We present our estimations and comparisons of the in situ Vp/Vs ratios and seismicity characteristics for the Parkfield segment of the San Andreas fault in northern California and the San Jacinto Fault Zone and its adjacent regions in southern California. Our results show that the high-resolution in situ Vp/Vs ratios are much more complex than the tomographic Vp/Vs models. They show similar variation patterns to those in the tomographic Vp models, indicating that Vp/Vs ratios are controlled by material properties but are also strongly influenced by fluid contents. In Parkfield, we observe velocity contrasts between the creeping and locked sections. In southern California, we see small-scale anomalous Vp/Vs variation patterns, especially where fault segments intersect, terminate and change orientations. In addition, our investigation confirms that the seismicity in Parkfield is more repeatable than in southern California. However, the earthquakes in the southernmost portion of the San Andreas fault, the trifurcation area of the San Jacinto Fault Zone and the Imperial fault are as much likely falling into clusters as those in Parkfield. The correlation of highly similar events with anomalous in situ Vp/Vs ratios supports the important role of fluids in the occurrence of repeating earthquakes. The high-resolution Vp/Vs ratio estimation method and the corresponding results are helpful for revealing roles of fluids in driving earthquake, fault interaction and stress distribution in fault zones.
The spatial geometry of seismicity encodes information about loading and failure processes, as well as properties of the underlying fault structure. Traditional approaches to characterizing geometrical attributes of seismicity rely on assumed locations and geometry of fault surfaces, particularly at depth, where resolution is overall quite poor. In this study, we develop an alternative approach to quantifying geometrical properties of seismicity using techniques from anisotropic point process theory. Our approach does not require prior knowledge about the underlying fault properties. We characterize the geometrical attributes of 32 distinct seismicity regions in California and introduce a simple four class classification scheme that covers the range of geometrical properties observed. Most of the regions classified as having localized seismicity are within northern California, while nearly all of the regions classified as having distributed seismicity are within southern California. In addition, we find that roughly 1 out of 4 regions exhibit orthogonal seismicity structures. The results of this study provide a foundation for future analyses of geometrical properties of seismicity and new observables to compare with numerical modelling studies.
more » « less- NSF-PAR ID:
- 10368757
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 231
- Issue:
- 1
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- p. 493-504
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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