Ionospheric disturbances are critically important because they directly affect the accuracy and reliability of GNSS-based navigation and radio communication systems. They also influence satellite operations and aviation systems, making them a key component of space-weather impacts. Geophysical phenomena such as Solar Flares, Geomagnetic disturbances, Large Scale Travelling Ionospheric Disturbance (LSTID), Medium Scale Travelling Ionospheric Disturbance (MSTID) and seismic activity cause the temporal as well as spatial variabilities of the ionosphere. Rapid variations in electron density during these events can introduce signal delays, scintillations and associated positioning errors. To assess these activities, Total Electron Content (TEC) is employed. This paper uses GPS derived TEC measurements to investigate ionospheric variations over a low latitude region. Differential Rate Of TEC (DROT) is used in this research which is an efficient algorithm designed for automatic variability detection is carried out. The proposed algorithm is validated with disturbances from solar Flares, geomagnetic activity, LSTID, MSTID and earthquake data from previous literature. The results are closely consistent with those reported in earlier studies. The IONOLAB-TEC measurements from HYDE and IISC stations over India are inspected with DROT to acquire wave-like oscillations, sudden disturbances, and other irregular variations. DROT demonstrates the ability to detect small, medium and large-scale variability throughout the ionosphere. The analysis expresses three DROT intervals that specify disturbance intensity: less than 50

A seismic refraction study has been carried out across the seismically active pericratonic Kachchh basin, formed as a failed rift during the Mesozoic breakup of Gondwanaland. The basin was significantly affected by the Reunion plume that emplaced massive Deccan flood basalts. Inversion of traveltimes derived from the seismic data reveals a six-layered complex structure, with P-wave velocities ranging between 1.90 and 5.20 km s-1 for sediments and 5.85-6.10 km s-1 for the basement. Synthesis of seismic refraction, reflection, gravity and well data indicates a variability in the basement depth from 2.2 km in the north to 5.7 km in the south. A basement upwarp of 1.3 and 2.2 km in the north and south is related to the Pachchham and Kachchh mainland uplifts. Traveltime skips of the first arrivals imply low-velocity Mesozoic sediments previously obscured beneath the high-velocity Deccan Traps. We propose thick sediments in the southern part with abundant source, reservoir and cap rocks, which are potential hydrocarbon prospects. This study also illuminates several faults, which act as stress concentrators. We postulate that the far-field compressional forces generated due to India-Eurasia collision and local stresses caused by the heterogeneous structure are responsible for the basement upwarp, uplift of the Kachchh mainland, reactivation of faults, and high seismicity of the region. We attribute the western plate boundary located similar to 400 km away from the Kachchh region as a probable causative for the Median High, a Hinze zone covering the basin and beyond. An evolutionary model of the basin is suggested.

GNSS-based communication systems, positioning accuracy, and space weather conditions are significantly influenced by Equatorial Plasma Bubbles (EPBs) and Traveling Ionospheric Disturbances (TIDs), especially in equatorial and low latitude regions such as India. The commonly used metric for tracing EPBs is the Rate of Total Electron Content Index (ROTI). Ground-based GNSS receivers are vital tools for detecting TIDs and EPBs by monitoring rapid variations in Total Electron Content (TEC). To accurately track irregularities, these receivers offer high-resolution temporal and spatial data in both quiet and geomagnetic conditions. In this study, the ionospheric disturbance index, namely the time-integrated ROTI (IROTI), is implemented to detect and classify both TIDs and EPBs, with a primary focus on its regional validation and application over the Indian sector. Seven geomagnetic storms which occurred on 17 March 2015, 23 April 2023, 10 May 2024, 8 September 2017, 23 June 2015, 19 April 2024, 11 October 2024 and eight quiet days on 21 May 2020, 7 July 2015, 15 September 2018, 28 January 2011, 1 January 2024, 10 April 2024, 12 June 2024, 25 October 2024 are counted for analysis using GNSS TEC data from Indian stations. IROTI Index on 17 March 2015 geomagnetic storm indicating both EPBs and TIDs with a higher spatial spread and clear temporal separation, with IROTI (EPB) values typically ranging from 8 to 40 TECU2 and IROTI (TID) values from 1.8 to 4.25 TECU2. The storm that occurred on 23 April 2023 exhibited IROTI (TID) activity between 0.64 and 3.1 TECU2 and IROTI (EPB) activity that typically ranged between 8 and 38.5 TECU2. During an extreme geomagnetic storm on 10 May 2024, IROTI values ranged between 8 and 60 TECU2 during EPBs. On 8 September 2017, a severe storm produced IROTI (TID) values between 0.3 and 2.12 TECU2 and IROTI (EPB) values ranging from 8 to 14 TECU2. The storm event observed on 23 June 2015 exhibited IROTI (TID) activity ranging from 0.2 to 4 TECU2. On 19 April 2024, a moderate geomagnetic storm, IROTI values were responsive between 8 and 40 TECU2 during EPBs, even on 11 October 2024, only EPBs were observed. Medium-scale TIDs (MSTIDs) were observed on eight quiet days mentioned above during post-sunset hours from 14:00–24:00 UT with magnitudes ranging from 0.2 to 2 TECU2. Statistical Analysis of ROTI and IROTI during TIDs periods is investigated for 7 geomagnetic storms and 8 quiet days, and a 92.3

