Geophysical Constraints on the Seismotectonics of the Sikkim Himalaya

The role of transverse lineaments is often invoked in strain partitioning to generate strike-slip-dominated earthquakes in the Sikkim Himalaya, including the recent M-w 6.9 earthquake of 18 September 2011. An integration of seismicity clusters and crustal structures in conjunction with gravity modeling helps to develop a unified seismotectonic model for the Sikkim Himalaya, where transverse lineaments appear to have little role in strain concentration. Instead, deflection of the arc normal slips by rigid eclogitized Indian crust in the 40-60 km depth range is the primary source to generate strike-slip-dominated, large-magnitude earthquakes. The clustering of relatively deep aftershocks and main event near the southern edge of the eclogitized lower crust is a clear manifestation of this deformation. This transcurrent deformation also reorients foliation planes in the low-velocity block immediately above to inflict anisotropy and promote strike-slip-dominated moderate-magnitude earthquakes in the 20-40 km depth range. The low-angle northeast-dipping detachment in the 10-12 km depth range forms a localized asperity that produces low-magnitude earthquakes beneath the Lesser Himalayan duplex. It is surmised that the competent and strong eclogitic layer in the lower crust serves as a repository of high stresses during an earthquake buildup cycle, wherein the fluid pressure in the fractured rock matrix above plays a key role in the earthquake generating process.