Journal of Geophysical Research: Solid Earth, vol. 39, issue May (2010) pp. 1-5 Published by Nature Publishing Group
We perform a systematic survey of triggered tremor along the San Andreas Fault in central California for the 31 teleseismic earthquakes with Mw 7.5 since 2001. We identify 10 teleseismic events associated with clear triggered tremor. About 52% of the tremor is concentrated south of Parkfield near Cholame, where ambient tremor has been identified previously, and the rest is widely distributed in the creeping section of the San Andreas Fault north of Parkfield. Tremor is generally initiated and is in phase with the Love wave particle velocity. However, the pattern becomes complicated with the arrival of the Rayleigh waves, and sometimes tremor continues after the passage of the surface waves. We identify two cases in which tremor is triggered during the teleseismic PKP phase. These results suggest that while shear stress from the passage of the Love waves plays the most important role in triggering tremor in central California, other factors, such as dilatational stresses from the Rayleigh and P waves, also contribute. We also examine the ambient tremor occurrence rate before and after the teleseismic events and find a transient increase of stacked tremor rate during the passage of the teleseismic surface waves. This observation implies that the occurrence time of tremor is temporally advanced by the dynamic stresses of the teleseismic waves. The amplitude of the teleseismic waves correlates with the occurrence of triggered tremor, and the inferred tremor-triggering threshold is 23 kPa. The relatively low triggering threshold indicates that the effective stress at the tremor source region is very low, most likely due to near-lithostatic fluid pressure.
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