The southeastern margin of the Tibetan Plateau is crucial to understanding the dynamics of the Indian–Eurasian continental collision and potential causes of the plateau uplift. In the current study, we use fundamental mode surface wave group velocity ( U_g ) tomography and the fast-marching method to examine the shear velocity structure of the crust and upper mantle beneath northeastern India and the Tibetan Plateau with 1^∘× 1^∘ resolution. The waveforms of 568 regional earthquakes that were detected at 326 seismic stations throughout the study region were combined to create the U_g dataset for periods of 4–70 s. A well-constrained quasi 3-D isotropic shear wave velocity tomographic image down to ∼ 100 km depth is produced utilising the non-linear damped least square method by inverting the dispersion curves extracted from each node point of the Rayleigh and Love wave U_g maps. Radial anisotropic maps across the study area observed from the disparity between vertically ( V_SV ) and horizontally ( V_SH ) polarized shear wave velocity measurements signify lateral differences within the crust. Consequently, the observed variations in the velocity structure and radial anisotropy along with the crustal thickness in the Tibetan Plateau support the concept that the region serves as a pathway for material migration moving east and southeastwards. Lower velocity in the Tethyan Himalayan upper crust ( ∼ 2.8 km/s), Lhasa middle crust ( ∼ 3.2 km/s) and lower crust in the Qiangtang and Songpan–Ganzi terranes ( ∼ 3.5 km/s) reflects the channel flow directed outwards from the Tibetan Plateau, southwards and southeast-wards across the Lhasa Terrane and rotate around the Eastern Himalayan Syntaxis, respectively. Observations of the velocity structure also suggest the possibility of a flow accumulation near the southern Yunnan province at 26 ^∘ N.

This study comprehensively examines the shear wave splitting measurements of XKS (SKS and PKS)-SKKS pairs on the same seismograms recorded at 357 broadband stations spanning India, to characterize anisotropy in the lowermost mantle. This resulted in the identification of 104 XKS-SKKS pairs at 62 stations, of which 27 pairs were found to be discrepant, based on the difference in splitting intensity of XKS and the corresponding SKKS phases. These discrepant pairs dominantly sample a portion of the lowermost mantle beneath Southeast Asia and the Indian Ocean. The majority of these pairs represent null-split and split-split cases, with the delay time of SKKS being larger than that of XKS for the latter. This suggests that the XKS phases primarily sample the isotropic (weakly anisotropic) or anisotropic regions with a cancelling effect in the lowermost mantle, while the corresponding SKKS phases sample the anisotropic region of the D '' layer. In addition, there are three discrepant pairs in the split-null category, suggesting anisotropy in the vicinity of southern Tibet, where discrepant pairs from other cases are not observed. This implies an apparent change in the anisotropy of the D '' layer for the regions sampled by XKS and SKKS, although they are associated with high-velocity anomalies. In these regions, the fast polarization azimuths of the discrepant pairs are in the NE-SW and ENE-WSW, and NNE-SSW directions, respectively. These do not coincide with the trend of mantle flow in the lowermost mantle, suggesting an association with paleo-subducted slabs. The observed deformation is probably due to phase transformation of bridgmanite to a more stable post-perovskite, causing Crystallographic Preferred Orientation of the lowermost mantle, which is the candidate mechanism for lowermost mantle anisotropy beneath Southeast Asia and the Indian Ocean.

