Shear Wave Splitting Measurements Research Articles

The expression of mantle seismic anisotropy in the global seismic wavefield

SUMMARY The dependence of seismic wave speeds on propagation or polarization direction, called seismic anisotropy, is a relatively direct indicator of mantle deformation and flow. Mantle seismic anisotropy is often inferred from measurements of shear-wave splitting. A number of standard techniques to measure shear-wave splitting have been applied globally; for example, *KS splitting is often used to measure upper mantle anisotropy. In order to obtain robust constraints on anisotropic geometry, it is necessary to sample seismic anisotropy from different directions, ideally using different seismic phases with different incidence angles. However, many standard analysis techniques can only be applied for certain epicentral distances and source–receiver geometries. To search for new ways to detect mantle anisotropy, instead of focusing on the sensitivity of individual phases, we investigate the wavefield as a whole: we apply a ‘wavefield differencing’ approach to (systematically) understand what parts of the seismic wavefield are most affected by splitting due to seismic anisotropy in the mantle. We analyze differences between synthetic global wavefields calculated for isotropic and anisotropic input models, incorporating seismic anisotropy at different depths. Our results confirm that the seismic phases that are commonly used in splitting techniques are indeed strongly influenced by mantle anisotropy. However, we also identify less commonly used phases whose waveforms reflect the effects of anisotropy. For example, PS is strongly affected by splitting due to seismic anisotropy in the upper mantle. We show that PS can be used to fill in gaps in global coverage in shear-wave splitting data sets (for example, beneath ocean basins). We find that PcS is also a promising phase, and present a proof-of-concept example of PcS splitting analysis across the contiguous United States using an array processing approach. Because PcS is recorded at much shorter distances than *KS phases, PcS splitting can therefore fill in gaps in backazimuthal coverage. Our wavefield differencing results further hint at additional potential novel methods to detect and characterize splitting due to mantle seismic anisotropy.

Anisotropic Characterization of the Chukchi Boardland Based on Ocean-Bottom Seismic Experiment during N11-CHINARE

Abstract The current research focus at Chukchi Boardland (CB) revolves around sediment stratification and crustal structure, but investigations into deep stress fields and mantle dynamics are limited. This article presents a study on the anisotropic characteristics of the CB. Shear-wave splitting measurements were conducted using the transverse energy minimization at six stations recovered from the 11th Chinese National Arctic Research Expedition. The observation period for these six stations ranged from 2 August 2020 to 8 September 2020. The results demonstrate significant anisotropy within the CB, with the fast shear-wave polarization direction ranging from N60°E to N70°E. The time delays between fast and slow shear waves were found to be ∼0.7 s. By comparing the anisotropy observed at the CB with that at land stations in Arctic Alaska, this study suggested that the genesis of anisotropy beneath the CB was related to the formation of the Amerasian basin. The tectonic processes of rifting during basin evolution and midocean ridge spreading led to the development of anisotropy in the lithosphere beneath the CB during expansion.

Seismic anisotropy and stress-field variations along the Dead Sea Fault zone in northern Israel

Geological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress (SHmax) direction along the Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances from a ∼ N-S, fault-parallel orientation in the south to an ∼E-W, fault-perpendicular orientation in the north. However, the association of the meso-scale markers to the present-day stress field is not well constrained as many of them could have formed at any stage during the DSF evolution. In this study, we use a large dataset of Shear-Wave Splitting (SWS) measurements, taken from local DSF-related earthquakes, to characterize the azimuthal seismic anisotropy of the upper-crust at this critical turning point of the DSF system, near the Lebanese Restraining Bend. The SWS directions commonly align parallel to the local SHmax as revealed by the meso-scale markers, supporting their association with the current stress field. In several stations, SWS fast directions indicate azimuthal anisotropy at high angles to the trace of the master fault, indicating that stress-induced anisotropy represents a more plausible mechanism for SWS than structure-controlled anisotropy. The SWS measurements suggest that a northward counterclockwise rotation of the crustal SHmax is present along this segment of the DSF, supporting previous notation on the transition from divergent to convergent strike-slip faulting in this area. Plain langauge summaryGeological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress direction along the plate-boundary Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances. However, the association of the meso-scale markers to the present-day local stress field is not well constrained since many of them could have formed at any stage of the DSF evolution. In this study, we use a common method of measuring local seismic anisotropy called “shear wave splitting” (SWS) to characterize the directional-dependent seismic wave speeds in the upper crust. The SWS measurements are obtained from local seismic and micro-seismic DSF-related events, and characterize the upper-crust seismic anisotropy at this critical turning point of the DSF system. The SWS directions commonly align parallel to the maximum compression as revealed by the meso-scale markers, supporting their association with the current stress field. The SWS measurements suggest that a northward counterclockwise rotation of the crustal maximum compression is present along this segment of the DSF.

