Focal depths and mechanisms of shallow earthquakes in the Himalayan–Tibetan region

Abstract

The complexity of the Himalayan–Tibetan lithospheric deformation is evident from widespread seismicity and diverse focal mechanism solutions. Here we investigate the focal depths and fault plane solutions of 97 moderate and shallow earthquakes in the Himalayan–Tibetan region by modeling teleseismic P-wave and its tailing surface reflections pP and sP. Earthquakes in central Tibet are restricted to the upper crust and originate dominantly by strike-slip faulting, in agreement with the low P-wave velocity layers in the lower crust and the strong S-wave attenuation zones in the uppermost mantle. In northern and southern Tibet, where the Asian and Indian plates descend beneath central Tibet, earthquakes appear to be distributed throughout the thickness of the crust and exhibit dominantly reverse faulting. We incorporate well-estimated focal depths of 127 additional earthquakes from previous studies to estimate the seismogenic thickness (Ts) of the study region. The resulting pattern of Ts is found to be rather flat for central and northeastern Tibet and highly variable along the strike of the Himalayan foreland.