We report two continental lithospheric mantle (CLM) earthquakes of M-W 3.8 and 3.0 beneath the South Indian Shield, which occurred on 25 and 27 July 2021, respectively. Probabilistic location, waveform modeling, and depth phase analysis confirm that both earthquakes occurred at a depth of 45 +/- 2 km along the western margin of the Cuddapah Basin (CB). Receiver function modeling reveals that the local Moho depth is at 33 +/- 2 km, which implies that the earthquakes occurred similar to 12 km below the Moho. Besides mantle earthquakes, 314 earthquakes (M-L 0.0-4.3) occurred throughout the entire crust beneath this region between 2008 and 2022. The source mechanisms of M-L >= 3.0 crustal and mantle earthquakes show dominant strike-slip and oblique mechanisms, respectively, on high-angle faults, striking NW-SE or NE-SW, with pressure axes aligning with the India-Eurasia convergence direction. The average spectral stress drops for both crustal and mantle earthquakes are indistinguishable. Geotherm modeling indicates temperatures of 522 +/- 33 degrees C at the source depth, aligning with the view that the CLM can be seismogenic up to temperatures of similar to 600 degrees C. The results suggest that the entire crust and about similar to 25 km of the upper mantle form a single seismogenic layer beneath the CB, while the mantle earthquakes concentrate along the lithosphere thickness gradient where the layer is at its weakest. The presence of CLM earthquakes beneath the South Indian Shield and Himalayan foreland might indicate that the strength of the Indian lithospheric mantle is optimal-sufficient to facilitate transmission of horizontal stresses without preventing seismogenesis.

This study attempts to understand the upper mantle deformation patterns beneath the Eastern Himalaya by performing shear wave splitting analysis of core-refracted phases. Out of the 83 broadband seismic stations used, data from 70 stations are analysed for the first time. This includes 21 stations which were newly deployed along two profiles in Arunachal Himalaya, to fill the gaps in the stations used for previous studies. In total, 172 well constrained new splitting and 215 null measurements are obtained in this study. Average delay time values of 0.64 and 0.76 s in the Bhutan and Arunachal Himalaya respectively, suggest weak anisotropy, probably due to a steep subduction of the Indian mantle lithosphere. There is a systematic variation in the orientation of fast polarization azimuths in the western (Bhutan Himalaya and western part of Arunachal Himalaya) and eastern segments (central to the eastern part of Arunachal Himalaya). In both these segments, the orientation of fast polarization azimuths varies dominantly from NE-SW or/and ENE-WSW, to E-W, from west to east. In the western and central parts of Bhutan Himalaya, the influence of absolute plate motion related strain in the asthenospheric mantle cannot be ruled out, while in its eastern part and Arunachal Himalaya, the azimuthal anisotropy can be explained by arc parallel mantle flow due to slab rollback. In addition, a few observations in the central part of Arunachal Himalaya indicate a slightly larger delay time, along NNE-SSW, which could be associated with mantle wedge flow. The eastern part of Arunachal Himalaya might be associated with a repulsive arc parallel flow from the Arunachal and Burmese arcs, resulting in null measurements. The optimal depth of anisotropy in Bhutan and Arunachal Himalaya is around 220-270 and 200-240 km respectively, suggesting that the source of anisotropy lies in the upper part of the asthenosphere.

