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

Earthquakes in the Himalayan arc occur due to the interaction of Indian and Eurasian plates, and a great majority of them are of interplate type, occurring on the Main Himalayan 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 (ML 4.3, MW 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 the Himalayan 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 flexure in the Indian plate caused by long term subduction, (iii) due to strong coupling on the MHT causing flexure in the foreland, and (iv) on the northward extension of the Delhi Haridwar 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.