The M w 6.9 Sikkim–Nepal earthquake of September 2011: a perspective for wrench faulting in the Himalayan thrust zone

Abstract

In the present study, we analyze the strike-slip-type earthquakes along the Himalayan arc that are transverse to the collision front, with a special emphasis on the Sikkim region. The study examines seismicity and focal mechanism data in conjunction with crustal structure derived from receiver functions and gravity gradients, supported by results from GPS data. We present a relation between the north–south depth distribution of the strike-slip earthquakes in Sikkim, with the northward dipping trend of the Indian crust beneath Eurasia deciphered from teleseismic receiver function analysis of data recorded by our seismic network. Results suggest that transverse faults caused by thrust partitioning along the Himalayan arc manifest as vertical strike-slip faults cutting across the crust of the descending Indian plate down to 60 km. Further, we argue in favor of a consistent right-lateral sense of slip among all the strike-slip-type earthquakes cutting across the Himalayan arc from Kumaon in the west through Sikkim and Bhutan Himalaya, to the Burmese arc and the Sagaing fault on the easternmost end. We infer a wrench fault tectonic model for the Indian plate described by a series of right-lateral strike-slip faults from west to east in the Himalaya, in conjunction with under-thrusting of the Indian plate beneath Eurasia.