The March 1999 Chamoli Earthquake in the Garhwal Himalaya: Aftershock Characteristics and Tectonic Structure

Abstract The aftershock sequence of the Chamoli earthquake (mb 6.3) of March 28,1999 provides an opportunity to study the aftershock characteristics and the seismotectonic structure in the Garhwal Himalaya. Detailed analysis of the spatial variation, the frequency-magnitude distribution (b-value), the activity decay-rate (p-value) and fault-plane solutions of the aftershocks throw new light on the aftershock characteristics and seismotectonics of the region. The observations suggest that the aftershock generating process and the b- and p-values changed with time, space and magnitude-ranges. The main shock occurred on the Basement Thrust by a thrust-fault mechanism. The aftershocks, on the other hand, are triggered by the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust as well as by strike-slip faulting, and all the events occurred above the plane of detachment.