2012 IRIS Workshop
Nonvolcanic Tremors and Deep Slow Slip Events in Central California
Aurelie Guilhem: Berkeley Seismological Laboratory now at Lawrence Berkeley National Laboratory, Robert M. Nadeau: Berkeley Seismological Laboratory
A) Location of NVT, low-frequency, and repeating earthquake activities. B) Tremor activity and recurrence times (Tr) of the episodes before (diamond) and after (dots) the 2004 Parkfield (PKD) earthquake. SS indicates 2003 San Simeon event. Bottom, times of M7.5 to M8 (dashed lines) and M8+ earthquakes (thick lines) are compared to the episodes’ (thin lines). C) Scaling of the displacement (d) and Tr with seismic moment (Mo). Parkfield episodes (stars) are compared to SSEs from other regions.
Full-resolution graphics file in original format: 0056.ai
We show evidences that recurring episodes of nonvolcanic tremors (NVT) along the San Andreas Fault (SAF) in central California near Parkfield are comparable to seismic events associated with slow slip events (SSE) observed in subduction zones. They are not uniformly periodic in time but instead, they 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 locating the episodic tremors in map view and at depth and comparing them with low-frequency event behavior, we note that the episodes mainly occur on a SAF parallel patch that is ~25 km long and 15 km wide centered at 25 km depth. By constraining their duration and correlated response with activity of repeating earthquake sequences along the SAF, we propose a simple SSE model for the 52 Parkfield episodes observed in ~ 9.7 years. Our results suggest that ~ 10-day period slow-slips corresponding to maximum slips of ~ 7.8 mm. In terms of magnitudes, they range between 5.0 and 5.4. This implies a stress drop of ~ 10 kPa. Despite the small displacements associated with these events and their greater depths, we also find that seismicity patterns above the SSE zone correlate with the SSE timing. Finally, we compare the source parameters of the SAF SSEs with SSEs of other regions and we show that they follow similar scalings when considering both their recurrence times and inferred displacements.
Acknoweldgements: Supported by the U.S. Geological Survey through award G10AC00093 and by the National Science Foundation through awards EAR-0738342 and EAR0910322.
For further reading: Submitted to Earth and Planetary Science Letters, 2012
Keywords: nonvolcanic_tremors, slow_slip, repeating_earthquake, san_andreas_fault