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This study investigates crustal anisotropy beneath the Koyna-Warna region (KWR) using waveforms of local earthquakes of magnitude M L ≥ 1.0, recorded at 24 surface and 6 borehole seismic stations during September 2005–February 2020. Waveforms of 1402 local earthquakes that fulfil the splitting criteria (i c < 45°) are used for shear wave splitting analysis, yielding 738 measurements. The study examines the spatial, temporal and depth variations in the fast polarization azimuths (FPAs) and delay times (δ t). The average delay times in the Koyna and Warna regions are 0.035 and 0.050 s, respectively, which did not show any significant correlation with depth. The FPAs are predominantly oriented in the NNW–SSE direction in the KWR, commensurate with the in situ maximum horizontal stress (SHmax) obtained using data from a 3-km-deep scientific borehole and individual focal mechanism solutions. The anisotropy beneath KWR is due to the alignment of fluid-filled cracks and pore throats along the SHmax, and is attributed to stress-induced anisotropy. A considerable variation in the orientation of FPAs from NW to NNW–SSE beneath KWR could be associated with heterogeneity in the lithological properties due to variation in the fracture orientation under regional stress. The non-parallel orientation of FPAs with SHmax or faults at some stations is attributed to structure-induced anisotropy, which seems to be associated with surface topography. Measurements at stations, even near the reservoirs, did not show any significant temporal variation in the splitting parameters due to the fluctuation in the water levels. [ABSTRACT FROM AUTHOR]

This study presents new results of shear wave splitting at 56 broadband stations installed in the Eastern Himalaya, Burmese Arc, and adjoining regions. The fast polarization directions (FPDs) within the Himalaya, Burmese Arc and the foredeep are parallel to the strike of the orogens, suggesting a coherently deformed lithospheric mantle under compression. The FPDs follow the Main Boundary Thrust and the Main Central Thrust in the central segment of the Arunachal Himalaya. Smaller delay times (∼0.7 s) can be best explained by multiple layers of anisotropy, where combined effects of the anisotropic fabric of a coherently deformed lithosphere of the Arunachal Himalaya and plate motion related strain are playing a role. Null measurements in the Siang window may be due to the Indian plate interaction with Eurasia and Burma plates, causing different layers of anisotropy. Another possibility is the coincidence of source polarization direction with the fast axis. In the Bengal Basin, the Nulls could be due to annihilation of frozen anisotropy in the downwelling Indian lithosphere and the APM‐related strain, or a highly heterogeneous mantle affected by the Kereguelen plume magmatism, at ∼116 Ma. Key Points: New results of shear wave splitting from the Eastern HimalayaCoherently deformed lithospheric mantle under compressionVariable nature of anisotropy along the Himalaya [ABSTRACT FROM AUTHOR]

This study presents 106 splitting and 40 null measurements of source side anisotropy in subduction zones, utilizing direct S waves registered at two stations sited on the Indian continent, which show null shear wave splitting measurements for S K S phases. Our results suggest that trench-parallel anisotropy is dominant beneath the Philippines, Mariana, Izu-Bonin, and edge of the Java slab, while plate motion-parallel anisotropy is observed beneath the Solomon, Aegean, Japan, and Java slabs. Results from Kuril and Aleutian regions reveal trench-oblique anisotropy. We chose to interpret these observations primarily in terms of mantle flow beneath a subduction zone. While the two-dimensional (2-D) slab entrained flow model offers a simple explanation for trench-normal fast polarization azimuths (FPA), the trench-parallel FPA can be reconciled by extension due to slab rollback. The model that invokes age of the subducting lithosphere can explain anisotropy in the subslab, derived from rays recorded at the updip stations. However, when downdip stations are used, contributions from the slab and supraslab need to be considered. In Japan, anisotropy in the subslab mantle shallower than 300 km might be associated with trench-parallel mantle flow resulting in the alignment of FPA in the same direction. Anisotropy in the deeper part, above the transition zone, is probably associated with 2-D flow resulting in trench-normal FPA. Anisotropy in the Mariana Trench might be associated with trench-parallel mantle flow in the supraslab region, with similar deformation in the upper mantle and the transition zone. [ABSTRACT FROM AUTHOR]