Ongoing fragmentation of the subducting Cocos slab, Central America

Abstract Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ~200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America.

Lithospheric mantle dynamics in Central and East Java Region, Indonesia from local shear wave splitting measurements

We have investigated the local shear wave splitting of 30–300 km depth earthquakes from 38 BMKG stations between 2009 and 2020 to determine upper mantle dynamics beneath the Central and East Java (CEJ) region, Indonesia. A total of 2338 measurements is obtained and divided the analysis into two focal depths, i.e., shallow (≤ 100 km) and deep (100 – 300 km) events. (1) Both individual station measurements and spatially averaged data using shallow events (≤ 100 km) show the trench-perpendicular fast direction in the northern CEJ region. Thus, anisotropy in this domain may be associated with the downdip subduction-induced 2-D corner flow in the mantle wedge allowing A-type olivine fabric to develop. Meanwhile, the trench-parallel fast directions in the southern CEJ region may reflect some possible causes of anisotropy: the presence of a serpentinized mantle wedge that promotes the development of B-type olivine fabric and anisotropy through alignment of the melt pockets. We also suggest a change in the hydration state of the subducting slab can cause the predominant trench-perpendicular fast directions in the eastern CEJ region. (2) For deep events (100 – 300 km), fast directions are relatively trench-parallel in the eastern CEJ region and trench-perpendicular in the western CEJ region, suggesting the presence of fossilized anisotropy and 2-D mantle flow-induced anisotropy, respectively.

Lowermost Mantle Structure Beneath the Central Pacific Ocean: Ultralow Velocity Zones and Seismic Anisotropy

AbstractUltralow velocity zones (ULVZs) and seismic anisotropy are both commonly detected in the lowermost mantle at the edges of the two antipodal large low velocity provinces (LLVPs). The preferential occurrences of both ULVZs and anisotropy at LLVP edges are potentially connected to deep mantle dynamics; however, the two phenomena are typically investigated separately. Here we use waveforms from three deep earthquakes to jointly investigate ULVZ structure and lowermost mantle anisotropy near an edge of the Pacific LLVP to the southeast of Hawaii. We model global wave propagation through candidate lowermost mantle structures using AxiSEM3D. Two structures that cause ULVZ‐characteristic postcursors in our data are identified and are modeled as cylindrical ULVZs with radii of ∼1° and ∼3° and velocity reductions of ∼36% and ∼20%. One of these features has not been detected before. The ULVZs are located to the south of Hawaii and are part of the previously detected complex low velocity structure at the base of the mantle in our study region. The waveforms also reveal that, to first order, the base of the mantle in our study region is a broad and thin region of modestly low velocities. Measurements of Sdiff shear wave splitting reveal evidence for lowermost mantle anisotropy that is approximately co‐located with ULVZ material. Our measurements of co‐located anisotropy and ULVZ material suggest plausible geodynamic scenarios for flow in the deep mantle near the Pacific LLVP edge.

Seismic anisotropy and geodynamics of the East Japan subduction zone

Seismic anisotropy in the East Japan arc has been extensively investigated by conducting shear-wave splitting measurements, receiver-function analyses, and tomographic inversions of body-wave travel times and surface-wave dispersion data, which have provided a wealth of information on dynamic processes associated with active subduction of the Pacific plate. Measuring shear-wave splitting is popular and effective to detect seismic anisotropy, but it has poor depth resolution. This problem has been overcome by conducting 3-D anisotropic tomography, which has high-resolution in both lateral and vertical directions. Both P and S wave anisotropies are revealed in the crust, which are caused by alignment or preferred orientation of crustal minerals and stress-induced microcracks related to active faults. Trench-normal fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the back-arc mantle wedge, reflecting subduction-driven convection there. Trench-parallel FVDs appear in the forearc mantle wedge under the land area, which may reflect deformation that results in B-type olivine fabric. The forearc mantle wedge offshore may lack anisotropy, suggesting that it is stagnant and decoupled from the subducting slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. The most significant findings of the body-wave anisotropic tomography are its constraints on the slab anisotropy. The subducting Pacific slab exhibits mainly trench-parallel FVDs, which reflect shape-preferred orientation of crystals and cracks related to normal faults produced in the outer-rise area before the plate subduction, overprinting the fossil anisotropy that the Pacific plate gained when it was produced at the mid-ocean ridge. Trench-parallel intraslab fast velocity planes of anisotropy intersect the slab upper surface at high angles (∼45–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults in the upper part of the slab may cause large intraslab earthquakes (M ≥7.0) that take place frequently beneath the forearc area. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction.

