Mount Erebus, the world’s southernmost active volcano in Antarctica, offers a unique opportunity to investigate crustal controls on persistent magmatism within a caldera environment. New seismic constraints reveal a thin (~ 20 km) and compositionally heterogeneous crust beneath Erebus, consistent with long-lived lithospheric extension associated with the Terror Rift and the Ross Sea. Bulk crustal Vp/Vs ratios vary from ~ 1.61–1.94, indicating pronounced crustal heterogeneity. Elevated Vp/Vs at mid-crustal depths, combined with localized reductions in shear-wave velocity, define thermally modified zones that are consistent with intrusive complexes or transient magma storage above the Moho. While these features align with the mafic-intermediate volcanic products of Mount Erebus, interpretations invoking partial melt or compositional variations remain inherently non-unique. Additionally, a CO2 rich magma reservoir and localised silica-enriched domains could further modify crustal properties and locally reduce Vp/Vs ratio. Although deeper mantle contributions to Erebus magmatism remain unresolved, the results emphasize the importance of crustal-scale processes in sustaining long-lived volcanism beneath Mount Erebus. This study highlights the importance of integrating geophysical data to understand complex melt distribution beneath caldera systems and offers new insights into magma plumbing in extensional intraplate settings.

In this study, we computed the Rayleigh wave group velocity tomography of northeast India (NEI) to a higher resolution of 2°×2° for a 15 to 80-second period. The group velocity dispersion obtained from the tomography was inverted using two ways – (a) inversion for every 0.2 degree of the study area to estimate the 3-D shear wave velocity, which overcome the constraint of sparse seismic station coverage in a few segments of the study region,(b) Joint Inversion of the computed dispersion with the Receiver Function from 22 stations spread across NEI, covering all major geological features, to deduce the shear wave velocity structure. Moho geometry showed significant variation in the region, with IBR (~ 43–62 km) and Himalaya (~ 40–53 km) showing deeper Moho; Assam Valley (~ 33–38 km), Shillong Plateau (~ 30–32 km) and Bengal Basin (~ 37 km) being comparatively shallower. Moho beneath Shillong Plateau is found to be the shallowest (~ 30 km). For stations, TAWA, RUPA, ITAN, and TZR significant back azimuthal variation in shear wave velocity structure is observed. The average crustal shear wave velocity Vs beneath Shillong Plateau (Vs ~ 3.16-3.27 km/s) and Assam Valley (Vs~3.14-3.35 km/s) is found to be lower than the average crustal Vs (~3.75 km/s) beneath the Indian shield. Shillong Plateau and proximal Assam Valley stations showed low uppermost mantle shear wave velocity (Vsn ~ 4.0-4.1 km/s), which might be attributed to factors such as rock composition, grain geometry, higher temperature or the presence of partial melt.The eastern segment of the Assam Valley is not in conformity with the western segment, as evident from the DIBR station at the eastern edge of Assam Valley which doesn’t show this decreased Vsn.Thus indicating prima facia towards different geodynamics along the eastern and western segment of the Assam valley, which might be attributed to the role played by the uplifted, uncompensated Shillong Plateau and/or the Kopli Fault. Relatively higher Vsn (~ 4.2-4.6 km/s) observed beneath the IBR stations can be associated with the deeper moho (~ 43–62 km). Thus the improvised Moho geometry, crustal velocities structure, Vsn could be crucial in understanding the geodynamics of the region and could provide better constraint on the quantification of seismic hazards in the region.