[Display omitted] • A large spatial variability in crustal deformation is found beneath the western and eastern parts of the Arunachal Himalaya. • Small delay times and large scattering in the fast axis of anisotropy characterize the western part. • Crustal anisotropy beneath the western part is due to a combination of structure- and stress-induced mechanisms. • Correlation of local S and XK(K)S measurements in the central part suggests coherent deformation. • Combined delay times from the central and eastern parts show a correlation with depth. Shear wave anisotropy of the crust beneath the Arunachal Himalaya is investigated by analysing the splitting of steeply incident (i c ≤ 45°) direct S-phases of 396 local earthquakes, recorded at 32 stations. The 76 well constrained splitting measurements reveal that the delay times between fast and slow waves range from 0.02 to 0.30 s, clustered around 0.07 s. A large variation in the orientation of fast polarization directions (FPDs) is found. The smallest delay times (mostly < 0.10 s) and large variation in the orientation of FPDs are found at stations located in the western part of the Arunachal Himalaya. This is likely associated with heterogeneities in the lithological properties and a combined effect of stress-induced and structure-induced anisotropy. For a small crustal block in the central part of Arunachal, the FPDs are parallel to the strike of the Himalayan arc and are associated with structure-induced anisotropy. In this region, the FPDs representing the upper mantle are also found to be parallel to the strike of the Himalayan arc, suggesting a coherent deformation. In the eastern part, a variation in the FPDs is observed from north to south, being parallel to the strike of the Siang river in the former and mostly parallel to the average maximum horizontal stress in the latter. The delay times show a correlation with depth when the observations in the central and eastern parts of the Arunachal Himalaya are combined. [ABSTRACT FROM AUTHOR]

This study attempts to understand the upper mantle deformation pattern beneath the northern part of Southern Granulite Terrain (SGT), located to the south of Archean Dharwar craton in south India. The study region comprises the tectonic blocks – Nilgiri, Biligiri Rangan, Shevaroy and Madras blocks – separated from the northern part of Madurai block by the Cauvery Suture Zone (CSZ), associated with subduction–accretion–collision tectonics. Shear wave splitting observed on high quality XK(K)S (SKS, SKKS, PKS) waveforms from 119 teleseismic events recorded by 19 broadband seismic stations, mostly within the CSZ and the northern part of Madurai block, is utilized in this study. The 122 splitting measurements obtained, show an average delay time of 0.8 s between the fast and slow waves. Strong lateral variations in the fast polarization azimuths (FPA) are observed, with the splitting parameters at the majority of stations showing a dependence on back azimuth. Importantly, excluding the Madras block, a poor correlation between the FPA and the present day absolute plate motion (APM) direction is observed. This dominance of non-APM strain reflects complex anisotropy preserved in the lithosphere. By plotting the fast polarization trends based on the ray piercing points at a depth of 150 km, three distinct splitting patterns associated with the (i) Biligiri Rangan block and the eastern part of the Nilgiri block, (ii) eastern part of the CSZ and the northern Madurai block (NMB), and (iii) Madras block and its vicinity within the CSZ, are observed. The strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions, two episodes of ocean closure and the collision that followed, plate motion related strain or a combination of these. Similar deformation patterns in the northern part of Madurai block might be related to remnants of subducted oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we investigate the upper mantle anisotropy beneath India using high quality SKS and SKKS waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations. In addition, we present evidence for anisotropy in the layer beneath southeast Asia using SKS and SKKS splitting discrepancies on the same seismogram. During this exercise, we obtain 200 new splitting measurements from 35 stations recently deployed in the Indo-Gangetic plains (IGP), central India and northeast India. While the delay times between the fast and slow axes of anisotropy (δt) range from 0.3 to 1.7 s, the fast polarization azimuths (Φ) at a majority of stations in the IGP and central India coincide with the absolute plate motion of India implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy. However, stations in NE India reveal fast polarization azimuths mainly in the ENE–WSW direction suggestive of lithospheric strain induced by the ongoing Indo-Eurasian collision. Our analysis for anisotropy yielded a total of 100 SKS–SKKS pairs, which can be categorized into those exhibiting (I) null measurements for one phase and significant splitting for the other phase, (II) null measurement for both the phases, (III) significant splitting for both the phases. A pair is considered to be anomalous if the splitting difference between SKS and SKKS is ⩾0.5 s and the individual split time is ⩾0.5 s. Using this criterion, we obtain 12 measurements under category III and 9 under category I that show a null measurement for SKS and large splitting for the SKKS phase. Further, we quantify the strength of the lower mantle anisotropy by correcting the SKKS measurement for the upper mantle anisotropy obtained by the SKS phase on the same seismogram. The SKS delay times are found to be consistently less than SKKS times, suggesting that the SKS phases do not capture the lower mantle anisotropy in comparison to their SKKS counterparts. Seven coherent measurements thus obtained reveal measurable anisotropy, with fast polarization azimuths oriented mainly in the ENE–WSW direction. These results suggest presence of a large region of deformation in the lowermost mantle beneath southeast Asia. A possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary (CMB) and is experiencing large shear deformation, resulting in lattice preferred orientation (LPO) of the lower mantle (Van der Hilst and Kárason, 1999; Long, 2009). The other possibility is the phase transformation from MgSiO3 perovskite to a more stable post-perovskite phase under favorable conditions, which results in LPO of the lower mantle. [Copyright &y& Elsevier]

