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Complexity of the deep San Andreas Fault zone defined by cascading tremor

Abstract

Weak seismic vibrations—tectonic tremor—can be used to delineate some plate boundary faults. Tremor on the deep San Andreas Fault, located at the boundary between the Pacific and North American plates, is thought to be a passive indicator of slow fault slip. San Andreas Fault tremor migrates at up to 30 m s−1, but the processes regulating tremor migration are unclear. Here I use a 12-year catalogue of more than 850,000 low-frequency earthquakes to systematically analyse the high-speed migration of tremor along the San Andreas Fault. I find that tremor migrates most effectively through regions of greatest tremor production and does not propagate through regions with gaps in tremor production. I interpret the rapid tremor migration as a self-regulating cascade of seismic ruptures along the fault, which implies that tremor may be an active, rather than passive participant in the slip propagation. I also identify an isolated group of tremor sources that are offset eastwards beneath the San Andreas Fault, possibly indicative of the interface between the Monterey Microplate, a hypothesized remnant of the subducted Farallon Plate, and the North American Plate. These observations illustrate a possible link between the central San Andreas Fault and tremor-producing subduction zones.

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Figure 1: LFE source family locations15 along the central San Andreas Fault and their interfamily interactions.
Figure 2: Migration patterns of LFE families.
Figure 3: LFE propagation velocities.
Figure 4: Tremor propagation example and conceptual model.
Figure 5: LFE family locations and interactions suggest deep fault zone complexity, perhaps related to an underlying fossil slab.

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Acknowledgements

I am very grateful for feedback and reviews from G. McLaskey, O. Kaven, D. Hill, J. Hardebeck, A. Thomas, D. Trugman and N. Beeler, all of which helped to improve the manuscript. The HRSN borehole seismic network, which was used to detect LFEs examined in this study, is operated by UC Berkeley. Data were obtained through the Northern California Earthquake Data Center (NCEDC).

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Correspondence to David R. Shelly.

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Shelly, D. Complexity of the deep San Andreas Fault zone defined by cascading tremor. Nature Geosci 8, 145–151 (2015). https://doi.org/10.1038/ngeo2335

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