First hand observations from the April 28, 2021 Sonitpur (M-W 6.4) earthquake, Assam, India: possible mechanism involved in the occurrence of widespread ground effects

摘要

On April 28 2021, an earthquake of M-W 6.4 occurred near Sonitpur, Assam, India. The epicenter was 43 km away from Tezpur, Assam with a focal depth of 34 km. The National Center for Seismology (NCS), Delhi reported the maximum intensity of MMI-V whereas the United States Geological Survey (USGS) reported the maximum intensity of MMI-VII. Preliminary reports suggest a reverse slip component for the earthquake, which occurred close to the previously reported Kopili fault. This fault is the cause of two damaging earthquakes in the past i.e., 1869 Cachar earthquake and the 1943 earthquake. The April 28 2021, Sonitpur earthquake caused widespread liquefaction, building damage, and lateral spreading as far as 90 km from the epicenter. The present study reports preliminary damages and ground effects observed soon after the earthquake along with a comparative analysis with previously reported damages around the area during the historical earthquakes. The farthest reported liquefaction occurred during the earthquake was plotted with other case studies which were well in agreement with the previous literature. The possible reason for the extensive liquefaction and ground cracks is inferred to be due to site amplification within a sedimentary basin. Therefore, the occurrence of the 2021Sonitpur, Assam earthquake emphasizes the seismic hazard scenario for the Brahmaputra Basin and Bengal Basin, which further requires more detailed study in terms of paleoseismology, liquefaction hazard zonation and seismic hazard assessment. In addition, the damages to the buildings highlight the implementation of proper building codes considering the liquefaction hazard zonation map for the study area.