Study of fault plane solutions and stress drop using local broadband network data: The 2011 Sikkim Himalaya earthquake Mw 6.9 and its aftershocks

Fault plane solutions of the September 18, 2011 Sikkim Himalaya earthquake Mw 6.9 and its four aftershocks (Mw>4.0) are studied by waveform inversion using the local broadband network data. The solutions show pure strike-slip mechanisms; one aftershock show thrust faulting with strike-slip component. Strike-slip mechanisms indicate predominant transverse tectonics in Sikkim, in the eastern Himalaya region, unlike predominant thrust tectonics in the western Himalaya. The 2011 main shock occurred at a much deeper depth (~47 km) compared to the shallower (<20 km) thrust events in the western Himalaya.  Further, analysis of ground acceleration spectra reveals low stress drop (14-38 bars) in agreement with the relatively long source duration and small co-seismic slip of the main shock as well as the aftershocks. We interpret the low stress drop in terms of lower energy release due to reactivation of the Tista fault in opposite sense to that of the Himalayan thrust tectonics.  The low stress drop also indicates a pre-existing brittle zone or fault zone at deeper depth or mantle depth.