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Summary: Differences in the evolution of seismic velocities
after the San Simeon and the Parkfield earthquakes
indicate that two different physical mechanisms may
be responsible for the changes in crustal properties: (i)
damage of shallow layers and fault zone caused by
the strong ground shaking and (ii) co-seismic stress
change followed by the postseismic relaxation. These
results demonstrate that measuring small velocity per-
turbations from correlations of seismic noise can be a
useful tool for studying the continuous time evolution
of the stress regime in the vicinity of seismogenic faults.
References and Notes
1. B. Allmann, P. Shearer, J. Geophys. Res. 112, B04305 (2007).
2. J. E. Vidale, Y. Li, Nature 421, 524 (2003).
3. Y. Li, J. E. Vidale, S. Day, D. Oglesby, E. Cochran, Bull.
Seismol. Soc. Am. 93, 854 (2003).
4. F. Niu, P. G. Silver, T. M. Daley, X. Cheng, E. L. Majer,
Nature 454, 204 (2008).
5. N. M. Shapiro, M. Campillo, L. Stehly, M. H. Ritzwoller,
Science 307, 1615 (2005).
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