Shear wave splitting measurements of North Sulawesi

Shear wave splitting measurements of North Sulawesi

Strongly Depth‐Dependent Ice Fabric in a Fast‐Flowing Antarctic Ice Stream Revealed With Icequake Observations

AbstractThe crystal orientation fabric of glacier ice impacts its strength and flow. Crystal fabric is therefore an important consideration when modeling ice flow. Here, we show that shear‐wave splitting (SWS) measured with glacial microseismicity can be used to invert seismic anisotropy and ice fabric, if represented in a statistical sense. Rutford Ice Stream (RIS) is a fast‐flowing Antarctic ice stream, a setting crucial for informing large‐scale ice sheet models. We present >200,000 SWS measurements from glacial microseismicity, registered at a 38‐station seismic network located ∼40 km upstream of the grounding line. A representative subset of these data is inverted for ice fabric. Due to the character of SWS, which accumulates along the raypath, we include information on the depth structure from radar measurements. We find that the following three‐layer configuration fits the data best: a broad vertical cone fabric near the base of RIS (500 m thick), a thick vertical girdle fabric, orientated perpendicular to flow, in the middle (1,200 m thick), and a tilted cone fabric in the uppermost 400 m. Such a variation of fabric implies a depth‐dependent strength profile of the ice with the middle layer being ∼3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ∼16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast‐flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models.

Lithospheric S Wave Velocity Variations Beneath the Mackenzie Mountains and Northern Canadian Cordillera

AbstractThe Mackenzie Mountains (MMs) in the Yukon and Northwest Territories, Canada, are an enigmatic mountain range. They are currently uplifting (Leonard et al., 2008, https//doi.org/10.1029/2007JB005456), yet are about 700 km from the nearest plate boundary. Their arcuate shape is distinct and extends over 100 km eastward from the general trend of the Northern Canadian Cordillera. To better assess the cause and conditions of the current uplift, we processed ambient seismic noise data from a linear array of broadband seismographs crossing the mountains, along with other regional seismic stations, to estimate Rayleigh wave phase velocities between 6 and 40 s periods. From this, we estimated phase velocity dispersion and performed a tomographic inversion to estimate VS. Tomography reveals a low‐velocity structure that extends upward from the base of the ∼50–66 km thick lithosphere to the upper crust, and we hypothesize that inferred low density and low rigidity associated with the VS anomaly localizes the ongoing uplift and thrust‐dominated seismicity of the MMs. Additionally, we find relatively low crustal velocities that extend to the west of the MMs, suggesting that strain transfer from the Gulf of Alaska plate boundary plays a driving role as the crust translates to the northeast and buckles up against the craton consistent with the orogenic float hypothesis of Mazzotti and Hyndman (2002, https//doi.org/10.1130/0091-7613(2002)030〈0495:YCASTA〉2.0.CO;2). Finally, we observe lithospheric azimuthal anisotropy with an NW‐SE fast direction. This is nearly orthogonal to teleseismic shear wave splitting measurements in the central MMs, and suggests that asthenosphere flow and lithospheric strain are not aligned in this region.

Northeastward expansion of the Tibetan Plateau: Seismic anisotropy evidence from shear-wave splitting measurements