This study attempts to characterize the nature of seismicity, anisotropy and coherency of deformation in the crust and upper mantle beneath the Doda- Kisthwar region (DKR), Jammu, located in the Kashmir seismic gap of NW Himalaya. Location of earthquakes using local waveforms, and shear wave splitting in local and teleseismic waveforms recorded at 5 temporary stations, are performed for this purpose. The relocation of 890 earthquakes reveals two clusters along the NE and SW part (referred to as Kisthwar and Doda cluster respectively), which is attributed to the under-thrusting of the Indian plate and uplifting of Kisthwar Window respectively. The shear wave splitting analysis yielded a total of 47 local S and 11 XK(K)S splitting measurements. The corresponding Fast Polarization Azimuths (FPAs) show two distinct spatial patterns. In the first pattern, the FPAs are oriented along the NW direction, are perpendicular to the maximum horizontal stress, and are parallel to the structural trend as a result of oblique compression in the NW Himalaya. In the second pattern, the FPAs are oriented along the ENE-WSW direction and the corresponding delay times are found to be larger than those in the DKR. This could be because of localized deformation; the strike of fluid-filled fractures in the fault zone is also along the ENE-WSW direction, parallel to the course of the Chenab river. Thus, for both the trends, structure-induced anisotropy seems to be the most possible cause for the observed anisotropy. Further, the splitting parameters show a slight variation with depth for the respective orientation patterns. The delay times of the XK(K)S phases vary from 0.3 to 1.3 s, and cluster around 0.5 s, with the majority of FPAs, oriented along the ENE-WSW direction. This can be reconciled in terms of a combined effect of absolute plate motion (APM) related strain and finite strain induced by compression. The inconsistency in the upper crust and upper mantle anisotropy does not favour a coherently distributed lithospheric deformation in this segment of the NW Himalaya. The source of anisotropy in the upper crust is complex compared to that in the upper mantle. • Analysed the local S and XK(K)S phases to examine the coherency of deformation in the crust and upper mantle • The fast axis of anisotropy in the upper crust does not correlate with the regional stress pattern. • The anisotropy in the upper mantle is due to the combined effect of basal shear and extension induced by collision. • Lack of coherent deformation in the crust and upper mantle suggests a decoupled nature. [ABSTRACT FROM AUTHOR]