismic station ranges from 0.8 to 1.3 seconds, demonstrating significant anisotropy and heterogeneity in the upper mantle under the studied region. Our observations predominantly reveal NE-SW FPDs throughout the majority of stations, which correlate with the Absolute Plate Motion (APM) of the Indian plate. The discrepancies between FPDs and APM direction at some stations suggest the presence of fossilised anisotropic fabrics resulting from prior subduction events during mesoproterozoic. The smaller δt (0.8 sec) at the seismic station in the Pachmarhi region may be attributed to the significant magmatism during the cretaceous period. Null measurements, in conjunction with splitting measurements, suggest that the stations may be located in a region characterized by multi-layered or complex anisotropy. Our observations indicate that the mantle flow beneath the CITZ is influenced by the contemporary APM direction of the Indian plate as well as lithospheric frozen anisotropy.
How to cite: Bishoyi, N., Tiwari, A. K., and Dubey, A. K.: Mantle Deformation Pattern Beneath Central Indian Tectonic Zone: A Seismic Anisotropy Study in Satpura Gondwana Basin and Surrounding Areas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-964, https://doi.org/10.5194/egusphere-egu25-964, 2025.