Radon concentration in soil, a well-known short-term precursor of an earthquake, was monitored for about five months in 2018, simultaneously at three monitoring stations in the eastern Himalayas and the Bakreswar-Tantloi geothermal region of east India. The eastern Himalayas is one of the world's six most seismically active regions. In contrast, the geothermal area, although considered 'non-seismic', has experienced small to moderate earthquakes over the years. The recorded data shows that various physical and meteorological parameters influence the emission of radon gas from soil, resulting in complex nonlinear nonstationary radon time series. A nonlinear two-step technique consisting of empirical mode decomposition and Hilbert-Huang transform was applied to remove influences of periodic factors. However, as this method alone is inadequate to identify the effects of geophysical factors, the networking approach was adopted in conjunction. The study looked for precursors for earthquakes of magnitude 5.0 or above within 500 km epicentral distance from each monitoring station; however, for earthquakes of smaller and larger magnitudes, smaller and larger epicentral zones, respectively, were considered. Apart from several seismicity-induced anomalies found in the radon time series of all three stations, one anomaly was observed almost simultaneously from all three monitoring centres preceding an M 5.3 earthquake that occurred on 12 September 2018, in Kokrajhar, Assam. The epicentre of this earthquake was found to be in the common region monitored by the three centres.

In the present study, the spatio-temporal variation of the seismic b-value in the vicinity of the Kopili fault and its surrounding area has been analysed using the unified and homogenous earthquake catalog of historical and instrumental (1950–2021) earthquake events. The study region is subdivided into 16 equisized square grids of 1° × 1° dimension, and the b-value is computed for each grid using the maximum likelihood method. The spatial distribution of the b-value varies from 0.58 to 1.14. The Kolmogorov–Smirnov (K-S) test has been conducted to check the significance of the spatial-temporal and depth-wise distributions of the b-value. The epicentral location of April 28th, 2021, lies in the low-b-value square grid. Likewise, the temporal b-value curve shows a decreasing trend before the occurrence of the April 28th, 2021 earthquake. The mean return period of the April 28th, 2021earthquake and the most probable maximum annual magnitude earthquake are also computed for this region. Meanwhile, the spatial associations and anomalous patterns between the b-value and factors like seismic moment or energy release and focal depth are assessed, as they contribute to a more comprehensive understanding of the seismicity in this area. The antithetical relationship between the b-value and seismic moment or energy release is established. While variation in b-value with depth provides new insights, low b-values are linked to the top of the crust, which could mean that the crust is uniform and that a lot of stress is building up.

The Indo‐Burma Ranges (IBRs) and its surrounding North‐east India is one of the seismotectonically active subduction systems in the world, where the Indian Plate is subducting beneath the Sunda Plate. This has resulted in major earthquakes in the past. In this study, spectral analysis of S‐wave has been used to investigate the source parameters for local earthquakes (3.3 ≤ MD ≤ 5.8) exclusively in the Indo‐Burma region fusing a network of six stations by adopting the Brune model. The corner frequencies (fc) of the events are varied from 0.6 to 3.2 Hz. The estimated source parameters lie in the range from 9 × 1013–3.7 × 1017 N m, 0.2–35.1 MPa, 410–1,956.4 m, 0.002–1.196 m, and 2.9–13.5 Hz for seismic momentM0, stress drop (Δσ), source radius, source dislocation, and maximum cut‐off frequency (fmax), respectively. The scaling relation betweenM0andfchas been derived asM0 = 1.622 × 1016 fc−5.298. The scaling relations ofM0withfmaxand stress drop have also been derived. The high‐stress drops (>10 MPa) were at a depth zone of ~40–60 km, while the stress drops were less than 4.0 MPa beyond this depth, indicating the shallow and deeper portion of the lithosphere to be relatively brittle. We postulate that this could occur due to a complex detachment process as a result of slab tearing in the region. The upper boundary of stress drop was found to have an increasing trend with the focal depth of the earthquakes. The empirical relations betweenM0 − MDandMW − MDhave also been derived for the IBRs. The estimated source parameters and scaling relations will be useful in estimations of lithospheric strength, simulation of strong ground motion in the IBRs, and calibrating the coefficient of the local earthquakes in the IBR and its surrounding region.

