Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault


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.

This is a preview of subscription content

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Example one-day tremor time series with superimposed tidal stresses.
Figure 2: Results of chi-square significance tests.
Figure 3: Tidal stress magnitude and rate distributions.
Figure 4: Percentage of excess events versus friction coefficient.


  1. Obara, K. Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296, 1679–1681 (2002)

    CAS  ADS  Article  Google Scholar 

  2. Shelly, D. R., Beroza, G. C. & Ide, S. Non-volcanic tremor and low-frequency earthquake swarms. Nature 446, 305–307 (2007)

    CAS  ADS  Article  Google Scholar 

  3. Shelly, D. R., Beroza, G. C., Ide, S. & Nakamula, S. Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip. Nature 442, 188–191 (2006)

    CAS  ADS  Article  Google Scholar 

  4. Rubinstein, J. L., La Rocca, M., Vidale, J. E., Creager, K. C. & Wech, A. G. Tidal modulation of nonvolcanic tremor. Science 319, 186–189 (2008)

    CAS  ADS  Article  Google Scholar 

  5. Audet, P., Bostock, M. G., Christensen, N. I. & Peacock, S. M. Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing. Nature 457, 76–78 (2009)

    CAS  ADS  Article  Google Scholar 

  6. Nakata, R., Suda, N. & Tsuruoka, H. Non-volcanic tremor resulting from the combined effect of Earth tides and slow slip events. Nature Geosci. 1, 676–678 (2008)

    CAS  ADS  Article  Google Scholar 

  7. Gomberg, J. et al. Widespread triggering of nonvolcanic tremor in California. Science 319, 173 (2008)

    CAS  ADS  Article  Google Scholar 

  8. Rogers, G. & Dragert, H. Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip. Science 300, 1942–1943 (2003)

    CAS  ADS  Article  Google Scholar 

  9. Brenguier, F. et al. Postseismic relaxation along the San Andreas fault at Parkfield from continuous seismological observations. Science 321, 1478–1481 (2008)

    CAS  ADS  Article  Google Scholar 

  10. Rubinstein, J. L. et al. Non-volcanic tremor driven by large transient shear stresses. Nature 448, 579–582 (2007)

    CAS  ADS  Article  Google Scholar 

  11. Nadeau, R. M. & Guilhem, A. Nonvolcanic tremor evolution and the San Simeon and Parkfield, California, earthquakes. Science 325, 191–193 (2009)

    CAS  ADS  Article  Google Scholar 

  12. Nadeau, R. M. & Dolenc, D. Nonvolcanic tremors deep beneath the San Andreas fault. Science 307, 389 (2005)

    CAS  Article  Google Scholar 

  13. Shelly, D. R., Beroza, G. C. & Ide, S. Complex evolution of transient slip derived from precise tremor locations in western Shikoku, Japan. Geochem. Geophys. Geosyst. 8 10.1029/2007gc001640 (2007)

  14. Lambert, A., Kao, H., Rogers, G. & Courtier, N. Correlation of tremor activity with tidal stress in the northern Cascadia subduction zone. J. Geophys. Res. Solid Earth 114 10.1029/2008JB006038 (2009)

  15. Nadeau, R. M. & McEvilly, T. V. Periodic pulsing of characteristic microearthquakes on the San Andreas fault. Science 303, 220–222 (2004)

    CAS  ADS  Article  Google Scholar 

  16. Dieterich, J. H. Nucleation and triggering of earthquake slip—effect of periodic stresses. Tectonophysics 144, 127–139 (1987)

    ADS  Article  Google Scholar 

  17. Agnew, D. SPOTL: Some Programs for Ocean Tide Loading〉 (1996)

    Google Scholar 

  18. Agnew, D. C. NLOADF: A program for computing ocean-tide loading. J. Geophys. Res. Solid Earth 102, 5109–5110 (1997)

    Article  Google Scholar 

  19. Shelly, D. R. et al. Precise location of San Andreas fault tremors near Cholame, California using seismometer clusters: slip on the deep extension of the fault? Geophys. Res. Lett. 36 10.1029/2008gl036367 (2009)

  20. Beeler, N. M. & Lockner, D. A. Why earthquakes correlate weakly with the solid Earth tides: effects of periodic stress on the rate and probability of earthquake occurrence. J. Geophys. Res. Solid Earth 108 10.1029/2001jb001518 (2003)

  21. Lockner, D. A. & Beeler, N. M. Premonitory slip and tidal triggering of earthquakes. J. Geophys. Res. Solid Earth 104, 20133–20151 (1999)

    Article  Google Scholar 

  22. Vidale, J. E., Agnew, D. C., Johnston, M. J. S. & Oppenheimer, D. H. Absence of earthquake correlation with Earth tides: an indication of high preseismic fault stress rate. J. Geophys. Res. Solid Earth 103, 24567–24572 (1998)

    Article  Google Scholar 

  23. Ide, S., Shelly, D. R. & Beroza, G. C. Mechanism of deep low frequency earthquakes: further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface. Geophys. Res. Lett. L03308 10.1029/2006gl028890 (2007)

  24. Cochran, E. S., Vidale, J. E. & Tanaka, S. Earth tides can trigger shallow thrust fault earthquakes. Science 306, 1164–1166 (2004)

    CAS  ADS  Article  Google Scholar 

  25. Blanpied, M. L., Lockner, D. A. & Byerlee, J. D. Frictional slip of granite at hydrothermal conditions. J. Geophys. Res. Solid Earth 100, 13045–13064 (1995)

    CAS  Article  Google Scholar 

  26. Becken, M. et al. A deep crustal fluid channel into the San Andreas Fault System near Parkfield, California. Geophys. J. Int. 173, 718–732 (2008)

    ADS  Article  Google Scholar 

  27. Hart, R. H. G. et al. Tidal calibration of borehole strainmeters: removing the effects of small-scale inhomogeneity. J. Geophys. Res. 101, 25553–25571 (1996)

    ADS  Article  Google Scholar 

  28. Devore, J. L. Probability and Statistics: for Engineering and the Sciences 6th edn, Ch. 14 (Brooks/Cole-Thomson Learning, 2004)

    Google Scholar 

Download references


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.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Amanda M. Thomas.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Figures

This file contains Supplementary Figure 1 with Legend. (PDF 100 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing