Seismotectonics of the Great and Large Earthquakes in Himalaya

The best known seismotectonic model of the Himalayan Seismic Belt (HSB) suggests that the great and large earthquakes in the Himalaya occur at a shallow depth (10-20 km) by thrust faulting on the Main Himalayan Thrust, i.e. on the plane of detachment. The plane of detachment is the interface between the Indian shield and the Himalayan sedimentary wedge. The recent earthquake data of the permanent and temporary local networks in the Himalaya, however, indicate bimodal seismicity at shallow (0-20 km) as well as greater depths (30-50 km). The source processes of the great and large earthquakes are reexamined in this article (the observations do not support a uniform seismotectonic model for the entire HSB). The four known great earthquakes (Ms ~8.0- 8.7) in the Himalayan region, from west to east are the 1905 Kangra, 1934 Bihar, 1897 Shillong and the 1950 Assam earthquakes that occurred by different tectonic processes; each occurred in its own unique complex tectonic environment. Most recently, the 1988 strong earthquake (Ms 6.6) in the Bihar/Nepal foothill Himalaya and the 2011 strong earthquake (Mw 6.9) in the Sikkim Himalaya show that these are not the plane of detachment events; these occurred by strike-slip faulting at mantle depth (∼50 km). A review of all these significant earthquakes in HSB is presented in this article.