Here, we report a Spatio-temporal analysis of the frequency magnitude distribution of earthquakes (b-value) before the 28th April 2021 (Mw 6.4) earthquake event observed in northeast India. To esti- mate the average b-value for the study region, a data set of 750 earthquake events with magnitude Mw ≥ 3.9 is extracted from the homogenous part of the earthquake catalog (1950-2021) documented by the United States Geological Survey (USGS) and International seismological center (ISC) in the region. For spatial analysis of the disparities in b-value, the whole study region is subdivided into 16 square grids of dimension 1o×1o and the b-value is calculated for each subsection. In congruence with other studies, this work yields b-values ranging from 0.66 to 1.25. After the calculation of the b-value for each grid, it is observed that the grid with the epicentral location of the 28th April 2021 (6.4) earthquake has a low b-value. Accordingly, the spatial correlation and aberrant pattern between b-value and focal depth have been comprehensively explored. It is observed that the b-value sig- nificantly dips within a depth range of ~15-35 km which implicates high-stress accumulation and crustal homogeneity. The depth-wise variation in b-value infers the antithetical relationship between b-value and crustal stress. Mostly interplate earthquakes are observed in the study region; thereby hinting at intense seismicity at the upper crust.

The Kopili Fault is one of the most active faults in northeast India, which is the causative source of several earthquakes. The zone is of great importance to the scientific community because of its seismic productivity in past as well in present, which includes two major earthquakes (M > 7), several moderate earthquakes, and most recent 28th April 2021 Mw 6.5 earthquake. In the present study, shear velocity crustal structure is mapped and the b-value is calculated near the Kopili Fault region using receiver function modelling and Gutenberg–Richter relation, respectively. Joint inversion of receiver function and Rayleigh wave group velocity dispersion data beneath Tezpur (TZR) station shows 40 km thick crust with 18 km upper and 22 km lower crust. However, thin crust (35 km) is observed beneath Mikir Hills. The overall b-value in the Kopili region is estimated to be 0.94. We observed an increase in b-value with depths, which may be due to the strength of crust and upper mantle material caused by the rise in temperature and pressure. The b-value variation with depth shows a sudden increase of b-value at a depth of 40–45 km, which corresponds to the crust–mantle transition, estimated from the receiver function modelling in the region. A sudden reduction of b-value is observed prior to 28th April 2021 Mw 6.5 earthquake, which underscores investigations of the b-value and its variations for the purpose of earthquake precursor studies in that area.

Activity of 222 Rn gas in soil has been recorded continuously at three monitoring centres, namely Ravangla and Diphu in the Eastern Himalayan region and Tantloi in the geothermal region of eastern India in order to determine possible precursors induced by seismic activity within a few hundred kilometres of the monitoring stations. The recorded data show that various physical and meteorological parameters affect the outflow of radon gas from soil, leading to very complex nonlinear non-stationary soil 222 Rn time series. Therefore, a two-step nonlinear technique consisting of empirical mode decomposition and Hilbert–Huang transform has been used for analysis of the simultaneously recorded soil radon data. A number of precursory anomalies caused due to earthquakes of magnitude around 5.0 within 500 km epicentral distance from each monitoring station have been found in the recorded time series of radon in soil at all three centres. An anomaly has been observed simultaneously from the three monitoring stations preceding an earthquake in the common region monitored by the centres. This demonstrates that monitoring of soil radon precursor by a network of stations can be a promising method for understanding earthquake generating processes.