The Réunion hotspot is the best example of a primary plume, manifested as intraplate-volcanism, a large igneous province and a geochemical anomaly. In this study, we investigate the mantle transition zone (MTZ) structure beneath the Réunion Island using 3D-migration of P-Receiver functions, to decipher the effect of the plume on the MTZ and its architecture. Results indicate a thin MTZ in the regions surrounding the Réunion, like Madagascar and its vicinity, eastern and south-eastern sides of the Réunion, suggesting high-temperature anomalies within, caused by the plume. Interestingly, we detect a depressed 410 km discontinuity exactly beneath the Réunion hotspot and a broader depression of 660 km discontinuity within and regions in its proximity. These maiden results shed-light on the high-temperature anomalies in the mid-mantle, probably sourced from the Réunion plume and provide evidence for the Majorite-garnet phase transformation at 660 km discontinuity. We postulate that an ascending Réunion plume has initially hit the 660 km discontinuity, got horizontally spread and further progressed to the 410 km discontinuity as a columnar structure.

The study determines the attenuation structure of the crust beneath Bhutan Himalaya using high-resolution 2-D Lg tomography. We utilized waveforms of 138 seismic events within an epicentral range of 250-2000 km, recorded at 37 broadband seismic stations installed in the Bhutan Himalaya. We categorized the Lg wave propagation as efficient, inefficient, or blocked based on visual examination of individual seismograms. We observed an effective propagation of the Lg wave across northern Myanmar, the Himalayas and southwestern region of China. We also noticed an obstruction or blocked propagation of Lg waves travelling through the sea in the southern part of the study region. Tibet has been found to exhibit ineffective propagation of Lg waves, as also seen in earlier studies. Subsequently, a 2-D LgQ tomographic model, along with the frequency dependent parameter eta has been produced using 582 high-quality station pairs. The results suggest a highly attenuating medium beneath Bhutan Himalaya. A difference in crustal attenuation is observed from east to west, in accordance with the tectonic characteristics of the region. Eastern Bhutan is found to exhibit low Q0 values whereas the western part mostly near the Paro window has higher Q structure. The central part, along 90.5 degrees E longitude exhibits lower values. These observations are quite consistent with the reported 3-D shear wave velocity structure in Bhutan Himalaya. Additionally, the variation in Q along two N-S profiles in the region shows significant correlation with Moho thickness. Our study does not capture any such strong lateral variations in eta across the region. A relatively high eta has been observed in the central part, while moderate to low values dominate the region. We interpret that both scattering and intrinsic attenuation contribute to high Lg wave attenuation beneath Bhutan Himalaya. The concurrence of our observations with the existing tectonic setting of the region lends credence to the degree of influence of crustal structures in wave propagation mechanisms. Knowledge of attenuation characteristics of the crust in Bhutan Himalaya will aid in future research to understand the dynamics of the actively deforming Himalayas.

We identify possible sources of seismic anisotropy beneath India by synthesizing 2064 well-constrained shear- wave splitting parameters determined from a consistent analysis of waveforms recorded at 357 broadband seismic stations. Our effort includes compilation of previous results, reanalysis of old data, analysis of new data from previous networks and new stations. Our results reveal that the average delay time for entire India and its constituent tectonic provinces is similar to 0.83 s suggesting moderate strength of anisotropy. Although the fast polarization azimuths (FPAs) are scattered, a NE trend appears dominant. Due to significant correlation of FPAs with the APM direction and lack of correlation between i) splitting parameters and backazimuths and ii) average delay times and lithospheric thickness, we conclude that the major contribution to anisotropy is from shearing in the upper part of the asthenosphere or a transitional layer from the base of the lithosphere to the upper part of the asthenosphere. Further, we postulate that a weakly anisotropic lithosphere in northern, central and south-eastern India is due to frozen anisotropy from past tectonic events. Northern and central India, Arunachal Himalaya and southern part of Burmese arc have simple anisotropy. Application of the spatial coherency technique reveals a source depth of 290 km for northern India. However, for south-eastern India and northern part of the Burmese arc, a two-layer model, with frozen-in and present-day anisotropy in the upper layer, and shearing and mantle flow in the lower layer, respectively, fits the anisotropy. In southern India, a large deviation of the FPAs from APM suggests imprints of deformation related to past tectonic events. A two-layer model, with frozen-in anisotropy in the upper and lower layers, is plausible. Variation in FPAs in the central part of the Indian shield is attributed to deflection in mantle flow at the northern edge of the lithospheric keel.

