Masters Thesis

Rayleigh wave tomography beneath the oceanic and continental margin of the North American and Pacific plate boundary

The North American and Pacific plates are separated by a unique transform plate boundary in southern California. The inception of the San Andreas fault system formed as a result of subduction of the East Pacific Rise spreading center, rifting of the Borderlands in the Miocene, and subsequent plate rotation that is ongoing today. However, the stresses surrounding this tectonic system are only partially understood due to the lack of offshore data which makes up half of this plate boundary. I used Rayleigh waves recorded by a marine seismic array of 34 ocean bottom seismometers (OBS) deployed as part of the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) project offshore southern California on 18-32 Ma seafloor. The marine seismic array recorded data for a 12 month duration from August 2010 to 2011, and are combined with 82 land stations from the CISN network which recorded earthquake data simultaneously. I analyzed 80 teleseismic events at distances ranging from 30° to 120° for Mw ≥ 5.9, filtered at periods between 16 and 78 s. Strong gradients in water depth, sediments, and crustal thickness are present across this plate margin; therefore, I performed amplitude corrections for OBS stations that account for velocity variations in water, sediment layer, crustal thickness, marine fossil layers, and lithospheric thickness as a function of sea floor age. I used a surface wave inversion that considers a two plane wave method to represent the incoming wave field and performed a grid search for inversion parameters. My results indicated that phase velocities averaged over my study area offshore are 1.6% lower than previous studies for the seafloor age bin 20-52 Ma that used oceanic ray paths recorded by land stations only. Phase velocities in the Borderlands are lower than the deep seafloor, but higher than the southern California land region at all periods. Phase velocities at lithospheric depths are 1.4% higher in the oceanic mantle compared to the continental mantle indicating compositional and structural differences due to formation history in the two tectonic environments. Anisotropy in the Southern California land region is very uniform at 1.5 % in an average direction N 69˚ W. The inner Borderland demonstrates different anisotropic structure compared to the outer Borderland at all periods. The fast directions in the inner Borderland are NW-SE below 50 s and change to NE-SW above 50 s. Anisotropy in the outer Borderland and deep seafloor displays an E-W fast direction at short periods that is perpendicular to seafloor magnetic anomalies, and consistent with the remnant fossil spreading direction. A transition is observed at long periods above 50s in the outer Borderland and deep seafloor that has a NW-SE fast direction which is parallel to active Pacific plate motion today. This change in anisotropic fabric from fossil spreading 15-35 Ma to active plate motion today will also constrain the base of the lithosphere.

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