The spatial distribution of the b‐value has been determined using a homogeneous earthquake catalogue from 1964 to 2018 to examine the characteristics of structural heterogeneities and their bearing on the style of faulting in the Indo‐Burma region of Northeastern India. The study region is associated with an uneven distribution of structural heterogeneities and demonstrates the mixed type of faulting, nature where the majority of earthquakes resulted mainly by thrust and strike‐slip fault. Our critical analysis demonstrates that the existence of two prominent zones is associated with high b‐values and low b‐values where the mixed type of faulting resulted in the genesis of seismicity in the study area. The high b‐values in the study area contain a large number of smaller earthquakes whilst the low b‐values comprise a smaller number of larger earthquakes distributed along with the Naga‐Disang Thrust, Kabaw Fault, and Sagaing fault systems. A close comparison between b‐values and different faulting styles reveals that the area containing a low b‐value suggests stronger seismogenic zones, where an accumulation of strain energy leads to the genesis of seismicity by thrust faulting with slight slip component whilst Indo‐Burma Ranges regions connected with b‐values (0.5–0.9) demonstrate strike‐slip to the normal mode of faulting, indicating relatively weak seismogenic zones where tensional forces are prevalent. The depth sections illustrate that lower b‐values for the upper part of the subducting slab correspond to more stress generated by tectonics compared to that of the lower part, and interestingly the deeper part of the subducting Indian slab is coupled with conspicuously very high‐b values, suggesting the weakening of the Indian subducting slab through dehydration.

Sri Lanka occupied a unique position in the Eastern Gondwana assembly, surrounded by its Gondwana neighbours: Madagascar, India, Antarctica and Africa. It comprises four distinct lithologic units: the Highland Complex, the Wanni Complex, the Vijayan Complex and the Kadugannawa complex. Crustal properties (crustal thickness and velocities) in Sri Lanka have been estimated by inversion modeling of receiver functions computed using earthquake data from three stations that operated between 2011 and 2018. Results reveal similar to 38 km crustal thickness and high average shear wave velocity (Vs similar to 3.7-3.8 km/s) beneath Sri Lanka, low average Vp/Vs ratio (similar to 1.72) in the Highland Complex, and high Vp/Vs ratio (1.79) in the Wanni Complex. This suggests that the crust beneath Sri Lanka is felsic to intermediate in nature and the probable rocks are of granite to granodioritic in composition. The probable rocks in Wanni Complex are felsic to mafic granulite in nature. The crustal thickness and average Vp/Vs ratio in the Highland Complex shows similarity with its Gondwana neighbours, denoting its unique position in Gondwana. The average crustal shear wave velocity in Sri Lanka is also found to be high (similar to 3.7 km/s), which is also comparable with its Gondwana neighbours.

The crust and upper mantle shear wave velocity structure beneath the northeast India is estimated by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data recorded at nine broadband seismic stations. The Assam valley and the Shillong-Mikir plateau are the two important tectonic blocks in the northeast India, which are surrounded by the Himalayan collision zone in the north, Indo-Burma subduction zone in the east and by the Bengal basin in the south. The joint inversion followed by forward modeling reveal crustal thicknesses of 30-34 km beneath the Shillong plateau, 36 km beneath the Mikir hills and 38-40 km beneath the Assam valley with an average shear wave velocity (Vs) of 3.4-3.5 km/s. The estimated low upper mantle shear wave velocity (Vsn) 4.2-4.3 km/s may be due to the rock composition or grain size or increased temperature and partial melt (< 1%) in the upper mantle, or an effect of all. Also, we report for the first time, the existence of the Hales discontinuity at depths 56-74 km with Vs similar to 4.4-4.6 km/s. Variable depth of the Hales discontinuity may be explained by the geotherm and/or addition of Cr3+ and Fe2+ in the spinel-garnet system.

Seismic anisotropy of crust beneath the Shillong–Mikir Plateau and the surrounding regions of northeast India have been investigated with the help of splitting analysis of S-wave of local earthquakes. We estimate a total 83 pairs of splitting parameters (Φ and δt) from 67 local shallow focus earthquakes (depth ≤ 32 km) recorded by the 10 broadband seismological stations operated in the study region. The results show delay times ranging from 0.02 to 0.2 s, which correspond to anisotropy up to 4%, suggesting significant strength of anisotropy in the study region. Fast polarization direction (Φ) in the Shillong Plateau shows mostly NW–SE trend in the western part and NE–SW trend in the northern part. Φs near Kopili fault (KF) follows NW–SE trend. Φ at most of the stations in the study region is consistent with the local stress orientation, suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field. On the other hand, anisotropy observed near the KF is due to aligned macroscopic fracture related to strike-slip movement in the fault zone.

