Seismic studies of the East African Rift System and the Tibetan Plateau: Implications for the rheology of lithosphere and the evolution of rifts in continents.

摘要

This dissertation investigates rheology of the continental lithosphere and evolution of continental rifts using earthquake source parameters, precisely determined from waveforms and travel-times in two target regions. First, I investigate the distribution of focal depths for earthquakes that occurred along the East African Rift System (EARS), using both new results of this study and reports from the literature. Three different patterns of distributions are observed and I infer them to correspond to three stages in the evolution of rifting. Under amagmatic regions without well-developed rift basins, seismicity extends down to about 30 km but peaks between 10 and 15 km, suggesting that before rift basins are formed, brittle deformation mainly concentrates in the shallow crust but with some extension at greater depths. Under well-developed amagmatic rift basins, the distribution of focal depths peaks at 10-15 km and at 30-40 km, indicating that as the rift develops; brittle deformation of the mantle lithosphere becomes more pronounced. Earthquakes beneath magmatic rifts occur only above depths of approximately 25 km, indicating that extended mantle lithosphere is greatly weakened by high temperature associated with magmatic activities so brittle deformation is limited to the crust.;Some seismicity occurs directly under the axis of the East African Rift System (EARS) at surprisingly large depths of up to about 44 km---a value too great to be in the crust which is being thinned along the most prominent, active continental rift system in the world. In addition, for at least two such earthquakes, I observed underside reflections off the Moho above the earthquake sources, leaving little doubt that these earthquakes occurred in the mantle lithosphere. As such, the mantle lithosphere must be strong enough to sustain the accumulation of elastic strain---a necessary condition for generating earthquakes. The bimodal distribution of focal depths under amagmatic, well-developed rift basins, with peaks centered near 10 and 35 km, strongly suggests a corresponding distribution in the mechanical strength of continental lithosphere under extension.;Additional evidence for this conclusion comes from my study of eleven intracontinental earthquakes, with magnitudes ranging from 4.9 to 6, which occurred in the mantle beneath the western Himalayan syntaxis, the western Kunlun Mountains, and southern Tibet (near Xigaze). High-resolution seismic waveforms show that some focal depths exceeded 100 km, indicating that these earthquakes also occurred in the mantle lithosphere, even though the crust has been greatly thickened.