Date of Award
Doctor of Philosophy (PhD)
For my thesis I use multiple types of seismic sources and methods to investigate the crust of northern Wyoming. In the first study, I use single-channel "Texan" geophones as passive-source receivers despite their intended use as active-source instruments. I show that these instruments are not only capable of recording distant earthquakes, but that the dense arrays that are typical with Texan deployments can be used to great effect to image basin geometry, successfully recovering the depth and dip of the Bighorn Basin and Powder River Basin using simple assumptions.
My second study involves finding the regional stress field using 89 local earthquakes that I identified and located using six months of continuous waveform data. I compute focal mechanisms for 14 earthquakes and find dominantly NE-SW directed extensional faulting throughout the mid to upper crust. I also observe one 63-km deep earthquake under the Wind River Basin, which is below the Moho in the area. This indicates a strong upper mantle capable of brittle failure, separated from the crustal earthquakes by a ~20 km thick aseismic zone.
I next use the earthquakes I have observed and located to study the effectiveness of P/S amplitude ratios as a seismic discriminant at close distances. This test is useful as global seismic monitoring continues to grow and networks expand nearer to known, and potentially unknown, explosion sources. I find that the P/S amplitude ratio remains a viable discriminant, but observe dramatic variation across the array due to geologic interference along the source-receiver path that could present significant problems for future discrimination studies.
Last I combine the active and passive sources used in these studies to create a 3D tomographic image of the crust and upper mantle. I invert over 22,000 P-wave arrivals to create the final velocity model. The result shows a surprising lack of fast 7.x material in the lower crust, material which had been first imaged by the nearby Deep Probe experiment. I then use the 3D velocity model to relocate the local earthquakes, resulting in a pattern that suggests reactivation of pre-existing Laramide-age thrust faults.
O'Rourke, Colin T., "Imaging and Investigating the Crust of Wyoming using Multiple Seismic Sources and Techniques" (2015). Geological Sciences Graduate Theses & Dissertations. 103.