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We investigated sedimentary thickness and shear wave velocity structure in the western part of the Indo-Gangetic Plain (Punjab and Haryana Plain) and adjoining Siwalik Himalaya with the help of receiver function inversion at 20 broad-band seismological stations. This region is one of the most seismically vulnerable zones of the world due to the presence of thick surface sediments in the foreland basin that can amplify seismic waves and cause huge damage due to the earthquakes of the Himalaya. The study reveals a progressive thickening of sediments from southwest to northeast. The basement depth varies from ∼1.5 to 1.7 km in the Central Alluvium Plain, ∼1.8 to 2.8 km in the Zone of Terminal Fans, and attains a maximum of ∼3.8 km near the Himalayan Frontal Thrust. The inverted models show the presence of soft alluvial with extremely low Vs (< 0.5 km s−1) and high Vp / Vs (∼2.5–3.0) at the top ∼400–700 m of the surface at most of the stations. A comparatively higher velocity of surface sediments observed at northern stations suggests the presence of compact sediments at the surface. The layered sedimentary structure revealed by the S -wave velocity models supports the previous geophysical investigations using borehole data. The velocity–depth structure obtained in this study is important for evaluating the seismic hazard of the densely populated urban areas spread over this region. [ABSTRACT FROM AUTHOR]

Crustal configuration beneath the indenting northeast corner of the Indian Plate in the Eastern Himalayan Syntaxis has been investigated with the help of receiver function (RF) analysis of teleseismic earthquakes recorded by 19 broad-band seismological stations. The common conversion point stacking of RFs and 1-D velocity models obtained through inversion provide new information on the intracrustal structure. The study reveals the signature of the Main Himalayan Thrust (MHT) beneath the Lohit Valley at ∼22–26 km depth. The MHT is not prominent in the Siang window plausibly due to large-scale crustal deformation related to the formation of the window and antiform folding. Unlike in the western and central Himalaya, the MHT does not play a major role in seismogenesis in the Lohit Valley and Siang Window, where seismicity is active up to the crustal depth of ∼40 km. The crustal thickness increases from ∼38 km at Pasighat in the south to ∼50 km at the northernmost station (Gelling) in the Siang window. In Lohit Valley, the crustal thickness increases from ∼40 km at Mahadevpur in the west to ∼54 km in the Tidding–Tuting suture zone, which again shallows to ∼51 km in the eastern Lohit Plutonic Complex (Walong station). The thinner crust beneath the Tidding–Tuting suture compared to the Indus Tsangpo Suture Zone of northwest Himalaya is caused due to the differences in convergence rate, higher exhumation rate and mechanisms to accommodate collision and rotational tectonics. [ABSTRACT FROM AUTHOR]

We estimate the source parameters (stress drop, corner frequency, source radius and scalar seismic moment) for local earthquakes that occurred during 2007–2008 in the Lohit Valley, Arunachal Himalaya in northeast India. The source parameters of 33 selected earthquakes (2.0 ≤ ML ≤ 4.4) are estimated by spectral analysis of P‐waves of the vertical component. The results are authenticated further by spectral analysis of S‐waves of horizontal component seismograms. Both P‐ and S‐wave spectral analyses show comparable results. The earthquakes' seismic moment varies from 2.66 × 1012 to 1.16 × 1015 N.m. The estimates of stress drop and source radius range from ~0.145 to 31.85 bar and ~242 to 647 m, respectively. The majority of the earthquakes show a low‐stress drop (≤10 bar). We interpret that the low‐stress drop is plausibly due to the presence of creeping patches in the thrust sheets in the study area within the upper ~10 km depth, which is not able to withstand much stress and release it in the forms of micro‐earthquakes. The scaling relations have been developed between important source parameters (ML, Mw, M0, ∆σ, and fc), which can serve as useful inputs for the assessment of earthquake hazards in the region. [ABSTRACT FROM AUTHOR]