In this study, we carry out detailed analysis of the sedimentary structures beneath the northeastern region (NER) of India, by inverting receiver functions from 9 broadband seismic stations, with applying a well known non-linear and derivative free direct search approach, Neighbourhood Algorithm (NA). Thick sediments (with low shear wave velocities and high Vp/Vs ratio) can amplify seismic waves, which can cause a huge damage due to moderate earthquakes in the region. Results show significant variability of sedimentary layer thicknesses from 0.2-1.0 km beneath the Shillong Plateau, which is the rupture area of great 1897 Shillong earthquake, 0.4-3.1 km beneath the Assam Valley region and 4.1-6.2 km beneath the Lesser Himalayan region. The average sedimentary layer of S-wave velocities for the majority of the stations in the study area varies between similar to 1.0 and 2.7 km/s with very high Vp/Vs ratio. The Sedimentary structure corresponds to thickness, shear wave velocity and Vp/Vs ratio (Poisson's ratio). Since NER lies in the seismic zone five of the country; these important seismic parameters can be useful in seismic hazard assessment and construction of earthquake-resistant structure to survive from strong earthquake shaking.

b-value is one of the most significant seismic parameters for describing the seismicity of a given region at a definite time window. In this study, high-resolution map of the Gutenberg-Richter b-value, seismic moment release, Bouguer gravity anomaly and fault-plane solutions containing faulting styles are analyzed in the Indo-Burma ranges of northeast India using the unified and homogeneous part of the seismicity record in the region (January 1964-December 2016). The study region is subdivided into few square grids of geographical window size 1 degrees x 1 degrees and b-values are calculated in each square grid. Our goal is to explore the spatial correlations and anomalous patterns between the b-value and parameters like seismic moment release, Bouguer gravity anomaly and faulting styles that can help us to better understand the seismotectonics and the state of present-day crustal stress within the Indo-Burma region. Most of the areas show an inverse correlation between b-value and seismic moment release as well as convergence rates. While estimating the b-value as a function of depth, a sudden increase of b-value at a depth of 50-60 km was found out and the receiver function modeling confirms that this depth corresponds to the crust-mantle transition beneath the study region. The region is also associated with negative Bouguer gravity anomalies and an inverse relation is found between Gravity anomaly and b-value. Comparing b-values with different faulting styles, reveal that the areas containing low b-values show thrust mechanism, while the areas associated with intermediate b-values show strike-slip mechanism. Those areas, where the events show thrust mechanism but containing a strike-slip component has the highest b-value.

Spatial variation of seismic b-value is estimated in the Indo-Myanmar subduction zone of northeast (NE) India using the homogeneous part of earthquake catalogue (1996-2015), recorded by International Seismological Center (ISC), consisting of 895 events of magnitude M-w >= 3.9. The study region is divided into 1 degrees x 1 degrees square grids and b-values are estimated at each grid by maximum likelihood method. In this study, the b-value varies from 0.75 to 1.54 in the region. Significant variation of low b-value in the respective location may indicate high stress accumulation in that region. Spatial variation reveals intermediate b-value anomalies around the epicenter of the M-w = 6/Manipur earthquake which occurred on 3rd January at 23:05 UTC (4 January 2016 at 04:35 IST). The variations of b-values are also estimated with respect to depth. The low b-value associated with the depth range similar to 15-55 km, which may imply crustal homogeneity and high stress accumulation in the crust. Since, NE India lies in the seismic zone V of the country; this study can be helpful to understand seismotectonics in the region.

