Seismic studies of the Blair Wallace Wycheg study area using ambient Rayleigh waves and the Tibetan Plateau using P to S wave conversions

Exploration into the Earth's subsurface structure was achieved by using passive source seismic waves. The first half of the thesis uses seismic ambient field data that was processed using seismic interferometry to investigate the critical zone. The purpose of this research is to generate a code to process Rayleigh waves that dominant the seismic ambient field that is inverted for shear wave structure to determine if geologically reasonable images can be produced in order to move forward with larger scale experiments. A drainage system located in Southeastern, Wyoming within a granite terrain was the area of study. Due to limited data coverage, only frequencies above 10 Hz were reliable which resulted in the upper 12 m shear velocity structure being accurately resolved. Even with this limitation, only one-hour of recording of the ambient seismic field was required to produce sufficient impulse responses and geologically reasonable seismic profiles from the ambient seismic field. Low velocity anomalies were found in the upper few meters that correspond with overlying utility roads showing high resolution in the near surface. Multiple hours of recording at one location allowed investigation into the repeatability of this technique. Ten different one-hour long datasets produced an average 7.3 m/s variation between the hour-long shear velocity images that shows the ambient seismic field is reliable and repeatable. In order to resolve all layers within the critical zone, larger station spacing would be required. This would allow lower frequency content to be accurately analyzed that is required to image to greater depths. The second half of the thesis is a crustal scale investigation of across Tibet and Nepal that was achieved by processing earthquake data using receiver function methodology. Currently, a "flab-slab" kinematic model is the leading explanation for the subduction of the Indian Plate beneath the Tibetan Plateau. For this research, a multi-channel deconvolution algorithm is implemented that is more sophisticated than the single three-component station spectral division method. Results indicate a steeper dip of the Indian Plate beneath the Himalayan Block and the presence of a negative velocity gradient that could be the result of dehydrating the lower crust of the Indian Plate as it enters the warm asthenosphere. Given this information, we hypothesize the Indian plate (without its upper crust) continues on a subduction path into the mantle beneath the Himalayan Block. This differs from the "flat-slab" model that underthrusts Tibet near the Yarlung Tsangpo Suture 250 km north of the Main Frontal Thrust and subducts into the mantle near the Banggong-Nujiang Suture further to the north.