MODELING OF ACTIVE LINEAMENTS FOR PREDICTING A POSSIBLE EARTHQUAKE SCENARIO AROUND DEHRADUN, GARHWAL HIMALAYA, INDIA

The Doon valley which consists of the Doon gravels and Siwalik rocks i s surrounded by the Main Boundary Thrust (MET) 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 wes t. Lineaments, identified from the aerial photographs, show that a few of them trend parallel to the Himalayan orogen, and the others are tr ansverse to it. The effect of the neotectonic activity is clearly refl ected in the Doon valley, implying that the Doon valley is tectonicall y unstable today, and that the possibility of earthquakes in this regi on cannot be ruled out. The ruptures along identified active lineament s are modelled three dimensionally. Peak accelerations are obtained by using a methodology, based on the semi-empirical method of Irikura, w hich was used by Midorikawa. The efficacy of this modelling technique for the Himalayan region is established by simulating peak ground acce lerations for the Uttarkashi earthquake of 20 October 1991 and compari ng 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 Mi dorikawa has been applied for the preparation of zonation maps in this seismically active region. Two different zonation maps showing, respe ctively, the effects of moderate as well as great earthquakes in this region are presented in this paper. The zonation map for moderate eart hquakes shows that the region can experience peak ground accelerations around 310 cm/s(2). This value is close to the peak ground accelerati on of 304 cm/s(2) recorded for the Uttarkashi earthquake (M-s = 7.1) a t Uttarkashi station, which is about 60 km north of Dehradun. The prep ared map shows that among all identified active lineaments, rupture al ong 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 greater t han or equal to 8) has been prepared by modelling a rupture in the Cen tral Gap region of the Himalaya. Seven different possibilities of rupt ure nucleation within the rupture plane are modelled and a zonation ma p containing three different zones is prepared. The prepared zonation map for this worst possible scenario shows that cities like Tehri, Utt arkashi, Srinagar, Deoprayag, Pauri and Dehradun fall within zone I an d can experience ground accelerations bigger than 600 cm/s(2). The reg ion like Hardwar and Rishikesh fall within zone 2 and can experience g round accelerations around 300 cm/s(2). The zonation map shows that Te hri can experience accelerations up to 0.84 g and this value is compar able with that obtained by other researchers. This study confirms that for very long periods, the destruction in the region will be dominant ly affected by ruptures having longer lengths and least effected by sh orter ruptures. Therefore, special consideration about the longer rupt ures should be taken while considering effect of earthquake hazards in this region.