Modelling of active lineaments for predicting a possible earthquake scenario around Dehradun, Garhwal Himalaya, India

Abstract

The Doon valley which consists of the Doon gravels and Siwalik rocks is surrounded by the Main Boundary Thrust (MBT) in the north, the Main Frontal Thrust (MFT) (Mohand Thrust and Bhimgoda Thrust) in the south, the Ganga Tear Fault in the east and the Yamuna Tear Fault in the west. Lineaments, identified from the aerial photographs, show that a few of them trend parallel to the Himalayan orogen, and the others are transverse to it. The effect of the neotectonic activity is clearly reflected in the Doon valley, implying that the Doon valley is tectonically unstable today, and that the possibility of earthquakes in this region cannot be ruled out. The ruptures along identified active lineaments are modelled three dimensionally. Peak accelerations are obtained by using a methodology, based on the semi-empirical method of Irikura, which was used by Midorikawa. The efficacy of this modelling technique for the Himalayan region is established by simulating peak ground accelerations for the Uttarkashi earthquake of 20 October 1991 and comparing it with recorded one. A seismic study of the region shows, that it is prone to moderate as well as great earthquakes. The technique of Midorikawa has been applied for the preparation of zonation maps in this seismically active region. Two different zonation maps showing, respectively, the effects of moderate as well as great earthquakes in this region are presented in this paper. The zonation map for moderate earthquakes shows that the region can experience peak ground accelerations around 310 cm/s 2. This value is close to the peak ground acceleration of 304 cm/s 2 recorded for the Uttarkashi earthquake ( M s = 7.1) at Uttarkashi station, which is about 60 km north of Dehradun. The prepared map shows that among all identified active lineaments, rupture along four lineaments can cause major damage in the region. Among these four, three lineaments are transverse to the major tectonic zones, viz., the MFT and MBT in the region, and one extends along the MFT lying south of Dehradun. The zonation map for great earthquakes ( M ≥ 8) has been prepared by modelling a rupture in the Central Gap region of the Himalaya. Seven different possibilities of rupture nucleation within the rupture plane are modelled and a zonation map containing three different zones is prepared. The prepared zonation map for this worst possible scenario shows that cities like Tehri, Uttarkashi, Srinagar, Deoprayag, Pauri and Dehradun fall within zone 1 and can experience ground accelerations bigger than 600 cm/s 2. The region like Hardwar and Rishikesh fall within zone 2 and can experience ground accelerations around 300 cm/s 2. The zonation map shows that Tehri can experience accelerations up to 0.84g and this value is comparable with that obtained by other researchers. This study confirms that for very long periods, the destruction in the region will be dominantly affected by ruptures having longer lengths and least effected by shorter ruptures. Therefore, special consideration about the longer ruptures should be taken while considering effect of earthquake hazards in this region.