Crustal configuration beneath the northeastern region of India has been investigated with the help of receiver function (RF) analysis of teleseismic earthquakes recorded by 19 broadband seismological stations. We adopted the H‐k stacking method to estimate crustal thickness and Poisson's ratio beneath each recording station. The study reveals a large variation in crustal thickness and Poisson's ratio which are correlated with the complex geology and tectonics of the region. The crust is observed to be thinner (36.5–41.6 km) beneath Bengal Basin, Shillong Plateau, and the Brahmaputra valley compared to the Indo‐Burma Ranges (IBR) (~40–54 km) and Arunachal Higher Himalaya (TAWA station, ~45 km) and Sikkim Himalaya (GTK station, ~46.5 km). A large variation of Poisson's ratio is observed in the region (~0.230–0.306). Poisson's ratio is generally low‐to‐intermediate in the Shillong‐Mikir Plateau, Bengal Basin, and the Brahmaputra Valley, while it is intermediate‐to‐high in the Tripura Fold Belt and the northern part of the IBR. The high Poisson's ratio in the Tripura Fold Belt is due to the presence of basaltic basement rock and clay minerals existing in the sedimentary rocks, whereas the presence of partially serpentinized rock in the ophiolitic mélange complex causes a high Poisson's ratio in the IBR. [ABSTRACT FROM AUTHOR]

Since 1934, two great earthquakes (Mw ≥8.0) and some eight large/strong earthquakes (Mw 6.5–7.8) occurred in the eastern Himalayas, from eastern Nepal to the Eastern Himalayan Syntaxis (EHS) Zone. All these events are instrumentally recorded by local as well as by the global network. From west to east these events are the 2015 two large/devastating earthquakes (Mw 7.8 and 7.3, respectively) in eastern Nepal, the 1934 and 1988 Bihar–Nepal earthquakes (Mw 8.1 and 6.8, respectively), the 2011 Sikkim (Mw 6.9), the 1967 and 2009 Bhutan (Mw 6.5 and 6.3, respectively), 1964 and 1947 Arunachal–Tibet (Mw 6.5 and 7.0, respectively), and the 1950 Assam–Tibet great (Mw 8.4) earthquakes. Most recently, on 28 April 2021, an earthquake Mw 6.1 occurred in the Assam foredeep. These events are documented as shallow (0–20 km) Main Himalayan Thrust (MHT) earthquakes in the International Seismological Center and National Earthquake Information Center—USGS catalogues. With the advent of local digital seismic networks and better precision of hypocentre locations, the eastern Himalayan earthquake source zones are found to be much different; these are not typically MHT events. Precise hypocentre locations indicate bimodal seismicity at shallower (0–20 km) as well as at deeper depths (40–70 km) in the eastern Himalayas. Seismic cross‐sections and fault plane solutions of the earthquakes do not support a uniform seismotectonic model for the entire Himalayas. All the instrumentally recorded past great earthquakes in the Himalayas and the recent destructive earthquakes in the eastern Himalayas study region are critically examined in view of the known seismotectonic model, and we suggest that each large/great earthquake occurred in its own unique complex tectonic environment. The transverse structures play a major role in generating the deeper earthquakes in the eastern Himalayas and the EHS; the shallower MHT may be flattened. [ABSTRACT FROM AUTHOR]

Eight medium magnitude (Mw 5.0–5.9) severely felt earthquakes, mostly at a shallower depth (<35 km), occurred in a short period of 7 months only, April–October, 2020, in Mizoram State, north‐east region (NER) of India, and it created much panic in the state. The increasing population growth as well as rapid urbanization in Aizawl, the capital city of Mizoram, may be under severe threat for an impending large earthquake in the region. The NER, India, is jawed between two arcs, the Himalayan Arc collision zone to the north and the Indo–Burma (Myanmar) Arc subduction zone to the east, and witnessed two great earthquakes (Mw ≥ 8.0, 1897 and 1950) and a total of 25 large earthquakes (Mw ≥ 7.0) in the past since 1869. The 2020 sudden burst of seismic activity in Mizoram, along with a strong (Mw 6.7) earthquake in Manipur in 2016 and a medium magnitude (Mw 5.7) earthquake in 2017 in Tripura, adjacent to the Mizoram State, are critically examined to understand the ongoing seismotectonics and the precursor implications, if any. The 2016 Manipur earthquake is interpreted to be an intra‐plate earthquake triggered by the transverse Kopili Fault by strike‐slip motion. The 2017 Tripura earthquake is also interpreted to be typically an intra‐plate strike‐slip event in the eastern margin of the Bengal Basin. The seismic activity in Mizoram, on the other hand, is found to have occurred in the accretionary Indo–Burma Wedge, between the Churachandpur Mao Fault and Eastern Boundary Thrust. The centroid moment tensor solutions of the events suggest the dextral‐slip motion of the wedge along the Indo–Burma Arc. A preliminary analysis of the Mizoram seismicity recognizes a precursor anomaly, a temporal variation of much higher seismic activity, for an impending large earthquake in the study area. [ABSTRACT FROM AUTHOR]

