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Science 12 September 2008:
Vol. 321. no. 5895, pp. 1478 - 1481
DOI: 10.1126/science.1160943
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Reports

Postseismic Relaxation Along the San Andreas Fault at Parkfield from Continuous Seismological Observations

F. Brenguier,1,2* M. Campillo,2 C. Hadziioannou,2 N. M. Shapiro,1 R. M. Nadeau,3 E. Larose2

Seismic velocity changes and nonvolcanic tremor activity in the Parkfield area in California reveal that large earthquakes induce long-term perturbations of crustal properties in the San Andreas fault zone. The 2003 San Simeon and 2004 Parkfield earthquakes both reduced seismic velocities that were measured from correlations of the ambient seismic noise and induced an increased nonvolcanic tremor activity along the San Andreas fault. After the Parkfield earthquake, velocity reduction and nonvolcanic tremor activity remained elevated for more than 3 years and decayed over time, similarly to afterslip derived from GPS (Global Positioning System) measurements. These observations suggest that the seismic velocity changes are related to co-seismic damage in the shallow layers and to deep co-seismic stress change and postseismic stress relaxation within the San Andreas fault zone.

1 Sismologie, Institut de Physique du Globe de Paris (IPGP) and CNRS, 4 Place Jussieu, 15252 Paris, France.
2 Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph Fourier and CNRS, 38041 Grenoble, France.
3 Berkeley Seismological Laboratory, University of California Berkeley, 215 McCone Hall, Berkeley, CA 94760, USA.

* To whom correspondence should be addressed. E-mail: fbrengui{at}ipgp.jussieu.fr

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Depth of Rock Damage from Strong Seismic Ground Motions near the 2004 Parkfield Mainshock.
N. H. Sleep (2009)
Bulletin of the Seismological Society of America 99, 3067-3076
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Temporal changes of surface wave velocity associated with major Sumatra earthquakes from ambient noise correlation.
Z. J. Xu and X. Song (2009)
PNAS 106, 14207-14212
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Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes.
R. M. Nadeau and A. Guilhem (2009)
Science 325, 191-193
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