Current deformation of the Himalaya–Tibet–Burma seismic belt: inferences from seismic activity and strain rate analysis

Abstract

A new measure of seismic activity in a region is defined, which is based on the concept of integrated fault surface area of earthquakes in the region. Using this concept, we analyze the Harvard CMT data of the Himalaya–Tibet–Burma seismic belt to estimate the relative proportions of seismic activity corresponding to reverse, strike-slip and normal faulting in the region. Further, strain rates are computed through summation of moment tensor elements of earthquakes. The study indicates that the deformation patterns of the Himalaya and the adjoining Tibetan plateau regions are distinct. For instance, the seismic activity of the reverse fault category changes from 93% in the Himalaya to a mere 2% in Tibet, which is dominated instead, by strike-slip faulting (59%). Strain rate computation indicates predominant crustal thickening in the Himalaya with a clear transition to crustal thinning in the Tibetan plateau region, just across the Indus-Tsangpo suture zone where EW extension is the predominant mechanism. A model of a thinning seismic upper crust in the EW direction decoupled from a thickening aseismic lower crust, both in equilibrium, in the Tibetan plateau, is proposed. In the Burmese arc region, crustal thickening is indicated, but coupled with NS compression. The observed seismic activity is predominantly of the strike-slip type (56%), uncharacteristic of subduction zones, which generally display up to 75% of seismic activity of the reverse fault category. This has implications for Indian plate motion along the Burmese arc, rather than in the direction of the subducted slab.