We analyse P -wave receiver functions across the Kumaon Himalaya and adjoining area to constrain crustal thickness, intracrustal structures and seismic velocity characteristics to address the role of the underlying structure on seismogenesis and geodynamic evolution of the region. The three-component waveforms of teleseismic earthquakes recorded by a seismological network consisting of 18 broad-band seismological stations have been used for receiver function analysis. The common conversion point (CCP) depth migrated receiver function image and shear wave velocity models obtained through inversion show a variation of crustal thickness from ∼38 km in the Indo-Gangetic Plain to ∼42 km near the Vaikrita Thrust. A ramp (∼20°) structure on the Main Himalayan Thrust (MHT) is revealed beneath the Chiplakot Crystalline Belt (CCB) that facilitates the exhumation of the CCB. The geometry of the MHT observed from the receiver function image is consistent with the geometry revealed by a geological balanced cross-section. A cluster of seismicity at shallow to mid-crustal depths is detected near the MHT ramp. The spatial and depth distribution of seismicity pattern beneath the CCB and presence of steep dipping imbricate faults inferred from focal mechanism solutions suggest a Lesser Himalayan Duplex structure in the CCB above the MHT ramp. The study reveals a low-velocity zone (LVZ) with a high Poisson's ratio (σ ∼0.28–0.30) at lower crustal depth beneath the CCB. The high value of Poisson's ratio in the lower crust suggests the presence of fluid/partial melt. The shear heating in the ductile regime and/or decompression and cooling associated with the exhumation of the CCB plausibly created favorable conditions for partial melting in the lower crustal LVZ. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Crustal thickness varied from ∼38 km beneath IGP to ∼41 km near the Vaikrita Thrust. • Poisson's ratio in the crust is extremely high in the Chiplakot Crystalline belt (CCB) • Presence of partial melt/fluid is the possible cause of high Poisson's ratio. • Clustered seismicity is observed in the CCB at mid crustal depth. Abstract Crustal thickness and Poisson's ratios were estimated at 13 broadband seismological stations established in a profile along the Kali river valley, Kumaon (Central) Himalaya. The seismological profile extends from the Indo-Gangetic plain (IGP) in the south to the Higher Himalaya in the north, passing through the Sub and Lesser Himalaya. The Receiver Function (RF) method has been adopted to investigate the crustal structure beneath the profile. Time domain iterative deconvolution method has been adopted for RF computation. The H- k stacking method has been adopted to analyze the RFs for obtaining average crustal thickness and Poisson's ratio (σ) of the crust beneath each station. The study reveals that the crustal thickness beneath the IGP is ∼38 km which gradually increases up to ∼41 km at the northernmost station located in the Higher Himalaya. The values of σ vary from 0.23 to 0.28. The value of σ is low in the Sub-Himalaya and outer Lesser Himalaya suggesting more of a felsic composition of crust. A Significantly high value of σ (∼0.28) is observed in the Dharchula region of the Chiplakot Crystalline belt which is coincident with a large number of micro-to-moderate magnitude earthquakes forming a cluster at shallow to mid-crustal depths. The high Poisson's ratio is possibly due to the presence of fluids/partial melts at mid-crustal depth. The presence of fluids influences the rheological property and controls the mechanical and shear strength of crustal rocks producing a cluster of seismicity observed beneath the Dharchula region. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Strong-motion records of recent Gorkha Nepal earthquake ( M 7.8), its strong aftershocks and seismic events of Hindu kush region have been analysed for estimation of source parameters. The M 7.8 Gorkha Nepal earthquake of 25 April 2015 and its six aftershocks of magnitude range 5.3-7.3 are recorded at Multi-Parametric Geophysical Observatory, Ghuttu, Garhwal Himalaya (India) >600 km west from the epicentre of main shock of Gorkha earthquake. The acceleration data of eight earthquakes occurred in the Hindu kush region also recorded at this observatory which is located >1000 km east from the epicentre of M 7.5 Hindu kush earthquake on 26 October 2015. The shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification. The strong-motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor ( Q ) at recording site. The frequency-dependent Q ( f) = 124 f is computed at Ghuttu station by using inversion technique. The corrected spectrum is compared with theoretical spectrum obtained from Brune's circular model for the horizontal components using grid search algorithm. Computed seismic moment, stress drop and source radius of the earthquakes used in this work range 8.20 × 10-5.72 × 10 Nm, 7.1-50.6 bars and 3.55-36.70 km, respectively. The results match with the available values obtained by other agencies. [ABSTRACT FROM AUTHOR]