Summary Globally, there is now a growing evidence for a low velocity layer in the deeper parts of the upper mantle, above the 410 km discontinuity (hereafter called LVL-410). The origin of this layer is primarily attributed to interaction of slabs or plumes with a hydrous mantle transition zone (MTZ) that results in dehydration melting induced by water transport upward out of the MTZ. However, the ubiquitous nature of this layer and its causative remain contentious. In this study, we use high quality receiver functions (RFs) sampling diverse tectonic units of the Indian sub-continent to identify Ps conversions from the LVL-410. Bootstrap and differential slowness stacking of RFs migrated to depth using a 3D velocity model reveal unequivocal presence of a deep low velocity layer at depths varying from 290 to 400 km. This layer appears more pervasive and deeper beneath the Himalaya, where detached subducted slabs in the MTZ have been previously reported. Interestingly, the layer is shallower in plume affected regions like the Deccan Volcanic Province and Southern Granulite Terrane. Even though a common explanation does not appear currently feasible, our observations reaffirm deep low velocity layers in the bottom part of the upper mantle and add to the list of regions that show strong presence of such layers above the 410 km discontinuity.

High-quality data recorded by a dense network of 53 seismic stations in the Garhwal- Kumaun Himalaya between February 2017 and December 2021 is analyzed. A total of 813 local earthquakes are relocated using a newly developed regional 1D velocity model incorporating station corrections. In addition, focal mechanism solutions of M >= 3.8 events are estimated using waveform inversion. The relocated seismicity patterns along with the focal mechanism solutions are utilized to present a seismotectonic scenario of the region. Almost 95% of the relocated seismicity is found to be clustered along the Himalayan seismic belt (HSB), down to - 24 km depth. Seismicity in this belt is interpreted to be caused due to interseismic stress loading associated with the ongoing India-Eurasia collision tectonics. A few scattered hypocenters in the deeper crust between 30 and 50 km depth attest the strength of the downgoing Indian plate. Focal mechanisms in the seismogenic upper crust reveal thrusting of the Indian plate beneath the Lesser Himalaya, with compression normal to the strike of the Main Central Thrust (MCT). The north-dipping thrust mechanisms can be associated with a near-horizontal Main Himalayan Thrust (MHT). In addition, more steeply dipping faults above it define the Lesser Himalayan duplex systems, similar to those in western and Nepal Himalaya. A prominent - 50 km wide seismicity gap region observed within the HSB is probably due to (1) a locally varying locking width of the MHT; (2) an unruptured, ductile segment at the eastern end of the rupture zone of the great 1803 earthquake (Mw 7.8 +/- 0.2); and (3) a slab tear in the MHT, similar to those in subduction zones.

Large and shallow earthquakes can trigger seismicity from long-distance ranges, ideally along significant plate boundaries and in active geothermal/volcanic regions. The present study aims to garner evidence for dynamically triggered events in the intraplate Surendranagar and Talala regions of Saurashtra Horst, Northwestern India, which are the premier sites of mining- and monsoon-induced activities, respectively. A routine catalogue analysis did not reveal any apparent dynamic triggering of earthquakes in the Saurashtra region. To investigate the possibility of triggered earthquake signatures in the waveform data, we applied the Matched Filter Technique (MFT) to the waveform data of 31 teleseismic earthquakes with Peak dynamic stresses ≥ 1 kPa, that occurred between 2007 and 2017. Results reveal that one (2017 Mw7.9 Papua New Guinea) event triggered seismicity in Surendranagar and four (2007 Mw7.9 Sumatra; 2009 Mw7.6 Sumatra; 2010 Mw8.8 Chile, and 2012 Mw7.6 Costa Rica) in the Talala region. β-statistics further confirm the triggering. Application of the MFT revealed 81 hitherto unrecognized local events in a 20-hour duration around the triggering mainshocks. Only ∼ 16

SUMMARY This study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broad-band stations. The fast polarization azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE–WSW, with a few along the NE and E–W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four subregions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE–WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE–WSW and NW, respectively. The strong ENE–WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220–240 km, implying that the dominant source of anisotropy could lie in the upper mantle.

