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Abstract: A moderate size earthquake (ML5.9) occurred near Mechuka in Siang Valley on 23rd April 2019 at 20:15:50.00 UTC in the rupture zone of the great Assam earthquake of 15th August 1950. This shallow-focused earthquake occurred close to NE trending Main Central Thrust has a thrust fault mechanism striking parallel to this major tectonics of the Himalaya. Including this event, a dataset of 46 local, micro and low magnitude earthquakes (1.5 ≤ ML≤ 5.9) of the Siang Valley of Arunachal Pradesh, NE India, are evaluated for the characteristics of the earthquake source, seismicity, stress drop and tectonic structures. The earthquake events were recorded by a close digital network of eight broadband seismic stations installed in December 2018 in the Siang Valley. The stress drop (Δσ) obtained for the Mechuka earthquake in the Siang Valley is 114.71 bars, the seismic moment (Mo) of 6.86E + 16 Nm and the source radius (r) of 1392.41 m. The seismic moments obtained from the microearthquakes vary between 2.44E + 11 and 6.86E + 16 Nm, and the source radius varies from 172.90 to 1392.41 m based on Brune’s circular source model 1970. The corner frequencies (fc) of the events range from 1.03 to 8.04 Hz. Seismically intense clustering of microearthquakes has a low-stress drop below 10 bar except for the Mechuka earthquake, which varies from 0.05 to 114.72. This high-stress drop of moderate earthquake compared to its low values of lower magnitude earthquakes indicates that the earthquake of 23rd April 2019 is associated with high-strength material accumulating high strain during the earthquake-building process. Moment magnitude of whole data set ranges from 1.52 to 5.16. The scaling relationship between Moand fcis obtained as 9.089E + 16fc–9.52, and for Moand Δσare modelled to be related as Mo= 8.96E + 10Δσ2.86. Evaluated low-stress drop for the smaller size of earthquakes shows brittleness of the upper crustal region. Research highlights:

Modelling of earthquake source parameters to obtain scaling relations for Siang Valley, Arunachal Pradesh.

The estimated Mo, fc, r, ∆σand Mware 2.44 × 1011to 6.86 × 1016Nm, 1.03–8.04 Hz, 172.90–1392.41 m, 0.05–114.71 bar and 1.52–5.16, respectively.

Scaling relation between Moand fc, Mo and Δσand Mwand fcis Mo= 9.089E + 16fc−9.52, Mo= 8.96E + 10 Δσ2.86and Log fc= –0.39Mw+ 2.39, respectively.

Abstract: 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 Mw5.5–7.8 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 Vsof 3.70 km/s. Our modelling result suggests that the Vsof the upper mantle is found to be 4.35 km/s below the study region. Research highlights:

Modelled average one-dimensional regional Vsstructure below northern part of India.

A 2-layered crustal velocity model below North India.

Upper crust at 20 km thick with Vsof 3.30 km/s.

And, lower crust at 19 km thick with Vsof 3.70 km/s.

The Moho depth is 39 km and Vsbelow the Moho depth is found to be 4.35 km.

Abstract: The Garhwal-Kumaun region continues to accumulate the built-up of strain energy like the other regions of the Himalaya. But this sector unlike the other sectors is yet to release this accumulated strain energy which can be in the form of great earthquake. The region has sufficient strain energy to generate earthquake of M>8. The analysis of seven hundred ten local events recorded by ten station broad band network between August’07 to February’10 shows that most of the seismic events recorded in this region continue to occur from shallow depths (< 25 km). The evaluation of source parameters from p-wave spectral analysis indicates that the events have low stress drop values. The region continues to release energy in the form of smaller magnitude earthquakes. The epicentral location map indicates that Munsiari Thrust, which is located south of the Main Central Thrust is more active.

Abstract: 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 2 spectral shape ranges from 1018 to 1023 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.