Seismotectonic Model of the Kangra–Chamba Sector of Northwest Himalaya: Constraints from Joint Hypocenter Determination and Focal Mechanism by Naresh Kumar, Jyoti Sharma, B. R. Arora, and Sagarika Mukhopadhyay Abstract The existing seismological network in the Kangra–Chamba sector has been upgraded with 12 three-component digital seismometers to obtain new insight on the nature and sources of continued clustered seismicity in this part of northwest Himalaya. A combination of travel-time–distance plots and travel-time inversion of P and S phases have been used to derive a 1D velocity model for the region. The mini- mum 1D velocity model divides the average 44 km thick crust into four layers. The top ∼10 km thick layer represents the metamorphosed sediments of the Chamba nappe that dominates the surface geology of the study area. Suggestion of a thin low-velocity layer at 15 km depth possibly marks the detachment zone separating the downgoing Indian plate from the overriding wedge. The improved locations of epicenters show close clustering of seismic events immediately northeast of the epicenter of the 1905 Kangra earthquake, while away from this zone the seismicity in the Chamba sector has more even distribution. In the later sector, space-depth distribution of hypocenters suggests that strain resulting from the ongoing collision of the Indian plate with Asia is being consumed by reverse-fault movement on the Chamba thrust. The clustered seismicity in the Kangra sector has three distinct source regions and mechanisms: (1) southward displacement of the thick Chamba nappe sheet over the Panjal imbricate zone along the Panjal thrust accounts for the seismicity at shallow depths of less than 7 km, (2) the nucleation of strains where the northeast dipping main boundary thrust (MBT) merges with the detachment plane produces focused seismicity near this junc- tion, and (3) the seismicity in a small pocket below the plane of detachment appears to be a consequence of stresses generated at the base of the northeast dipping detachment plane by the transverse structure. Introduction The Himalayan region is one of the most active seismic intracontinental regions where devastating earthquakes result due to the continued continent–continent collision between India and Asia. Studies on the space-depth patterns of seis- micity in conjunction with focal mechanisms have not only been used to understand the earthquake generating processes but have also served as a key to constrain alternative tectonic evolution models of Himalaya (Fitch, 1970; Molnar et al., 1973; Chandra, 1978). Pioneering work of Seeber et al. (1981) and Ni and Barazangi (1984) showed that hypocen- ters of the well-constrained large earthquakes (M 5–6) tend to align on a low-angle north-dipping decollement plane, in- terpreted as a basement thrust fault (BTF) separating the top of the underthrusting Indian plate and the overriding sedi- mentary wedge of the Himalayan region. Most of the pre- vious studies relied on the observations of moderate to large earthquakes recorded at regional stations, largely outside Himalaya. Lack of information on small magnitude earth- quakes coupled with large errors in hypocenter locations pre- vented correlation of observed seismicity of the Himalayan region with megathrusts and other tectonic features of Hima- laya. However, the spatial distribution of regionally located epicenters shows a broad zone of concentrated epicenters bounded between the main central thrust (MCT) and the main boundary thrust (MBT), designated as the Main Himalayan Seismic Belt, extending over the entire length of the Hi- malayan arc (Ni and Barazangi, 1984). Modeling of Global Positioning System (GPS) measurements suggests that this concentration is associated with interseismic stress accumu- lation around the down-dip end of the locked portion of the 95 Bulletin of the Seismological Society of America, Vol. 99, No. 1, pp. 95–109, February 2009, doi: 10.1785/0120080220