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The seismological data recorded at our four semi-permanent broadband seismographs installed by the CSIR-NGRI, Hyderabad, Telangana in the Aravalli mobile belt (AMB) and Rajasthan craton in the northwest Indian shield during the period 2014–2016 are used in the study. The 16 selected regional Indian earthquakes of Mw 5.5–7.8 that occurred in the North India region recorded at the four seismic stations are used to compute the surface wave (both Rayleigh and Love waves) group velocity dispersion characteristics and average 1-D regional shear wave velocity (Vs) structure beneath the North India which covers the northern part of India between the whole Himalayan region and Rajasthan craton including AMB. First, we compute Rayleigh waves (at 7–87 s) dispersion curves and Love waves (at 7–82 s) group velocity dispersion curves by utilizing the standard frequency time analysis (FTAN) of 3-component broadband data recorded at these four 3-component broadband seismic stations. After that, using the logarithmic stacking technique, we compute the final average stacked fundamental mode dispersion curves of surface waves (both Love and Rayleigh waves) for our study area. Then, we finally invert the final average dispersion curves of surface waves (both Love and Rayleigh waves) to compute the average one-dimensional regional shear velocity structure beneath North India. Our modelling result in North India suggests a two-layered crustal structure with a crustal thickness of 39 km. The thickness of the first crustal layer (upper crust) is 20 km with a shear wave velocity (Vs) of 3.30 km/s. On the other hand, the second crustal layer (lower crust) marks a 19-km thickness with a Vs of 3.70 km/s. Our modelling result suggests that the Vs of the upper mantle is found to be 4.35 km/s below the study region. Research highlights: Modelled average one-dimensional regional Vs structure below northern part of India. A 2-layered crustal velocity model below North India. Upper crust at 20 km thick with Vs of 3.30 km/s. And, lower crust at 19 km thick with Vs of 3.70 km/s. The Moho depth is 39 km and Vs below the Moho depth is found to be 4.35 km. [ABSTRACT FROM AUTHOR]

Between 2017 and 2019, the CSIR-NGRI, Hyderabad, Telangana, established a broad-band seismic-network with fifty-five 3-component broadband seismometers in the Himalayan region of Uttarakhand, India. Out of 55 three component broadband seismic (BBS) networks, we chose 17 for the present study. Using digital waveform data from twenty-one (21) regional Indian earthquakes of Mw 5.0–6.2 that were recorded in the 17 broadband seismometer, we compute fundamental mode group-velocity dispersion (FMGVD) characteristics of surface waves (Love and Rayleigh waves) and the average one-dimensional regional shear-wave velocity (Vs) structure of the Uttarakhand Himalayan region. First, we compute FMGVD curves for Love waves (6–73 s) and Rayleigh waves (at 6.55–73 s) period, and then, we finally invert these dispersion curves to compute the final average one-dimensional regional crustal & sub-crustal shear-wave velocity (Vs) structure below the Uttarakhand Himalaya. Our best model in Uttarakhand Himalayan region, India, reveals the 8-layered crust with a mid-crustal low velocity layer (MC-LVL) (approximately a drop of 1.5–2.3% in Vs) between 8 and 20 km depth in the proximity of MCT (Main Central Thrust). In the upper crustal part (0–20 km depths), our modelling suggests shear velocities (Vs) varies from 3.1 to 3.9 km/sec while shear velocities (Vs) in the lower crustal part (20–45 km depth) are modelled to be varying from 3.7 to 4.69 km per sec. The Moho-depth is calculated to be 45 km deep below the K-G Himalaya, and the shear-velocity (Vs) in the sub-crustal sector is 4.69 km/sec. Our estimated mid-crustal low-velocity layer (MC-LVL) could be linked to the presence of metamorphic fluids in the fractured Main Himalayan Thrust (MHT), resulting from the weakening of the crustal material at the interface between the overriding Eurasian plate and upper part of the underthrusting Indian plate. [ABSTRACT FROM AUTHOR]

Landslides and related mass movement activities along with earthquakes are common in the Himalayan terrain. The distribution of seismic activities in the vicinity of the Main Central Thrust (MCT) is well known. However, in order to assess the relationship between these seismic events and landslides, the spatial distribution of landslides and earthquake events (magnitude >1.5) on either side of the MCT zone was evaluated. It has been observed that seismic events of magnitude <2.1 are clustered around the 1991 Uttarkashi earthquake rupture, 1999 Chamoli earthquake rupture and around Munsiyari. The zone surrounding 1999 Chamoli earthquake rupture indicates the highest earthquake and landslide density of ~75% and 53% respectively. [ABSTRACT FROM AUTHOR]

