ACTIVE LOWER CRUSTAL FLOW BENEATH TIBET TRIGGERS SEISMOGENIC FAULTING

The large, unexpected, May 12, 2008, Wenchuan earthquake in the western Sichuan Basin along Longmen Shan, China, caused 80,000 fatalities. Historic records indicate that almost all the big earthquakes that have occurred in the northeastern Tibetan Plateau, including the Wenchuan earthquake, were located along a zone of sudden change in topography. Here we show, based on new Rayleigh wave tomography, a close correspondence between the lower crustal thickness underneath the Tibetan Plateau and the change in topography. More significantly, the tomography reveals a distinct low velocity zone in the lower crust of diminishing thickness towards the northeast that we interpret as a zone of hot ductile lower crust that is progressively being injected northeastward. Our new data, combined with previous work, suggests a model for the formation and evolution of lower crustal flow beneath Tibet. Lower crustal flow, plateau uplift, and upper crustal seismogenic faulting are presently active and closely related.