Tectonics of the Area
The study area, Uttarakhand, is a state located in northern part of India in a latitude of 28-320N and longitude of 77-810E (Fig. 1). The seismicity of the area is governed by seismicity of Himalaya. Himalaya is situated near the Indo-Eurasian plate boundary and is formed due to continental-continental collision in Cenozoic era. Currently Indian plate is moving at a rate of about 5cm/year (Kumar et al. 2007). Due to collision of plates a complex network of thrusts, faults and suture is formed in this region. The seismicity of the plate boundary region is mainly influenced by different prominent Himalayan tectonic thrusts named main boundary thrust (MBT), main central thrust (MCT) and main frontal thrust (MFT). Uttarakhand almost lies in the central part of Himalaya and possess significant segment of MBT, MCT and MFT. Again a large part of central Himalaya was identified as central seismic gap (Bilham and Szeliga, 2008). Seismic gaps supposed to have high potential for future earthquake in any region. Besides these Himalayan thrusts, several faults are found in and around Uttarakhand. Among those Karakoram fault is important one that runs more than 1000km from central Pamir to north of Uttarakhand Himalaya. Other than these huge thrusts and faults, other seismically important geological tectonic features of the study area are Jwalamukhi thrust (JMT), Drang Thrust(DT), Ramgharh thrust (RT), North Almora Thrust (NAT), South Almora Thrust (SAT), Great Boundary Fault (GBF), Alaknanda Fault (AF), Mahendragarh Dehradun Fault (MDF), Moradabad Fault (MF), Manali Fault (MF), etc. Along with these important thrust and faults a large number of smaller thrusts, faults and lineaments are also present in and around Uttarakhand.
The seismic activity in Uttarakhand is related to all the regional tectonic features like thrust and faults. Many earthquakes of moderate to large magnitude have occurred in this area in the past. In addition to 1991-Uttarkashi earthquake of magnitude Mw 6.8 (USGS) and 1999-Chamoli earthquake of magnitude Ms 6.6 (USGS), many earthquakes have occurred in nearby locations out of Uttarakhand along the Himalayan thrusts and faults. Among these the great Kangra earthquake of 1905 of magnitude 8 having epicenter in the vicinity of MBT, the Kashmir earthquake of magnitude 7.6 are large ones and the 1945 Chamba earthquake, 1975 Kinnaur earthquake and 1986 Dharmasala earthquake are of moderate magnitude earthquakes. Most of the earthquakes in this region are shallow crustal earthquakes with focal depth less than 60km from surface.