Graduate Thesis Or Dissertation
 

A Genetic Algorithm Approach to Imaging the Internal Southern San Andreas Fault Zone

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gq67k063q

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  • The southern San Andreas fault (SSAF) is the fault segment with the highest 30-year probability of causing a moderate to large magnitude earthquake in California, yet key structural characteristics of the fault remain unconstrained, particularly in the Coachella Valley where the fault splits into the Banning (BF) and Mission Creek (MCF) faults. Questions remain about whether the SSAF dips northeast or is more vertical, how strain is partitioned and released along it, and which strand of the fault is presently more active. To address these questions, wavefield variations from local earthquakes were analyzed across an array of 322 three-component seismic nodes deployed near Thousand Palms Oasis Preserve in California. Beamforming was performed on early P waves propagating across subarray halves near the BF and MCF, yielding beams representing P wavefront arrivals on either side of each fault strand. 24 events met coherency criteria for both subarrays. Because tomographic seismic velocity models lack resolution in the area and poorly capture the sharp velocity contrasts expected within the fault zone, a genetic algorithm was designed to find fault damage zone structures that can be inserted into existing seismic velocity models to better reproduce the measured beams from these 24 events. Using a four-block model of fault zone geometry and synthetic datasets with and without small errors, the algorithm successfully produces a family of solutions clustered around the depths of the four blocks, their velocity perturbations, and the dips of the two faults. Damage zone depths appear less well resolved than velocity perturbations and fault dips. Despite spurious depth results, when applied to the measured beams, the algorithm consistently yields a family of solutions dominated by moderate velocity increases in the blocks northeast of the BF and MCF and southwest of the BF, small velocity decreases in the block southwest of the MCF, a slightly northeast dipping BF, and a moderately southwest dipping MCF. These results can be incorporated into seismic hazard analysis of the critically strained SSAF and the developed tool can be applied to other sections of the same fault as well as similar faults and large-N array datasets globally.
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