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AGU: Geophysical Research Letters

 

Keywords

  • triggered seismicity
  • Parkfield
  • annual modulation

Index Terms

  • Mathematical Geophysics: Time series analysis
  • Physical Properties of Rocks: Microstructure
  • Structural Geology: Role of fluids
  • Tectonophysics: Continental tectonics: strike-slip and transform
  • Tectonophysics: Stresses: crust and lithosphere

Abstract

Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA

Lizet B. Christiansen

U.S. Geological Survey, Menlo Park, California, USA

Shaul Hurwitz

U.S. Geological Survey, Menlo Park, California, USA

Steven E. Ingebritsen

U.S. Geological Survey, Menlo Park, California, USA

We analyze seismic data from the San Andreas Fault (SAF) near Parkfield, California, to test for annual modulation in seismicity rates. We use statistical analyses to show that seismicity is modulated with an annual period in the creeping section of the fault and a semiannual period in the locked section of the fault. Although the exact mechanism for seasonal triggering is undetermined, it appears that stresses associated with the hydrologic cycle are sufficient to fracture critically stressed rocks either through pore-pressure diffusion or crustal loading/unloading. These results shed additional light on the state of stress along the SAF, indicating that hydrologically induced stress perturbations of ∼2 kPa may be sufficient to trigger earthquakes.

Received 2 November 2006; accepted 22 January 2007; published 23 February 2007.

Citation: Christiansen, L. B., S. Hurwitz, and S. E. Ingebritsen (2007), Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA, Geophys. Res. Lett., 34, L04306, doi:10.1029/2006GL028634.

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