Active Tectonics in the Northwestern Outer Himalaya: Evidence of Large-Magnitude Palaeoearthquakes in Pinjaur Dun and the Frontal Himalaya

The Himalayan region has experienced a number of M8 and M5-M7.8 magnitude earthquakes in the present century. Apart from the release of strain builtup due to convergence of the Indian and Tibetan plates by seismic activity and aseismic slip, the tectonic activity in the current tectonic regime has also effected morphotectonic changes due to uplift, tilting of drainage basins, shifting or diversion of rivers and their tributary channels. Seismicity is mainly due to activity along numerous active faults, which trend parallel or transverse to the Himalayan mountain belt. In the outer Himalaya or the foothills, lying between the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT), some active faults have generated major earthquakes. The present article illustrates two such faults in the Pinjaur Dun and in the HFT zone at Kala Amb, Himachal Pradesh. Palaeoseismological study carried out at Nalagarh in Pinjaur Dun has revealed Late Pleistocene earthquakes along the Nalagarh Thrust (NT) that separates the Palaeogene rocks from the Neogene Siwaliks. The study shows evidences of at least two large magnitude earthquakes that rocked this region. The repeated reactivation of NT and HFT substantiates high seismic potential of the northwestern outer Himalaya and calls for more extensive study of palaeoearthquakes in this vastly populous mountainous region.