Microearthquake spectra from the Kopili region, one of the seismic prone areas of northeast India, are examined to observe the effect of attenuation on these spectra. The spectral ratio method is utilized in order to estimate Q values for both P and S waves in the subsurface layer wherein the ratio of spectral amplitudes at lower and higher frequencies are taken into consideration for six stations at varying epicentral distances. Average values of Q(p) and Q(s) are computed to be similar to 481 +/- 36 and 966 +/- 98, respectively. Q(s)/Q(p) having a value greater than 1 is contemplated in major parts of Kopili and the neighboring area, which can be ascribed to underlying sediment deposits. The estimates of attenuation parameters are well substantiated by site geology. The values obtained in this study are in good conformity with the worldwide average. The estimates will help in seismic hazard estimation of this seismically active area.

We estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast India, by using H-Vp/Vs stacking method and a non-linear direct search approach, Neighbourhood Algorithm (NA) technique followed by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data. Results show significant variations of thickness, shear velocities (Vs) and Vp/Vs ratio in the crust of the study region. The inverted shear wave velocity models show crustal thickness variations of 32–36km in Shillong Plateau (North), 36–40 in Assam Valley and ∼44km in Lesser Himalaya (South). Average Vp/Vs ratio in Shillong Plateau is less (1.73–1.77) compared to Assam Valley and Lesser Himalaya (∼1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1–2km thick sediment layer with low Vs (2.5–2.9km/s) and high Vp/Vs ratio (1.8–2.1), while it is observed to be of greater thickness (4km) with similar Vs and high Vp/Vs (∼2.5) in RUP (Lesser Himalaya). Both Shillong Plateau and Assam Valley show thick upper and middle crust (10–20km), and thin (4–9km) lower crust. Average Vp/Vs ratio in Assam Valley and Shillong Plateau suggest that the crust is felsic-to-intermediate and intermediate-to-mafic beneath Shillong Plateau and Assam Valley, respectively. Results show that lower crust rocks beneath the Shillong Plateau and Assam Valley lies between mafic granulite and mafic garnet granulite.

We focused on validation of applicability of semi-empirical technique (spectral models and stochastic simulation) for the estimation of ground-motion characteristics in the northeastern region (NER) of India. In the present study, it is assumed that the point source approximation in far field is valid. The one-dimensional stochastic point source seismological model of Boore (1983) (Boore, DM. 1983. Stochastic simulation of high frequency ground motions based on seismological models of the radiated spectra. Bulletin of Seismological Society of America, 73, 1865–1894.) is used for modelling the acceleration time histories. Total ground-motion records of 30 earthquakes of magnitudes lying between MW 4.2 and 6.2 in NER India from March 2008 to April 2013 are used for this study. We considered peak ground acceleration (PGA) and pseudospectral acceleration (response spectrum amplitudes) with 5% damping ratio at three fundamental natural periods, namely: 0.3, 1.0, and 3.0 s. The spectral models, which work well for PGA, overestimate the pseudospectral acceleration. It seems that there is a strong influence of local site amplification and crustal attenuation (kappa), which control spectral amplitudes at different frequencies. The results would allow analysing regional peculiarities of ground-motion excitation and propagation and updating seismic hazard assessment, both the probabilistic and deterministic approaches.

In this study, we aim to improve the scaling between the moment magnitude（MW）, local magnitude（ML）, and the duration magnitude（MD） for 162 earthquakes in Shillong-Mikir plateau and its adjoining region of northeast India by extending the MWestimates to lower magnitude earthquakes using spectral analysis of P-waves from vertical component seismograms. The MW-MLand MW-MDrelationships are determined by linear regression analysis. It is found that, MWvalues can be considered consistent with MLand MD, within 0.1 and 0.2 magnitude units respectively, in 90 % of the cases. The scaling relationships investigated comply well with similar relationships in other regions in the world and in other seismogenic areas in the northeast India region.

Modified form of Nakamura method,H/Vratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated fromH/Vratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness andVS. With the help of this, the probableVSvalues for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city.