Seismic anisotropy of the crust beneath the eastern Ladakh-Karakoram zone has been studied by shear wave splitting analysis of S -waves of local earthquakes and P -to- S or Ps converted phases originated at the crust-mantle boundary. The splitting parameters (Φ and δ t ), derived from S -wave of local earthquakes with shallow focal depths, reveal complex nature of anisotropy with NW-SE and NE oriented Fast Polarization directions (FPD) in the upper ∼22 km of the crust. The observed anisotropy in the upper crust may be attributed to combined effects of existing tectonic features as well as regional tectonic stress. The maximum delay time of fast and slow waves in the upper crust is ∼0.3 s. The Ps splitting analysis shows more consistent FPDs compared to S -wave splitting. The FPDs are parallel or sub parallel to the Karakoram fault (KF) and other NW-SE trending tectonic features existing in the region. The strength of anisotropy estimated for the whole crust is higher (maximum delay time δ t : 0.75 s) in comparison to the upper crust. This indicates that the dominant source of anisotropy in the trans-Himalayan crust is confined within the middle and lower crustal depths. The predominant NW-SE trending FPDs consistently observed in the upper crust as well as in the middle and lower crust near the KF zone support the fact that the KF is a crustal-scale fault which extends at least up to the lower crust. Dextral shearing of the KF creates shear fabric and preferential alignment of mineral grains along the strike of the fault, resulting in the observed FPDs. A Similar observation in the Indus Suture Zone (ISZ) also suggests crustal scale deformation owing to the India-Asia collision. [ABSTRACT FROM AUTHOR]

The seismotectonic scenario of the northwest part of the India-Asia collision zone is studied by analyzing the local earthquake data (M ~ 1.4–4.3) recorded by a broadband seismological network consisting of 14 stations. Focal Mechanism Solutions (FMSs) of 13 selected earthquakes were computed through waveform inversion of three-component broadband records. Depth distribution of the earthquakes and FMSs of local earthquakes obtained by waveform inversion reveal kinematics of the major fault zones present in eastern Ladakh. A most pronounced cluster of seismicity is observed in the Karakoram Fault (KF) zone down to a depth of ~ 65 km. The FMSs reveal a transpressive environment with an inferred strike slip fault plane parallel to the KF. It is argued that the KF penetrates down to the lower crust and is a manifestation of active underthrusting of Indian lower crust beneath Tibet. Two clusters of microseismicity are observed at a depth range 5–20 km at the northwestern and southeastern fringes of the Tso Morari gneiss dome, which can be correlated to the activities along the Zildat fault and Karzok fault, respectively. The FMSs obtained for representative earthquakes show thrust fault solutions for the Karzok fault, and normal fault solutions for the Zildat fault. It is suggested that the Zildat fault is acting as a detachment, facilitating the exhumation of the Tso Morari dome. On the other hand, the Tso Morari dome is underthrusting the Karzok ophiolite at its southern margin along the Karzok fault due to gravity collapse. [ABSTRACT FROM AUTHOR]

Teleseismic data recorded at 13 broad-band seismological stations across northwest part of the Tethyan Himalaya and eastern Ladakh are analysed to determine the seismic characteristics of the crust and upper mantle beneath the northwest India-Asia collision zone. The receiver functions computed from teleseismic P-waveform for a wide range of backazimuth show strong azimuthal variation in the Indus suture zone (ISZ), the zone which marks the collision and subsequent subduction of both the Tethyan oceanic plate and Indian continental plate beneath Eurasia. The teleseismic waves piercing the ISZ do not show clear P-to-S (Ps) converted phase at the depth of Moho. In contrast, the waves piercing the Karakoram zone, Ladakh batholith and the Tethyan Himalayan region south of the ISZ clearly show the Moho converted Ps phase and corresponding inverted models reveal variation of crustal thickness from ~60 km beneath the Tethyan Himalaya to ~80 km beneath the Karakoram fault zone. A prominent intracrustal low velocity zone (IC-LVZ) is detected in the shear wave velocity models within the depth range ~15-40 km. The IC-LVZ identified at the stations both north and south of the ISZ can be interpreted as due to presence of fluid/partial melt. Our study provides compelling evidence that the mid-crustal low velocity zone does extend across the suture zone, in to the Tethyan Himalaya. The contact between this serpentinized ultramafic rocks and the eclogitized Indian continental crust in the suture zone is identified at ~47-50 km depth. [ABSTRACT FROM AUTHOR]

