Seismic velocity structure in the western United States from shear-wave splitting and receiver functions of teleseismic earthquakes

In this dissertation, while I use seismic anisotropy to provide constraints on upper mantle deformation and its relation to surface tectonic features through examination of shear-wave splitting of teleseismic shear waves, I use combined analysis of teleseismic receiver functions and surface waves to estimate detailed crustal and upper mantle shear-wave velocity structure in the western United States. The first part of the shear-wave splitting study is on the development of a double layer waveform inversion technique to retrieve the splitting pairs of two anisotropic layers, each possessing hexagonal symmetry with differing horizontal symmetry axes from high quality SKS, SKKS, and S waveforms.; In the next study, I present a systematic analysis of more than 250 teleseismic shear waveforms for shear-wave splitting from 26 broadband stations in the western U. S. The major findings of this study are: (1) the correspondence between the fast directions and plate tectonic deformations, (2) San Andreas fault-parallel anisotropy caused by the finite strain field associated with relative plate motion between the North American and Pacific plates, (3) possible eastward displacement of this strain field in the.southern California region, (4) anisotropy in the Gorda plate region induced by subduction, (5) regional variations in the fast direction of anisotropy within the Basin and Range Province, and (6) a deeper, widespread, E-W oriented fast direction of anisotropy presumably caused by asthenospheric flow in the slabless window left behind the Farallon plate.; In the final study, I present a new, modified method based on the joint inversion of receiver function and surface wave phase velocity data to obtain well-determined one-dimensional shear-velocity structures. This new inversion method takes advantage of average velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus minimizing the non-uniqueness problem resulting from the velocity-depth trade-off. This method results in well-determined estimates of the crustal and upper mantle shear-velocity structures which are in excellent agreement with the wide angle refraction and reflection and earthquake tomographic compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. Simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a near classical Airy type of compensation in the province.; A detailed analysis of the shear-wave velocity model derived from the inversions at station BMN and compressional-wave velocity models from the 1986 PASSCAL experiment is presented for northwestern Nevada. A crustal high pore pressure mechanism is proposed to explain both shear and compressional velocity features. This mechanism favors layered fluid porosity models proposed to explain extensive middle to lower crust continental seismic reflections and high electrical conductivity. I also present evidence for an upper mantle shear-wave low-velocity zone. This low-velocity zone is interpreted to be partial melt that may be associated with magmatic underplating in this region.