In Situ Vp/Vs Ratios from Microearthquakes in Northern and Southern California
Abstract
Recent studies have identified different seismicity features (e.g., seismicity streaks and fault creep) in northern and southern California. Seismic velocities and Vp/Vs ratios are often used to help discriminate lithologies and saturation state of rocks and to estimate variations in fracturing and fluid content in active fault zones. It has been shown that when both P- and S-wave differential times from waveform cross-correlation are available, it is possible to estimate high-resolution near-source Vp/Vs ratio with high precision because of the great accuracy of the waveform cross-correlation data. In order to investigate the structural differences between northern and southern California and improve our understandings of the role of fluids in determining diverse fault slip behaviors, we estimate near-source in situ Vp/Vs ratios in the Parkfield section of the San Andreas Fault and the San Jacinto Fault Zone (SJFZ), the most seismically active regions in northern and southern California. We calculate differential times from waveform cross-correlation and perform similar event cluster analysis based on newly-detected earthquake catalogs by applying a matched filter technique. Over 87% and 77% of earthquakes fall within event clusters in Parkfield and the SJFZ, respectively and each cluster is assumed to have a constant Vp/Vs ratio. Our preliminary results in Parkfield show variable Vp/Vs ratios along the creeping section and relatively uniform ratios in the locked section. In the SJFZ, the Vp/Vs ratios in the intersection areas of fault segments are more fluctuating than other regions. Our study will reveal fine-scale structure that cannot be resolved by the traditional Vp/Vs estimation methods and we will present detailed Vp/Vs patterns in both regions and their comparison.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS070...03L
- Keywords:
-
- 7299 General or miscellaneous;
- SEISMOLOGY