Highlights: [•] Shear-wave velocity structure of the Shillong–Mikir plateau using teleseismic P-wave receiver function analysis. [•] Thinner Moho beneath the Shillong Plateau than the Brahmaputra valley. [•] Crustal thickness beneath the Himalayan foredeep region ∼46–48km. [•] A high velocity layer is observed in NE backazimuth of SHL. [•] Underthrusting of Bengal sediments at the segment of the Dauki fault zone. [Copyright &y& Elsevier]

SUMMARY Crustal structure across the India-Asia collision zone (Tidding Suture) in the northeast Himalaya bounded by the Eastern Himalayan Syntaxis (EHS) is investigated using the P-wave receiver function (RF) method. The analysed data included three-component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad-band seismic stations. The RFs and inverted shear wave velocity models reveal azimuthally varying crustal structure. The RFs for earthquakes from the northeast back azimuths are conspicuous by absence of P-to- S converted phase at the Moho discontinuity. Inverted velocity model ascribe this to absence of the typical step velocity jump at the Moho in a narrow section of the EHS bordering the indenting Indian Plate and pierced by travelling waves. In contrast, teleseismic waves arriving from southeast back azimuth sample different litho-tectonic blocks of the Himalayan collision zone and inverted models show northeast dipping Moho across the Tidding Suture. Compared to an overall thickness of >70 km in the northwest and central Himalaya , the crust across the Tidding Suture is only about 55 km thick. This is attributed to a slower rate of convergence in this part of the collision zone. The Moho structure beneath Indian Plate and southeastern Tibetan Plateau reveal opposite dip directions with their colliding margin placed just east of the Walong Thrust. The inverted shear wave velocity models show evidence of intracrustal low-velocity layer whose strength varies across the Tidding Suture. The magnitude of velocity reduction beneath Lohit Plutonic Complex (Trans Himalaya) favour partial melt as a possible mechanism whereas south of the Tidding Suture, where velocity reduction is comparatively less, fluids generated by dehydration reactions appear to be the source for velocity reduction. [ABSTRACT FROM AUTHOR]

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east-west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region. [ABSTRACT FROM AUTHOR]

