ALBERT

Description: 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 minimum 1D velocity model divides the average 44 km thick crust into four layers. The top approximately 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 junction, 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.

Description: The role of transverse lineaments is often invoked in strain partitioning to generate strike-slip-dominated earthquakes in the Sikkim Himalaya, including the recent M (sub w) 6.9 earthquake of 18 September 2011. An integration of seismicity clusters and crustal structures in conjunction with gravity modeling helps to develop a unified seismotectonic model for the Sikkim Himalaya, where transverse lineaments appear to have little role in strain concentration. Instead, deflection of the arc normal slips by rigid eclogitized Indian crust in the 40-60 km depth range is the primary source to generate strike-slip-dominated, large-magnitude earthquakes. The clustering of relatively deep aftershocks and main event near the southern edge of the eclogitized lower crust is a clear manifestation of this deformation. This transcurrent deformation also reorients foliation planes in the low-velocity block immediately above to inflict anisotropy and promote strike-slip-dominated moderate-magnitude earthquakes in the 20-40 km depth range. The low-angle northeast-dipping detachment in the 10-12 km depth range forms a localized asperity that produces low-magnitude earthquakes beneath the Lesser Himalayan duplex. It is surmised that the competent and strong eclogitic layer in the lower crust serves as a repository of high stresses during an earthquake buildup cycle, wherein the fluid pressure in the fractured rock matrix above plays a key role in the earthquake generating process.