Seismotectonics of the himalaya, and the continental plate convergence

Abstract

The relationship between seismicity and tectonics of the northwestern, central and part of the eastern Himalaya lying between longitudes 74–88°E and latitudes 26–35°N has been studied. A seismicity map of the area for the period 1890–1970 is presented. A strain-energy release map of the Himalaya was prepared following the method discussed by Allen et al. (1965). On the basis of this map it is suggested that the Himalaya does not behave as a single unit so far as the seismic activity is concerned. It could be divided appropriately into three different units; 1. (1) the Panjab Himalaya; 2. (2) the Kumaon Himalaya; and 3. (3) the Nepal Himalaya. It is suggested that the extension of Aravalli structures into the Himalayan regions has played a role in the tectonics of the Kumaon Himalaya, and probably is the cause of the complex nature of seismicity of the region. In the Kumaon Himalaya the maximum strain-energy release is related to the Main Central Thrust (MCT), whereas in the Panjab as well as in Nepal Himalaya it is related to the activity of the Main Boundary Fault (MBF). The strain-energy release characteristics of the two thrusts show that their mechanisms of storage and release of energy are different. Strain-energy release through the MCT seems to be more uniform as compared to the MBF along which the release of energy has been mostly abrupt, through large magnitude earthquakes. Focal-mechanism solutions of Himalayan earthquakes located north as well as south of the Indus Suture Zone indicate that the Indian plate is underthrusting the Tibetan plate towards the north, wehereas the latter is underthrusting the Indian plate towards the south. P-axes lying to the north as well as south of the Indus Suture Line are shallow dipping. This confirms the view that the present-day seismicity of the area is a result of continental-continental collision and not of lithospheric subduction.