We estimate frequency-dependent attenuation of P and S waves in Garhwal Himalaya using the extended coda normalization method for the central frequencies 1.5, 2, 3, 4, 6, 8, 10, 12, and 16 Hz, with earthquake hypocentral distance ranging from 27 to 200 km. Forty well-located local earthquake waveforms were used to study the seismic attenuation characteristics of the Garhwal Himalaya, India, as recorded by eight stations operated by Wadia Institute of Himalayan Geology, Dehradun, India, from 2007 to 2012. We find frequency-dependent P and S wave quality factors as defined by the relations Q = 56 ± 8 f and Q = 151 ± 8 f by fitting a power-law frequency dependence model for the estimated values over the whole region. Both the Q and Q values indicate strong attenuation in the crust of Garhwal Himalaya. The ratio of Q/ Q > 1 obtained for the entire analyzed frequency range suggests that the scattering loss is due to a random and high degree of heterogeneities in the earth medium, playing an important role in seismic wave attenuation in the Himalayan crust. [ABSTRACT FROM AUTHOR]

The Tehri Dam is located in a seismotectonically active region in the Indian Himalayan belt. This 260.5 m high dam has a live water storage of 2615 × 10 m and is capable of generating crustal deformation corresponding to water fluctuation. Filling of the reservoir started in October 2005. Seismic data around the dam between 2000 and 2010 shows that seismicity is corresponding to drawdown levels of the reservoir rather than to higher water levels. GPS data at twelve local benchmarks were collected from 2006 to 2008 during filling and drawdown reservoir levels. The velocity vectors show ground motion to be between ∼0.69-1.50 mm in the different filling-drawdown cycles with reference to the permanent station at Ghuttu. The motion appears to be inwards into the reservoir when the reservoir is filled and outwards when the reservoir is drained. This ground motion corresponds to elastic deformation and rebound due to effect of the oscillating water levels. [ABSTRACT FROM AUTHOR]

We report an optimum ID velocity model of the Kumaon-Garhwal Himalaya, based on analysis of --5000 seismograms generated at 50 digital broadband seismic stations in the region during April 2005 and June 2008. These include data from 385 local earthquakes with azimuth gaps of < 180°, each of which had a mini-mum of seven P- and five S-phase readings. This velocity model was then used to map the locations of 1150 earthquakes (1 < M < 5) that occurred in the region. A majority (~75%) of these earthquakes occurred in the upper 20 km of the crust, largely above the Main Himalayan Thrust, and forms an ~10-km-wide band along the surface trace of the Munsiari Thrust. However, we also find another parallel band of earthquakes, about 70 km to its southwest through the Tehri dam in the Lesser Himalaya, and a significant number (~15%) both in the Tethys Himalaya and within the flexed crust of the underthrusting Indian plate, beneath the Ganga basin. Furthermore, unlike the reported absence of seismicity in the lower crust of the Nepal Himalaya, and its reappearance in the shallow mantle, the Kumaon-Garhwal Himalayan crust is found to be seismically active throughout, with no event reliably recorded in the shallow mantle of this region during the epoch of our seismic recording. [ABSTRACT FROM AUTHOR]

In the early morning hours of 23 July 2007 at 04 : 32 : 13.5 (IST), a moderate (ML = 4.9, Mw = 5.0) earthquake occurred in the Garhwal Himalaya near Kharsali, about 50 km northwest of the epicentre of the strong (M 6.4) Uttarkashi earthquake of 20 October 1991. Sourced in the upper crust (15 km focal depth), this earthquake was followed by a series of shallow-focused aftershocks of magnitudes ≤ 3.4. Data from the local broadband seismic network were used for understanding the source characterization. The fault plane solutions of the main shock and the biggest aftershock (M 3.4) are almost similar, suggesting that the deformation occurred by reverse faulting with a significant strike-slip component. For the main shock, the trend/plunge of the major and minor principal axes P/T was 166°/1° and 258°/67° respectively. In these solutions one fault plane is dipping to the NE, and the strike of the fault plane coincides with the major tectonic faults as well as the longest axis of isoseismals. The seismic moment, source radius and stress drop of the main shock are estimated to be 4.15 × 1016 Nm, 1660 m and 4.15 MPa respectively. [ABSTRACT FROM AUTHOR]

