Disaster Advances Vol. 7 (9) September 2014 28 Earthquake Source Parameters and their Scaling for the Uttarakhand region of North-West Himalaya Kumar Arjun 1* , Kumar Ashwani 2 , Gupta S. C. 2 , Mittal Himanshu 3 and Sen Arup 2 1. Department of Civil Engineering, Arni University, Kathgarh (Indora), Himachal Pradesh, INDIA 2. Department of Earthquake Engineering, Indian Institute of Technology, Roorkee-247667, Uttarakhand, INDIA 3. Centre of Excellence in Disaster Mitigation and Management, Indian Institute of Technology, Roorkee-247667, Uttarakhand, INDIA *arjundeq@gmail.com Abstract The Uttarakhand region of North-West Himalaya exhibits high historic and instrumental seismicity as evidenced by the occurrence of several moderate to large-sized earthquakes in the region. Source parameters of 16 small and moderate-sized earthquakes (3.1≤Mw≤6.7) have been estimated and a scaling law for the region has been developed. In this study software EQK_SRC_PARA 33 has been used that considers Brune’s model that yields a fall-off of 2 beyond corner frequencyf c with high frequency dimunition factor presented by Boore 7 to estimate f max . The estimated seismic moments range from 5.1×10 13 to 1.10× 10 19 Nm. The source radii are confined between 200 m to 9.7km, the stress drop ranges between 2.59MPa to 8.34MPa respectively. Using the data set of 16 events, a scaling law, M 0 f c 3 = 3.0 x 10 16 Nm/s 3 has been developed for the region. From the plot between seismic moment and f max , the values of f max seem to be dependent on the source size and vary from 4 Hz to 18 Hz at various sites falling in the Uttarakhand region of NW Himalaya. Comparing the average stress drop of 6.0 MPa obtained in the present study, with the global average of 3.0 MPa for inter-plate earthquakes, it can be inferred that the average stress drop associated with the small and moderate earthquakes is almost double in the Uttarakhand region of NW Himalaya. The estimates of stress drops, f max and scaling law will help to simulate strong ground motion using stochastic methods for this region. Keywords: Source parameters, f max, Scaling Relation, Uttarakhand, NW Himalaya. Introduction The Uttarakhand region of NW Himalaya lies approximately between latitude 29.0º N to 31.5º N and longitude 77.5º E to 81.0ºE. The mighty rivers like Ganga, Yamuna, Bhagirathi, Bhilangna and Alakhananda traverse through the region and possess immense potential for the generation of hydroelectric power. In view of this many hydropower projects have either been constructed or are under investigation and planning stage. The region falls in the seismic zones IV and V as per IS Code [IS 1893 (Part 1):2002]. Two most well studied moderate earthquakes of the region are: the Uttarkashi earthquake of 1991 (m b 6.6 IMD) and the Chamoli earthquake of 1999 (m b 6.3 USGS). Attempts have been made to study the seismotectonics of this region using teleseismic data 39 and local earthquake data 27 and to estimate the source parameters of the earthquakes occurring in the region using local earthquake data. 31,34,45 These are essential requirements to estimate the design earthquake parameters for the design of hydropower projects as well as to estimate the peak ground motions for the seismic hazard assessment. Fifteen earthquakes have occurred in the Uttarakhand region of NW Himalaya and one earthquake has occurred near Roorkee in the Ganga in deep, south of MBT. In this study both the strong motion data and weak motion data have been analyzed using software EQK_SRC_PARA 28 to determine the source parameters and scaling law for the region. Acceleration and displacement spectrums of SH component of ground motion generated by earthquakes have been analyzed considering Brune’s earthquake source model yielding a fall-off of 2 beyond corner frequency considered with high frequency dimunition factor, a Butterworth high-cut filter presented by Boore 7 to estimatef max . Geology, tectonics and seismicity of the area The Uttarakhand region which forms part of the northwestern Himalaya and lies between the rupture zones of the two great earthquakes viz. the Kangra earthquake of 1905 and the Bihar-Nepal earthquake of 1934. The broad geologic and tectonic framework of the study area that falls in the Uttarakhand region of NW Himalaya Himalaya is shown in figure 1. This figure depicts the surface trace of the main boundary thrust (MBT), the main central thrust (MCT), main frontal thrust (MFT) and regional tectonic features such as South Almora Thrust (SAT), North Almora Thrust (NAT), Moradabad fault (MT), Great boundary fault (GBF) along with local tectonic features like Alaknanda fault (AF), Ramgarh thrust (RT) and Martoli thrust (MT) 18 . The seismicity of the Uttarakhand region of NW Himalaya in particular is of interplate type. A significant feature of seismicity of the Himalaya is that the distribution of locations of earthquakes follows the trend of the mountain range. Epicentres of most of the moderate-sized earthquakes lie between the main boundary thrust (MBT) and the main central thrust (MCT) 44 and their occurrence is