Focal depths and fault plane solutions of earthquakes under the Tibetan Plateau

Abstract

We compare synthetic and recorded P wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts (>4000 m) of the Tibetan plateau. Fault plane solutions for all 16 events show combinations of normal and strike slip faulting with T axes oriented approximately east–west. None of these solutions show thrust faulting. Thus the data corroborate previous inferences that the active tectonics are dominated by east–west extension. Focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km. This style of deformation and these depths of faulting are similar to those in the Basin and Range province of the western United States. Two intermediate depth events below the crust of southern Tibet also show primarily normal faulting with east–west T axes. The solution for one, discussed by Chen et al. (1981), is unambiguous. The solution for the other, the event of August 1, 1973 (27.59°N, 89.17°E, 85±10 km, mb = 4.9) is less certain. Both apparently occurred in the mantle beneath a thick, aseismic lower crust, and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath Tibet.