Common conversion point imaging of the western United States: Improved methods for receiver function stacks

Receiver functions are an important tool to study the upper mantle and its discontinuities. Due to the regional complex geologic history and ongoing tectonism, the western United States has been a difficult area to image using traditional receiver functions methods. This thesis will develop improved methods to produce receiver functions and attempts to determine which style of receiver function generation works best in this complex area. To generate receiver functions, we interpret Ps (P-wave to S-wave) conversions and reverberations from broadband seismograms of teleseismic events from 30 to 90 degrees away. This is accomplished by deconvolving the horizontal component of the seismogram by the vertical (which, because of the near vertical ray paths, is assumed to be the P-source function) leaving Ps conversions that arise from velocity discontinuities in the upper mantle (from 0 to 800 km). Receiver function methods employing traditional source equalized deconvolution were tested alongside more recent beamforming and simultaneous deconvolution strategies. Three dimensional velocity models were used during raytracing of events from source to receiver locations to allow for the most precisely calculated raypaths. Methods involving both beamforming and simultaneous deconvolution of teleseismic events proved to be the most robust way to generate images of the upper mantle. Depths to the Moho, 410, and 660 km discontinuities were mapped and interpreted. A commonly held view is that temperature variations are the sole contributing factor to depths of discontinuities. This study however shows that there must be other controlling factors affecting the transition zone boundary depths, for the majority of the discontinuities mapped did not correlate as they should have with inferred temperature anomalies. The anomalies on the 410 km discontinuity seemed to correspond to some tectonic features, such as the propagation of the Gulf of California, Basin and Range province extension, and/or warm mantle presumed to provide elevation in the Rocky Mountains and the Colorado Plateau. The 660 km discontinuity did not seem to correlate with these features (if depth variations are indeed controlled by temperature). Therefore, either temperature anomalies do not cross vertically through the transition zone, or the depths to these boundaries are not solely dependent on temperature.