Earthquake mechanisms in the Himalayan, Burmese, and Andaman Regions and continental tectonics in central Asia

Focal mechanisms are presented for thirteen earthquakes between Hindu Kush and Sumatra, including four shallow-focus events along the Himalayan mountain front and two events at intermediate depth beneath the Burmese mountains. All mechanisms are based on first-motion P- and S-wave data recorded by long-period instruments. The mechanisms along the Himalayan front confirm the existence of thrust faulting for which there is post-Mesozoic geologic evidence. Additional evidence for thrust faulting in central Asia comes from focal mechanisms based on other seismic data published recently by Russian investigators. The axis of minimum compression (the T axis) at intermediate depths beneath the Burmese mountains is oriented approximately down the dip of the seismic zone, as are T axes at intermediate depths in several other seismic zones. A mechanism solution consistent with strikeslip faulting and another consistent with normal faulting were derived from two shallow earthquakes in western China. One shallow earthquake within the Himalayas yielded a normal faulting mechanism, as did one event from the western margin and two events near the northern border of the Andaman Sea. Slip vectors consistent with underthrusting beneath the Himalayas have a nearly uniform north to northeasterly strike. This evidence, as well as the frequency of occurrence of large-magnitude earthquakes, suggests that seismic slip at shallow depths may account for the convergence between the Indian Ocean and the Eurasian plates along the Himalayan mountain front.