• Shear-wave splitting in and around the NE Tibetan Plateau mainly reflects anisotropy in the lithospheric mantle. • Dominant NW-SE anisotropy in the lithospheric mantle indicates NE-SW compression. • Fossil anisotropies exist in the interior Alxa Block and the western Ordos Block. The northeastern Tibetan Plateau (NETP) is a key region to clarify the nature and mechanism of the Tibetan Plateau expansion. This region includes the eastern Qilian Orogenic Belt, the Alxa Block, the Helan-Yinchuan Tectonic Belt and the Ordos Block. We make shear-wave splitting measurements using teleseismic waveforms recorded at 17 portable broadband stations that we newly deployed in the study region. Our measurements provide new information on seismic anisotropy and deformation patterns in the lithospheric mantle, which can be used to constrain the extent of the NETP northeastward expansion. The dominant fast polarization direction (FPD) is NW-SE in the eastern Qilian Orogenic Belt, the southern Alxa Block and the Helan-Yinchuan Tectonic Belt, subparallel to the prominent orientation of tectonics in the NETP. The same NW-SE FPD in the three tectonic units indicates NE-SW compression in the lithospheric mantle there caused by the NETP northeastward expansion. NE-SW FPDs are revealed in the interior Alxa Block, which may reflect fossil anisotropy probably formed by subduction and closure of the Paleo-Asian ocean. ENE-WSW FPDs exist in the western Ordos Block, reflecting fossil anisotropy probably caused by convergence of the South China and North China Cratons. Combined with the NW-SE FPDs in the southwestern margin of the Ordos Block revealed by previous studies, our results suggest that the NETP northeastward expansion has obviously affected the southern Alxa Block and the southwestern margin of the Ordos Block. However, its recent northeastward expansion mainly occurred along the Helan-Yinchuan Tectonic Belt.

Seismic constraints and geodynamic implications of differential lithosphere-asthenosphere flow revealed in East Asia

During the last 50 Ma, the East Asian continent has been a zone of massive continental collision and lithospheric deformation. While the consequences of this for Asian surface and lithospheric deformation have been intensively studied over the past 4 decades, the relationships between lithospheric deformation and underlying asthenospheric flow have been more difficult to constrain. Here we present a high resolution 3-D azimuthal anisotropy model for the northeastern Tibetan Plateau and its eastward continuation based on surface-wave tomography and shear-wave splitting measurements. This model shows that eastward lateral flow of asthenosphere beneath the northeastern Tibetan Plateau is being blocked by thick Ordos and Sichuan cratonic keels. The damming effect of these keels induces flow to first rotate around the Ordos keel and then transition into strong east-west flow beneath the thinner lithosphere that forms the lithospheric suture between the two cratonic keels. We further find that asthenosphere flow directions can differ from those of overlying lithosphere, with the asthenosphere neither being passively dragged by overlying lithosphere, nor being able to drag the overlying plate to mimic its subsurface flow. Finally, the region of eastward-channeled asthenospheric flow from Tibet underlies a belt of stronger intracontinental deformation in eastern China.

Crust-mantle decoupling beneath Afar revealed by Rayleigh-wave tomography

The Afar triple junction accustoms the diverging plate dynamics between the Arabian, Nubian, and Somalian plates along the Red Sea, Gulf of Aden, and East African rifts. The average anisotropy obtained from shear-wave splitting measurements agrees with the surface motion recovered by geodetic analyses. However, the vertical layering of anisotropy in this region is yet to be accurately determined. Here, we use earthquake seismic data to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath the East African Rift System. Our results suggest that a layering of anisotropy is present around the East African Rift System. At shorter periods that sample the crust, rift-parallel anisotropy is present in the vicinity of the rift, but in the central part of the rift, rift-normal anisotropy is found. At longer periods, sampling the lithospheric mantle, the anisotropic pattern is quite different. These observations suggest that the crust and lithospheric mantle are mechanically decoupled beneath the environs of the East African Rift System. Similarly, these results suggest complex dynamics within the crust and lithosphere in the region of the Afar triple junction.

Upper mantle and continental crust anisotropy in southeastern Mexico determined from shear-wave splitting measurements using local intraslab earthquakes

