Seismotectonic Model of the Sikkim Himalaya: Constraint from Microearthquake Surveys

The seismotectonic model in the Sikkim Himalaya does not fit well with the proposed steady state or evolutionary models. The main boundary thrust (MBT) is seismogenic and is a mantle-reaching fault. The earthquakes are not confined to shallower depths (<25 km) above the plane of detachment as proposed in the models, the seismic activity continues from surface to the lower crust (0-45 km) to the north of MBT, and earthquakes are produced by a thrust mechanism. The earthquakes to the east of Sikkim, in the Bhutan Himalaya, on the other hand, are produced along a 200-km-long northwest-southeast-trending lineament by transverse tectonics; the seismogenic lineament cuts across the Himalayan major thrusts and extends to the Goalpara wedge in the southeast. The earthquakes occur by strike-slip mechanism in the midcrust, at a depth range of 10-25 km, along this long active lineament.