Rupture of the Indian Slab in the 2011 Mw 6.9 Sikkim Himalaya Earthquake and Its Tectonic Implications

Abstract

Unlike the other Himalayan plate boundary segments, the eastern Nepal to Bhutan Himalayan region is not known to have generated prominent shallow thrust faulting earthquakes, typical of the ongoing convergence. This region features strike-slip earthquakes over the depth ranges of 40�120 km, indicating intraslab deformation. Here we present for the first time a slip distribution model for the largest recorded intraslab strike-slip earthquake in this region, the Mw 6.9 Sikkim event that occurred on 18 September 2011. Relying on kinematic source process modeling, our results indicate a NE-SW trending, steeply dipping sinistral source zone within the underthrusting Indian slab. The rupture propagated radially, with a low rupture velocity of 1.7 km/s, breaking a large asperity of 20 A� 20 km 2 with a maximum slip of 1.6 m. The rupture nucleated at a depth of 45 km and reached upper mantle depths. The computed coseismic stress drop value is 13.6 MPa. We suggest that most of the aftershocks occurred on the conjugate plane, possibly due to stress triggering. Stress inversion of focal mechanisms indicates a transpressive stress regime throughout the crust and pure strike-slip regime in the upper mantle. We observed a unimodal distribution of earthquakes beneath the Higher Himalaya. This indicates a strong, brittle Indian slab and unravels a scenario of an eventual breakup of the lithosphere; the key trigger might be variation in the convergence rates along the Himalayan arc. ©2019. American Geophysical Union. All Rights Reserved.