Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA
Article first published online: 23 FEB 2007
Copyright 2007 by the American Geophysical Union.
Geophysical Research Letters
Volume 34, Issue 4, February 2007
How to Cite
2007), Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA, Geophys. Res. Lett., 34, L04306, doi:10.1029/2006GL028634., , and (
- Issue published online: 23 FEB 2007
- Article first published online: 23 FEB 2007
- Manuscript Accepted: 22 JAN 2007
- Manuscript Revised: 9 JAN 2007
- Manuscript Received: 2 NOV 2006
- triggered seismicity;
- annual modulation
 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.