Seismotectonics of the eastern Himalayan and indo-burman plate boundary systems

Abstract

The eastern Himalayan and Indo-Burman plate boundary systems are distinct from the rest of the India-Eurasia continental collision, due to oblique convergence across two orthogonal plate boundaries resulting in a zone of distributed deformation both within and away from the plate boundary. To understand the seismotectonics of this region, we model the source mechanism of 44 earthquakes using waveform inversion and combine them with source mechanism of 30 previously studied earthquakes. Depth distribution of these earthquakes reveal that the entire crust beneath northeast India is seismogenic. From spatial distribution and source mechanism it is evident that the N20'E convergence between India and Tibet is accommodated by N-S convergence and E-W subduction. The N-S convergence is accommodated through (a) shallow thrust earthquakes within the eastern Himalayan wedge, (b) lower crustal thrust earthquakes along the northern edge of Shillong Plateau, (c) lower crustal dextral strike-slip earthquakes in the Kopili fault zone, and (d) sinistral strike-slip earthquakes within the Bengal Basin crust. The E-W subduction results in shallow thrust earthquakes to intermediate depth strike-slip earthquakes and deep focus thrust earthquakes underneath the Indo-Burman convergence zone. Orientation of the fault plane and slip vectors point to downdip extension and along arc compression of the subducted Indian plate in response to slab pull forces and buckling at depth. Earthquake slip vectors are in good agreement with the GPS velocity vectors across northeast India and conforms to the clockwise rotating "microplates" model.