Seismotectonics of the Kangra region, Northwest Himalaya

Abstract

The occurrence of major and moderate earthquakes having thrust and strike-slip mechanism along the longitudinal and transverse feature within the same seismogenic zone requires us to study the area in detail to infer the seismotectonics. The epicentral distribution of earthquakes >4.0 (mb) between Chamba and Sundernagar region shows the high rate of activity either north of Chamba, i.e. north of the Main Central Thrust (MCT) or north of Dharamsala in between the Panjal Thrust (PT) and MCT in the Dhauladhar Range. It is, however, surprising that all the moderate earthquakes that have triggered in the Kangra region have either been in the frontal belt along the transverse features or have followed the arcuate belt close to the Main Boundary Thrust (MBT). The spatio-temporal variation of the seismicity from 1973 to 2000 (>4.0mb) reveals a migration trend between Chamba and Sundernagar. During the seventies and early eighties, the seismicity was concentrated in the Chamba region and in the late eighties and early nineties, the seismicity shifted around Dharamsala and later further north.In this arcuate tectonic belt of the Chamba–Sundernagar region, the foreland region has a characteristic tectonic setting. All the fault and fold structures in the Oligocene–Miocene and Neogene–Pliestocene sediments in the foreland region show relative movement in the area of transverse tectonics. The surface structures, i.e. thinning and thickening of the sediments, are controlled by the basement cofiguration controlled by the subduction process. The MBT and the PT are closely spaced and meet each other just northwest of Dharamsala. The width of the Lesser Himalaya in the Chamba–Sundernagar region is also much narrower as compared to the region in the east, in Garhwal Kumaun and western Nepal. This suggests that the convergence is taken up largely by underthrusting of the Lesser Himalayan formations along the PT. The occurrence of a moderate earthquake of magnitude 6.5 in 1978 and the 1986 Dharamsala earthquake along the SE-dipping and SW-dipping nodal planes within the NE-dipping thrust regime and displacement of the MBT and PT from 4 to 12km reflects the active involvement of the NE–SW trending fault system in controlling the seismic behavior of the Kangra region. The microearthquake migration in time and space in the Chamba–Sundernagar region also reflects the time differential segmental collision behavior of the basement.