Cocos intraslab earthquakes were used to make shear-wave splitting measurements to explore the factors that control seismic anisotropy and to study the mantle wedge flow patterns in southeastern Mexico, where the Cocos plate subducts beneath the North American plate. Cocos intraslab earthquakes reach depths of 250 km, making it possible to sample the mantle wedge. The Silver and Chan (1991) covariance method was used to measure the splitting parameters: the fast polarization direction (φ) and the delay time (δt). The measurements can be divided into three regions: (1) northwest and (2) southeast of the Tehuantepec Ridge extension (i.e., the subducted Tehuantepec Ridge within the Cocos slab) and (3) the region above the subhorizontal Cocos slab. (1) In the first region, northeast of the 100 km isodepth contour of the Cocos slab, the fast axes are trench-perpendicular. This can be explained assuming the development of A-type olivine fabric and the existence of 2-D corner flow driven by the downdip motion of the Cocos slab. Southwest of the 100 km isodepth contour, measurements show trench-parallel fast polarization directions that are also consistent with corner flow, albeit in a serpentinized mantle wedge. Right above the Tehuantepec Ridge extension (northeast of the 100 km isodepth contour of the subducting slab), a change in the fast polarization directions from trench-normal to trench-parallel while going from northwest to southeast is observed and signals a change in the mantle flow pattern possibly through a vertical tear in the Cocos slab. 3-D toroidal flow could drive subslab mantle material around this slab edge and into the mantle wedge. (2) In the second region, the measured fast polarization directions show a trench-parallel orientation that is interpreted to result from southeastward trench-parallel flow through a serpentinized mantle wedge tip and also through a mantle wedge core made up of A- or C-type olivine fabrics with their fast axes oriented parallel to the flow direction. Trench-perpendicular fast polarization directions are observed beneath the fore-arc region of the Central America Volcanic Arc, near Tacaná Volcano, and trench-parallel polarizations are observed beneath the arc. These orientations could be explained by assuming the presence of B-type olivine fabric in the mantle wedge tip, A- or C-type olivine fabric in the mantle wedge core, and trench-parallel flow, so that the orientations of the fast axes become perpendicular to the mantle flow direction beneath the fore-arc and parallel to it beneath the arc. Lastly, (3) in the third region, over the flat slab, the observed delay times (0.04–0.42 s) are consistent with crustal anisotropy magnitudes, and the fast polarization directions seem to be controlled by the orientations of fault systems and alignments in foliations. Therefore, crustal faults and folds seem to be the dominant factors controlling the observed anisotropy.

New SKS splitting anisotropy measurements point to tearing beneath Central Italy

In the middle of the Mediterranean, the partly still active Apennines subduction system has been usually defined using tomographic images and available shear wave splitting measurements. In this paper we describe the new seismic anisotropy dataset for Central Italy, the region where the transition between Northern and Southern Apennines occurs. The new measurements show NW-SE fast polarisation directions beneath the belt, due to the retreat of the slab, NNE-SSW orientations from proper Adriatic mantle sources, and E-W directed anisotropy, attributed to mantle convection flow at the Thyrranian side. Additionally, the new data suggest the presence of a toroidal mantle flow through a tear in the Apenninic slab, from the Adria to the Tyrrhenian side. However, mantle circulation and flows, identified by the pattern of shear wave splitting results, seem different from what was proposed in previous geodynamic models. Indeed, our results support the presence of a vertical slab tear with limited dimension. In the geodynamic model we propose, the tear acts to accommodate a differential slab retreat. The slab partitioning results in a different pattern and strength of seismic anisotropy traced from the Central Apennines with respect to the adjacent Northern and Southern Apennines.

Scattered QL Surface Waves Reveal Edge-Driven Convection Mantle Flow beneath the Magma-Poor Malawi Rift Zone, East Africa

Abstract How the initial rupturing of the magma-poor rifts started remains a controversial scientific issue due to the lack of definitive studies on the lithospheric structure and mantle flow beneath the lithosphere from these rift systems. We constrain the anisotropic structure of the mantle in the magma-poor Malawi rift zone (MRZ) by observing quasi-Love (QL) waves, which are abnormal waveforms with Rayleigh wave polarization characteristics formed by the scattering of Love waves through lateral gradients in anisotropic structures. Here, we observed clear QL waves at stations in the western MRZ, indicating that there are significant lateral gradients in anisotropy beneath the western MRZ. The stations that recorded QL waves are close to the eastern edge of an unexposed Niassa craton revealed by the latest high-resolution tomographic model AF2019. Comparing the observations of QL waves with recently published shear-wave splitting measurements in the MRZ, we infer that the cratonic lithosphere of the Niassa induced edge-driven convection (EDC) of the asthenospheric mantle on its eastern boundary, resulting in lateral gradients in seismic anisotropy beneath the western MRZ. As the EDC is a small-scale, relatively weak form of mantle convection that is easily overwhelmed by strong active upwelled asthenosphere, its presence indicates the lack of large-scale mantle convection associated with asthenospheric upwelling beneath the MRZ. The magma-poor MRZ could be an example of passive rifting due to the local asthenospheric mantle flow.

