Microearthquake activity in some parts of the Himalaya and the tectonic model

Microearthquake data from temporary/permanent networks in different parts of the Himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region. The microearthquake activity in Arunachal Pradesh, northeastern Himalaya, is found to be pronounced at the Main Boundary Thrust (MBT) and to its south; the subcrustal earthquakes (depth 50-80 km) occur much below the plane of detachment of the tectonic models proposed by Seeber et al. (1981) and Ni and Barazangi (1984). The MBT is not the seismogenic fault; the earthquakes are generated by strike-slip movement on deep seated hidden faults, transverse to the MBT. The high seismic activity in the Shillong Plateau, about 200 km south of the MBT in the northeast region, is due to the influence of Himalayan collision tectonics to the north and Burmese are subduction tectonics to the cast. The activity in the Plateau is not directly related to the Himalayan thrust belt or seismic belt. These are mostly crustal earthquakes (depth 10-30 km), and are caused by local active faults/lineaments. In the eastern Himalaya, in the Sikkim and Darjeeling area, the seismic activity is found to be clustered mostly to the north of the MBT. The earthquakes occur at a depth range 0-50 km; the majority of them occur below the detachment plane by thrust-faulting. In the central part, in the Nepal Himalaya, lateral variations of the seismic activity are observed, which represent lateral segmentation of the MBT by transverse tectonic features. In the western Himalaya, however, the tectonic models fit well with the microearthquake data. In the Himachal Pradesh of the western Himalaya, the microearthquakes are mostly recorded in the MBT zone, and the hypocentres (depth 0-20 km) are confined above the plane of detachment or on the Basement Thrust. The earthquakes mostly occur to the south of the Main Central Thrust (MCT), which suggests that the MCT is not seismogenic; it is rather a dormant fault. No single tectonic model explains the Himalayan earthquakes. (C) 2001 Elsevier Science B.V. All rights reserved.