Broadband waveform modeling and its application to the lithospheric structure of the Tibetan Plateau

This thesis presents a study of the lithospheric structure of the Tibetan Plateau. The data are broadband seismic waveforms recorded during the 1991-1992 Sino-US Tibet PASSCAL experiment. Several techniques are developed to retrieve the structural information from these waveforms at ranges from near-field to teleseismic distances.; First, a 1-D average crustal velocity model is derived from regional earthquakes, based on travel times of various phases and modeling waveforms of Love waves. The source mechanisms and depths of 62 events in Tibet and surrounding areas are determined using this 1-D model. The result is that most earthquakes occur at shallow depths, between 5 and 15 km. Thrust faulting source mechanisms are dominant on the margins of the plateau. Within the plateau but at locations with surface elevation less than 5 km, source mechanisms are a mixture of strike-slip and thrust. In areas with surface elevation higher than 5 km, all events show consistently normal faulting, which indicates that a large portion of the high plateau is under EW extension. I also found three sub-crustal earthquakes at depth range between 70 and 80 km in southern Tibet. Their existence suggests relatively cold uppermost mantle in the region.; The lateral variations are investigated using teleseismic waveforms. Crustal thickness and Vp/Vs ratio at each station are estimated using receiver function analysis. I found crustal Vp/Vs ratios to range from 1.75 to 2.0 and crustal thicknesses from 55 to 80 km. On average the northern Tibetan crust is 20 km thinner and has a higher Vp/Vs ratio than the southern part. Teleseismic P and S arrival delays exhibit strong azimuthal and lateral variation. The uniform surface elevation of Tibet coupled with large variations of crustal thickness and upper mantle velocity suggest that the north-central plateau is supported partly by a hot upper mantle. A low velocity layer is found in the mid-crust of northern Tibet. Modeling the anomalous "double-pulse" P waveform at the northernmost station reveals a 15 to 20 km Moho offset between the plateau and the Qaidam Basin.