Abstract: Himalayas, one of the most seismically active belts, have been formed due to continent–continent collision and are probable locations for future moderate to great earthquakes. It is of paramount importance to recognize precursory swarm activity considered to be long predictors of main events. Recently two earthquake swarms occurred across Main Central Thrust (MCT) in Garhwal Himalayan region in a time interval of nearly one month. The first swarm of April 2009 had epicentral locations lying north of MCT on hanging wall side, which has shown less seismic activity in recent times, while the second swarm occurred in May 2009 near Gopeshwar having epicenters on active seismic zone, located south of MCT. The approximate distance between the two swarms is about 50km. In present study preparation zone of 100×100 km2 has been demarcated recognizing these swarms as precursors to moderate to great earthquake in the Garhwal Himalaya. [Copyright &y& Elsevier]

This paper presents the computation of time series of the 22 July 2007 M 4.9 Kharsali earthquake. It occurred close to the Main Central Thrust (MCT) where seismic gap exists. The main shock and 17 aftershocks were located by closely spaced eleven seismograph stations in a network that involved VSAT based real-time seismic monitoring. The largest aftershock of M 3.5 and other aftershocks occurred within a small volume of 4 × 4 km horizontal extent and between depths of 10 and 14 km. The values of seismic moment ( M) determined using P-wave spectra and Brune's model based on f spectral shape ranges from 10 to 10 dyne-cm. The initial aftershocks occurred at greater depth compared to the later aftershocks. The time series of ground motion have been computed for recording sites using geometric ray theory and Green's function approach. The method for computing time series consists in integrating the far-field contributions of Green's function for a number of distributed point source. The generated waveforms have been compared with the observed ones. It has been inferred that the Kharsali earthquake occurred due to a northerly dipping low angle thrust fault at a depth of 14 km taking strike N279°E, dip 14° and rake 117°. There are two regions on the fault surface which have larger slip amplitudes (asperities) and the rupture which has been considered as circular in nature initiated from the asperity at a greater depth shifting gradually upwards. The two asperities cover only 10% of the total area of the causative fault plane. However, detailed seismic imaging of these two asperities can be corroborated with structural heterogeneities associated with causative fault to understand how seismogenesis is influenced by strong or weak structural barriers in the region. [ABSTRACT FROM AUTHOR]

On the 27th of August 2016, a seismic swarm activity consisting of 58 earthquakes (1.5 ≤ M L  ≤ 4.4), which occurred in Rampur area of the Kullu-Rampur Tectonic window of Himachal Pradesh in Northwest Himalaya. The epicenters of these events are located at the northern front of the Berinag Thrust in its hanging wall. To better understand the seismotectonics of this region, we analyzed the spectral source parameters and source mechanism of this swam activity. Spectral analysis shows the low stress drop values (from 0.05 to 28.9 bars), suggesting that the upper crust has low strength to withstand accumulated strain energy in this region. The Moment Tensor solutions of 12 earthquakes (≥2.7M L ) obtained by waveform inversion yield the shallow centroid depths between 5 and 10 km. All these events are of dominantly thrust fault mechanism having an average dip angle of ∼30°. The P-axes and the maximum horizontal compressive stresses are NE-SW oriented; the relative motion of the Indian Plate. The present study reveals that the swarm activity in the Himachal region of NW Himalaya is related to the out-of-sequence thrusting or the Lesser Himalayan Duplex system. [ABSTRACT FROM AUTHOR]

On 7 February 2021, Chamoli district (Uttarakhand, India) was devastated by a deadly rock-ice avalanche that led to a large causality of more than 200 people and a huge economic loss. We found noteworthy sequence of precursory signals of main failure/detachment preceded by a dynamic nucleation phase. The rock-ice avalanche appears to have been initiated by seismic precursors which were continuously active for 2:30 h prior to main detachment. The seismic amplitude, frequency characteristics and signal-to-noise ratio variation of detected tremors indicate static to dynamic changes in nucleation phase located at the source of detached wedge. The characteristics of seismic data distinguished debris flow and hitting obstacles from other seismic sources and allowed the estimations of debris flow speed. We analyzed and verified the seismic signals with field evidences to estimate the associated impacts and velocity of dynamic flow. The proximal high-quality seismic data allowed us to reconstruct the complete chronological sequence and evaluate impacts since the initiation of nucleation phase to its advancement. Furthermore, we suggest that real-time seismic monitoring with existing network and future deployment of integrated dense network can be used for forecasting of flow events and hazard mitigation in the downstream., (© 2022. The Author(s).)