Stress Variations in the Delaware Basin from Shear-Wave Splitting Analysis

Abstract The northwestern Delaware basin has experienced an increase in seismicity, including one M 5.0 earthquake in 2020, correlated to hydraulic fracturing and wastewater injection activities. We conduct shear-wave splitting (SWS) analysis from local earthquakes to understand the increasing and intensifying seismicity in this area. Over 840 robust SWS parameters, the fast polarization orientation and the delay time, are obtained at five Texas Seismological Network stations by analyzing seismograms of more than 4000 events from 2019 to 2021. The fast orientations from individual events vary in a broad range for all stations, indicating a complex fracture system in the upper crust, even though the averages are consistent with the local fault strikes or the maximum horizontal stress. Fast orientations with large angles from the local stress appeared after the 2020 M 5.0 earthquake, evidencing an increase of pore pressure that facilitates slips on less favorable fracture planes by the stress field. Stress change caused by this earthquake could also contribute to the increasing diversity of SWS measurements. Future data analysis and stress modeling are needed to distinguish the different mechanisms.

Mantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves

Shear-wave splitting measurements map the orientation of geodynamically induced fabrics in the upper mantle. However, due to the limited depth resolution of shear-wave splitting investigations from core-mantle refracted phases, it has so far been difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To improve this differentiation, we take advantage of backazimuthal variations of the fast orientation ϕ and delay time Δt that is expected for an olivine-mantle block with vertical b-axis as compared to vertical c-axis. This suggests that shear-wave splitting observations can in principle provide constraints that have so far not been recognized. The two end member cases can occur depending on the type of deformation, for example, a subhorizontal foliation in the case of a Simple Asthenospheric Flow and a subvertical foliation when considering transcurrent/transpressional circumstances during Vertical Coherent Deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern without large azimuthal variations per station, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the entire study area, our procedure indicates b-up olivine and thus favors an asthenospheric source of the anisotropy, with a horizontal flow plane of deformation. We also test the influence of position relative to the (European) slab under the Central Alps, distinguishing northern and southern subarea based on vertically-integrated travel times calculated from a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of their variation δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of planar Poiseuille flow around the Alps, the origin in the southern subarea remains uncertain, but it is likely to be affected by the slab itself.

Slab Induced Mantle Upwelling Beneath the Anatolian Plateau

AbstractThe subducting African Plate in the easternmost Mediterranean is actively tearing and deforming beneath the Anatolian Plate as the margin transitions from long‐lived subduction to collision. In central Anatolia, the subducting slab is characterized by both lateral and vertical slab tears. We investigate patterns of mantle flow around the edges of a contorting and fragmenting African slab segment, called the Cyprean slab, using measurements of shear wave splitting. We observe three distinct regions of coherent shear wave splitting that correlate with the segmentation boundaries of the Cyprean slab. Regionally coherent mantle flow occurs near both the eastern and western the edges of the slab. These regions of coherent splitting are separated by an area of null splitting that encompasses the Central Anatolian Volcanic Province near the easternmost edge of the slab. The null measurements likely result from mantle upwelling due to the displacement of asthenosphere from the vertical Cyprean slab.

Upper crustal anisotropy in the Southeastern Sichuan Basin, China from shear-wave splitting measurements

The Sichuan basin which embodies salt and shale gas resources, is characterized by low seismicity. Seismicity in the southeastern Sichuan basin increased dramatically in recent years, including a series of moderate to strong earthquakes. We investigate seismic anisotropy in the upper crust by performing shear wave splitting analyses on local earthquakes recorded at seventeen stations around the Changning-Xingwen area, southeastern Sichuan basin between January 2016–February 2021. We perform an automatic shear wave splitting analysis and obtain 2926 high-quality splitting measurements by examining 5174 local earthquakes. The delay times range between 0.08 and 0.20 s, and the anisotropy magnitude ranges between 1.9% and 5.5% with a mean of 4.2%, indicating that the study area has significant strength of anisotropy. The fast polarization directions are highly variable reflecting a combined effect of complex local structure and stress field in this area. In the southeastern part of the study area, fast directions are aligned parallel to NW- and NE-oriented surface structures, suggesting structural anisotropy. In the northwestern part of the study area, fast-polarization directions align parallel to the strike of folds and the direction of maximum horizontal compressive stress, probably reflecting the combination of both stress- and structural-controlled anisotropy. Whereas at these stations of central part, fast directions tend to align parallel to the direction of maximum horizontal compressive stress, suggesting stress-induced anisotropy. We examined the temporal variation of seismic anisotropy associated with the 2019 MS 6.0 Changning earthquake. The significant changes in splitting parameters during the Changning earthquake period may only reflect the spatial changes in seismic anisotropy.