Evidence for seismicity in the lower crust and upper mantle in the Nepal Himalaya, implication for the rheology of the lithosphere

Abstract

It has recently been argued that the strength of the continental lithosphere lies in the crust with the upper mantle being extremely weak. Pivotal to this argument is the observation that most of the seismicity occurs in the crust and that the elastic thickness of continental plates is generally comparable to the depth range of the seismicity. This hypothesis contradicts the earlier view that seismicity is bimodal with earthquakes occurring either in the shallow crust or in the upper mantle, a view that was first promoted based on the seismicity in the Himalaya and Tibet, and on model of the lithosphere rheology derived from experimental rock mechanics. Here we test whether this hypothesis applies in the Nepal Himalaya in view of recent progress on the Moho geometry using the seismicity recorded by the National Seismological Centre of Department of Mines and Geology, Nepal. The seismicity of Nepal Himalaya is characterized by a linear and continuous belt of micro-seismic activity which runs due NW-SE. Majority of the earthquakes have indeed shallow depths and the depth ranges between 10 and 25 km. However a number of earthquakes are relatively deeper (23