Geometrical heterogeneity of the subducting Indian continental crust along the Himalaya-Tibet collision zone remains enigmatic. Mass budget estimates describing shortening across the orogen are partly derived from observations made from seismic imaging of the deep earth. Here, using data from 38 broadband seismic stations covering Sikkim Himalaya, we produce high resolution seismic images in order to fill crucial gaps in our understanding of the evolution of Himalayan collision zone. We used 12,288 high quality receiver functions computed using waveforms of earthquakes having magnitude >5.5 in the distance range of 30 degrees-100 degrees. Our results reveal a highly imbricated and heterogeneous crust beneath Sikkim Himalaya. The Main Himalayan Thrust (MHT) responsible for large scale earthquakes in the Himalayan collision zone is not so distinct in the migrated images, but intermittent. A dominant cluster of earthquakes at shallower depths is associated with the MHT, marked by negative amplitude arrivals. The Moho is found to be gently dipping, reaching depths of similar to 60 km beneath the Higher Himalaya compared to similar to 40 km in the Himalayan foredeep. Interestingly, the Moho in this part of Himalaya has offsets and overlapping segments, indicating crustal imbrication in response to active shortening. Clusters of lower crustal earthquakes coincide with the junction of offsets in the Moho.

The Arunachal and Bhutan Himalaya, which are tectonically distinct from other regions of the Himalaya, have a structure that is quite intricate. The eastern Himalayan segment is a component of the region where the Indian and Eurasian plates collided 50 Myr. The Indian Plate goes beneath the Eurasian Plate in the north, and in the eastern part of the region, the Indian Plate subducts under the Burmese Plate. Here, we studied the seismic attenuation of the uppermost mantle by measuring the quality factor of the S-n wave (S(n)Q) to understand the dynamics of the lithospheric mantle and the cause of the seismic anomalies found in this area. The upper mantle Q structure has significant lateral differences in Arunachal and the Bhutan Himalaya. Arunachal Himalaya's central region is characterized by a very low Q(<= 150). The successive low-high-low S(n)Q values in eastern Arunachal Himalaya near Siang region have been observed. The western Arunachal region, close to the Bhutan border, exhibits a contrast in Q values. We notice that low Q values (<= 200) predominate in the central to eastern Bhutan Himalaya. The western part of Bhutan Himalaya exhibits relatively high Q(>= 200) values, mostly near Paro and Thimpu. Interestingly, a clear boundary between low and high Q has been observed near Kakthang thrust (KT) in the Bhutan Himalaya. We found significant lateral variation of frequency dependent parameter (eta) across the study region. They range from 0.25 to 0.75, with low values (<= 0.5) found mostly in the central Bhutan Himalaya and in a few isolated areas of the Arunachal Himalaya. Low Q and a relatively higher Q(>= 0.5) might suggest that the scattering attenuation is the controlling mechanism for S-n wave attenuation in the upper mantle beneath Arunachal Himalaya. On the contrary, dominant low Q values across the central segment of the Bhutan Himalaya, along with a low to moderate body wave velocity and dominating low eta values, subsequently corroborate that intrinsic attenuation is the dominant factor in the upper mantle of the central Bhutan Himalaya.

Seismic attenuation structure of the uppermost mantle is investigated using Sn waves in Bhutan Himalaya. Sn phase is the uppermost mantle-refractedphase, which travels with a velocity of 4.3 - 4.7 km/s. Visual inspection of all the seismograms are conducted to examine the efficient, inefficient and blocked paths in the region. The inefficient, and blocked S n phases are mainly observed from the western side of our study region. Sn attenuation is determined using the two-station Methodology (TSM ). We have generated 460 station pairs from 1539 seismograms with magnitude ≥ 4 within an epicentral distance of 200 - 1650 km recorded at 38 seismic stations. Furthermore, a 2D Sn Q model is produced to understand the upper mantle rheology of the area. The central part shows a low Q (≤100) value while high Q dominates in northern and western parts of Bhutan Himalaya. The overall results correlate well with the tectonic setting beneath the study region. A comparison study is also made with the adjacent Arunachal Himalaya for better understanding.

