Paleoseismic context of the 1950 earthquake: Implications for seismic gaps in the Eastern Himalaya

Abstract

We evaluate the seismotectonic setting in the Upper Assam and Arunachal/Bhutan segments to obtain a critical understanding of the 1950 earthquake, and its predecessors. The paleoseismological findings from the region inform that among the earthquakes prior to 1950, the one that occurred either in 1697 or in 1714, may belong to the class of a major earthquake (M~8), and it may have activated part of the Bhutan-Arunachal segment. The Assamese historical reports are much vocal on the effects of the 1697 earthquake (known as the Sadiya earthquake) in the Upper Assam Valley than that of the 1714 event. The evidence provides more credence to the 1697 event that may tentatively be given the status of a major earthquake in the Upper Brahmaputra region, and the 1714 event appears to be more of a Bhutan-specific event. The geological record indicates that the great Upper Assam earthquake of 1950 does not have any predecessors mimicking its magnitude nor the projected complex style of rupture at least during the last 1000 years or may be more. The question is whether the 1950-type earthquake involving two orthogonal structures with variable strain accumulation is kinematically a repeatable activity. The 1697 earthquake with a likely magnitude (Mw) of ≥8 must have ruptured the eastern-most part of the Arunachal frontal thrust. The records from the Assam Valley do not conclusively establish the evidence of any major impact in the Brahmaputra Valley, commensurating with a magnitude ≥8.0 assigned to the 1714 event sourced in the Bhutan segment.