Articles written in Journal of Earth System Science

    • Magnitude estimation of regional earthquakes in India and its adjoining region


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      To provide reliable and quick estimation of magnitude for moderate to large earthquakes at regional distances, two magnitude relations specific to the peninsular shield have been proposed based on long-period magnitude ($M_{A}$) and energy magnitude ($M_{E}$), using broadband velocity data of 23 regional events recorded at 18-station seismic network in the state of Telangana and Andhra Pradesh, India. $M_{A}$ is estimated using amplitude of filtered (0.03–0.08 Hz) broadband velocity seismograms, while $M_{E}$ is estimated based on radiated energy using broadband velocity spectra. It is observed that $M_{A}$ for larger events with $M_{w}$ >7.2 saturates, whilst $M_{E}$ does not suffer from saturation even for larger events. Thus, it is apparent that these two magnitude relations can provide magnitude estimates without saturation for all moderate to large regional earthquakes, which, in turn, can provide a homogeneous catalogue for moderate to large regional Indian earthquakes. The data transmission from remote stations to the central server at CSIR-National Geophysical Research Institute (NGRI) is quasi-real-time since it is connected by GPRS and VSAT. Using the proposed region specific magnitude relationships it becomes possible to estimate reliable magnitudes for moderate to large regional Indian earthquakes ($M_{w} \leq 7.2$) within 30 min of the occurrence of an event.

    • Seismotectonic significance of the December 1, 2020 Haridwar, India earthquake (M 4.3), a lower crust event near the Himalayan topographic front


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      Earthquakes in the Himalayan arc occur due to the interaction of Indian and Eurasian plates, and a great majority of themare of interplate type, occurring on the MainHimalayan Thrust (MHT). Some earthquakes, however, occur south of the Himalayan arc within the subducting Indian plate and majority of these earthquakes occur on the subducting ridges of the Indian plate, the most prominent of which is the Delhi–Haridwar ridge. The December 1, 2020 ($M_L$ 4.3,$M_W$ 3.8) earthquake is one such event whose source parameters are very well constrained by the local network installed in the region. The earthquake occurred close to theHimalayan Frontal Thrust at a depth of 36 km. The estimated focal mechanism from moment tensor inversion shows a strike-slip mechanism, with P-axis orientation concurrent with Indian plate motion with respect to Eurasia. The stress drop of 9.4 ± 3.7 MPa is consistent with relatively higher stress drop in intraplate earthquakes. Based on the estimated parameters, we qualitatively evaluated whether it occurred (i) on the newly discovered southernmost deformation front, referred as the piedmont fault, which developed in response to the southward propagation of the Himalayan wedge, (ii) due to Cexure in the Indian plate caused by long term subduction, (iii) due to strong coupling on theMHTcausing Cexure in the foreland, and (iv) on the northward extension of theDelhiHaridwar ridge.We propose that it probably occurred on the northward continuation of the Delhi–Haridwar ridge as similar earthquakes occur on this ridge in and around the Delhi region. We also suggest that the 1988 Udaipur (Nepal) earthquake, which had a similar focal depth, location, and focal mechanism, occurred on the Munger–Saharsa ridge’s northward continuation. The strong coupling on the MHT in the adjoining Himalayan segments might have helped in the occurrence of both earthquakes.

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