• Complex pattern of anisotropy is observed in NW Himalaya and Ladakh-Karakoram zone. • Anisotropy direction (FPD) is orogen parallel in frontal part of the Himalaya. • The NE oriented FPDs in NW Himalaya suggests plate motion related anisotropy. • The FPDs observed in Karakoram Fault (KF) zone are roughly parallel to the KF. • The KF is inferred as a lithospheric scale fault. Seismic anisotropy of the upper mantle beneath the northwest Himalaya and eastern part of the Ladakh-Karakoram zone has been investigated using shear-wave splitting of core-mantle refracted SKS waveforms from 94 teleseismic earthquakes recorded at 28 broadband seismic stations. The SKS splitting parameters (Φ and δ t) show a significant strength of anisotropy with a wide range of delay time of split waves (∼0.75−2.94 s). The distinct variations in Fast Polarization Directions (FPDs) along the study section suggest a complex source of anisotropy that cannot be explained by a simple model. In the frontal part of the Himalaya, the FPDs are mostly parallel or sub-parallel to the strike of the Himalayan orogeny suggesting deformation in the shallow lithospheric mantle under compression owing to the India-Asia collision. On the other hand, FPDs observed in the Lesser, Higher, and Tethyan Himalaya largely follow NE-oriented absolute plate motion (APM) of the Indian plate which can be attributed to basal shear as the Indian plate moves over the asthenospheric mantle with a minor contribution from shallow lithospheric deformation. A complex anisotropy pattern is observed in the Indus Suture Zone. The FPDs near the Karakoram Fault Zone (KFZ) are parallel or sub-parallel to the strike of the KF. The strike-slip or transpressional deformation in the lithospheric mantle in the KFZ is considered as the major source of anisotropy beneath the KFZ. The study envisaged that the KF is a lithospheric scale fault that largely accommodates the India-Asia collision and extrusion in the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• Crustal thickness varies within the range ~46–55 km beneath the Lohit Valley. • Poisson's ratio is high near Tidding Suture and Lohit Plutonic Complex. • The possible cause of high Poisson's ratio if the presence of partial melt/fluid. • Bi-modal depth distribution of seismicity is observed beneath the profile. Crustal thickness and Poisson's ratio were estimated at 11 broadband seismic stations established in a profile along the Lohit river valley of Arunachal Himalaya passing across the Tuting-Tidding Suture Zone (TTSZ). The average crustal thickness (H) and Poisson's ratio were estimated with the help of H- k stacking analysis of receiver functions at each station. The study reveals a thickening of crust from ~46 km beneath the Brahmaputra valley in the west to ~55 km in the western part of the Lohit Plutonic Complex (LPC). Marginal uplifts of Moho depths (~50–52 km) are observed at some stations to the eastern part of the profile (LTL, HWI and SDL stations). The estimated Poisson's ratio in the Brahmaputra valley is low (0.23), suggesting the felsic composition of the crust. The Poisson's ratio is intermediate in the Mishmi Thrust zone (0.249–0.261) and in some parts of the LPC. High Poisson's ratio (0.277–0.293) is observed beneath the TTSZ and western part of the LPC, indicating the effect of aqueous fluid/partial melt present in the crust. The seismicity in the study area has been investigated based on data recorded by our network and data obtained from the reviewed catalogue of the International Seismological Center (ISC) bulletin. Seismicity is observed to be distributed mainly at a shallow depth range ~1–15 km. However, earthquakes of M > 4.0 are primarily concentrated at ~25–35 km depths. This bimodal distribution of seismicity has a significant role in understanding subsurface configuration beneath the profile. [ABSTRACT FROM AUTHOR]

The article discusses the precaution observed in soil and water radon data at a multi-parametric geophysical observatory (MPGO) at Ghuttu, Garhwal Himalaya in India for the Nepal earthquake. Topics include measuring the radon at MPGO in the soil and underground water, the influence of environmental and hydrological effects on the data, and the potential of the observation for earthquake precursory research.

Over years, soil radon (Rn-222) measurement was started at Multi-parametric Geophysical Observatory (MPGO), Ghuttu, Garhwal Himalaya to assess the earthquake precursory signatures. We carried out a statistical analysis to examine anomalous soil radon emanation during the occurrence of local earthquakes. Twenty earthquakes of moderate and bigger magnitude are occurred within 300 km from MPGO, Ghuttu during 2009–2017. Continuous time series highlight a high effect of rainfall precipitation on the soil radon emanation measured at 10 m depth. During monsoon period (June to September), high rain precipitation at the recording site cause a high variation in the radon emanation. In spite of our best efforts, it is difficult to isolate the complex behavior of heavy and abrupt occurrence of rain from the soil radon data. Preferably, the data of 11 events of the pre- and post-monsoon are evaluated to identify the seismic origin effect on soil radon. To examine these anomalous variations, statistical analysis of soil radon data is carried out by calculating mean (m), and then obtaining standard deviation (σ) from mean values. Changes in the soil radon concentration are treated anomalous for values exceeding one and two standard deviations (m±σ and m±2σ) from the mean value for the selected duration. Two nearby and the strongest events suggests pre-seismic variations which are related to the earthquake precursory signatures. The observed results are explained in the light of dilatancy- diffusion model. • Nine years soil radon data, measured at 10 m depth during 2009-2017 is assessed to investigate earthquake precursors. • Temporal variations in radon highlight earthquake precursor related to very close (300 km) and strongest earthquakes (M≥4.5). • Changes in the radon concentration are treated anomalous for values exceeding one and two standard deviations (m±σ and m±2σ). • Seismic index and PGA measurements quantify the seismic effect depending up on earthquake size and closeness. • Significant anomaly of moderate magnitude earthquakes is observed during nearest M5.8 earthquake of February 06, 2017. [ABSTRACT FROM AUTHOR]