Episodic tremors and deep slow-slip events in Central California

Observations of episodic tremor and slip (ETS) events have given insight into the seismic hazards along subduction zones by demonstrating that strain release was possible from slow-slip events (SSEs). Here, we show evidences that recurring episodes of nonvolcanic tremors (NVT) along the San Andreas Fault (SAF) in the Parkfield, California area, are reminiscent of seismic events associated with ETS and SSEs observed in subduction zones. Nonetheless, the Parkfield episodes are not uniformly periodic through time but exhibit systematic differences between aperiodic and periodic recurrences that are related to the 2004 M6.0 Parkfield earthquake and the long-term SAF slip rate. By constraining the locations of the NVT episodes, their duration, and the correlated response between episodes and repeating earthquake activity along the SAF, we propose a simple SSE model for the 52 Parkfield episodes observed in ∼9.7 yr. Our findings suggest that ∼10-day period slow-slips corresponding to maximum slips of ∼7.8 mm occur on a SAF parallel patch that is ∼25 km long and 15 km wide centered at ∼25 km depth. SSE moment magnitudes range between 5.0 and 5.4 and imply a stress drop of ∼10 kPa. We also show that seismicity patterns above the SSE zone show correspondence with the SSE timing, and this emphasizes the importance of NVT episodes as in situ measurements to inferred fault slip transients below the seismogenic zone. Finally, we compare the source parameters of the SAF SSEs with SSEs of other regions and we show that they follow similar scalings.