&lt;p&gt;&amp;#160; The exact role of subducting Indian continental crust in the formation of Himalaya-Tibet collision zone remains enigmatic. The mass budget estimates describing shortening across the orogen is partly derived from the observations made from seismic imaging of deep earth. Here using data from 38 broadband seismic stations covering Sikkim Himalaya, we produce high resolution seismic images in order to fill the crucial gaps in our understanding of the formation of Himalayan collision zone. We have used 11,594 high quality receiver functions using earthquakes of magnitude &gt;5.5 in the distance range of 30-100&amp;#176;. Our data demonstrates a highly imbricated and heterogeneous crust beneath Sikkim Himalaya. The Main Himalayan thrust responsible for large scale earthquakes in the Himalayan collision zone is not so vivid in the migrated images, but is observed intermittently. The main cluster of earthquakes at shallower depths linked to the Main Himalayan thrust is marked by low amplitude arrivals. Overall trend suggests a gently dipping Moho attaining crustal depths of &amp;#8764;60 km beneath Higher Himalaya compared to &amp;#8764;40 km in the Himalayan foredeep. Moho as we see in this segment of Himalaya is with possible offsets and overlapping segments. Imbrication is well reported in the Himalayan orogenic wedge forming upper crust, we also observe this in the lower crust indicating lithospheric imbrication in response to collision. Interestingly, the lower crustal clusters of earthquakes fall at the juncture of offsets in the Moho. The offset positions at lower crustal depths seem more prone to earthquakes in response to active shortening. Seismic images reveal differences in amplitude of receiver functions and presence of conversions at deeper depths in the lithospheric mantle across Dhubri-Chungthang Fault Zone, possibly related to the segmentation of Himalaya.&amp;#160;&amp;#160;&lt;/p&gt;

SUMMARY The dynamic stresses generated during the passage of surface waves perturb most major plate boundaries and active volcanic/geothermal regions. The present investigation aims to understand and find evidence for dynamic triggering in the intraplate Kachchh Rift Basin, northwestern India. We analysed the local earthquake catalogues and waveforms of 36 teleseismic earthquakes with Mw ≥ 7.5, which occurred between 2007 and 2017 and generated peak dynamic stresses of at least 1 kPa. The study manually examines the waveforms 24 hr before and after the teleseismic P wave and detects missing earthquakes by applying the matched filter technique. Notably, the analysis reveals that 6 out of 36 earthquakes (17 per cent of the analysed earthquakes) are responsible for triggering non-volcanic tremors or microearthquakes during the passage of surface waves and later. Although the triggering potential in the region seems insensitive to the amplitude of dynamic stresses imparted by the teleseismic surface waves, the direction of the incoming waves appears critical. The surface waves of events whose backazimuths are parallel to the strike of the active faults are capable of triggering. The triggered earthquakes lie along the Kachchh Mainland Fault, North Wagad Fault and South Wagad Fault, in the vicinity of the epicentre of the 2001 January 26, Mw 7.7 Bhuj earthquake. The largest triggered event of magnitude 3.2 occurred ∼7 hr after the 2009 October 7, Mw 7.7 Vanuatu event.

A Monte Carlo based algorithm is developed to improve the 1D velocity inversion routines and minimize bias due to the choice of a starting model. Using this algorithm, a well-resolved six-layer minimum 1D velocity model, down to -24 km depth, is determined for the Garhwal-Kumaun Himalaya. A total of 4765 P-and 4724 S -phase travel times of local earthquakes recorded at 53 broadband seismic stations are used for this purpose. The travel time-distance curves from these carefully analyzed phase data of events are used to subsequently derive a prior 1D velocity model. Forward modeling of the travel time-distance curve yields an average Moho depth of -46 km and bulk crustal P-and S -wave velocity values of 7.60 and 4.47 km/s, respectively. To circumvent the subjectivity due to manual intervention in the inversion and automate the process, we propose a Monto Carlo style semirandom generation of initial trial velocity models, guided by the initial values derived from forward modeling. The estimated minimum 1D model reveals P-and S -wave velocities increasing from 5.17 to 6.85 km/s and 3.12 to 3.82 km/s, respectively, from the surface to a depth of 26 km. Subsequently, an optimum model is constructed for the region by incorporating the Moho layer in the minimum 1D model.