Since its initial discovery nearly a decade ago1, non-volcanic tremor has provided information about a region of the Earth that was previously thought incapable of generating seismic radiation. A thorough explanation of the geologic process responsible for tremor generation has, however, yet to be determined. Owing to their location at the plate interface, temporal correlation with geodetically measured slow-slip events and dominant shear wave energy, tremor observations in southwest Japan have been interpreted as a superposition of many low-frequency earthquakes that represent slip on a fault surface2,3. Fluids may also be fundamental to the failure process in subduction zone environments, as teleseismic and tidal modulation of tremor in Cascadia and Japan and high Poisson ratios in both source regions are indicative of pressurized pore fluids3,4,5,6,7. Here we identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. We suggest that this tremor represents shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. There are a number of similarities between tremor in subduction zone environments, such as Cascadia and Japan, and tremor on the deep San Andreas transform3,4,5,6,7,8,9,10,11,12, suggesting that the results presented here may also be applicable in other tectonic settings.
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We thank D. Agnew, N. Beeler, G. Beroza, F. Bonetto, J. Gomberg, A. Guilhem, R. Nakata, Z. Peng, E. Roeloffs and D. Shelly for discussion and comments that improved the manuscript. Funding was provided in part by the National Science Foundation through a Graduate Research Fellowship and awards EAR-0537641 and EAR-0544730, and by the US Geological Survey through awards 06HQGR0167, 07HQAG0014 and 08HQGR0100. Data used in this study were obtained from the Northern California Earthquake Data Center (NCEDC). This is Berkeley Seismological Laboratory contribution #10-23.
Author Contributions A.M.T. performed the analysis. A.M.T. and R.B. wrote the paper. R.M.N. developed the tremor and repeating earthquake catalogues and performed analysis related to Figure 3. All authors contributed to the interpretation and final manuscript preparation.
The authors declare no competing financial interests.
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Thomas, A., Nadeau, R. & Bürgmann, R. Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault. Nature 462, 1048–1051 (2009). https://doi.org/10.1038/nature08654
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