Regional Mapping of the Crustal Structure in Southern California

Abstract

This thesis presents a study of the lateral variations of the crustal structure in Southern California from the waveform data recorded by the Southern California Seismic Network (SCSN) stations, the LARSE I and II surveys. The Receiver Function method is used to process the teleseismic waveform data to map the lateral variations of the crustal structure. A first arrival analysis of the LARSE II refraction data is used to determine the upper crustal velocity structure (less than 4 km). A 2-D upper crustal structure with seven steeply dipping faults was constructed from the LARSE II seismic refraction survey. The regional upper plate complexes of the granitic and gneissic crystalline rocks are observed to yield consistently anomalous low velocities of 3.7-4.3 km/s as compared with the expected velocities for the constituent rock types, which is inferred to result from the extreme shearing, brittle fracturing and related retrogressive hydration reactions. The faults are identified from the distinctive features in the first arrivals and they correlate very well with the geologically mapped faults. Lateral variations of the crustal structure in Southern California are imaged from the back-azimuthal-grouped receiver functions (RFs) of the SCSN stations and the LARSE I, II passive stations. Large variations in the crustal structure are commonly observed beneath the San Gabriel Mountains (SGM), the western Peninsula Ranges, and the eastern Mojave Desert, such as the transition from 2-layered crustal structure to 3-layered from the eastern to the western SGM and the large offsets on the Moho among the different RF station groups. Deep Moho of 34-39 km is observed beneath the western Peninsula Ranges (WPR), Sierra Nevada and the San Bernardino Mountains and no regional root is observed beneath the San Gabriel Mountains. Synthetic waveform modeling of the anomalous features in the RFs for two stations in the eastern San Gabriel Mountains indicates the existence of a flat-topped notch structure on the Moho. Moho is inferred to get shallowed from 37-39 km north of the San Andreas Fault, 33-35 km south of the San Gabriel Fault to a depth of ~29 km beneath the Mt. Baldy block.