Stress Accumulation

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Science  01 Jun 2001:
Vol. 292, Issue 5522, pp. 1611
DOI: 10.1126/science.292.5522.1611a

The magnitude 7.8 San Francisco earthquake occurred on the San Andreas fault in 1906. For about 70 years afterward, no earthquake greater than magnitude 6 occurred in the San Francisco Bay Area, except for the magnitude 6.5 Morgan Hill event in 1911. This quiescent period has been attributed to the 1906 earthquake having caused shear stress relaxation on other faults in the area, most of which run parallel or subparallel to the San Andreas. The regional perturbation to the stress field, which acts to extend the time until the next fault failure, is called the stress shadow, and almost all of the faults in the stress shadow showed delayed failure. The Morgan Hill earthquake was the exception because it ruptured the Calaveras fault, which lies inside the stress shadow of the San Francisco earthquake.

To explain this discrepant rupture, Hori and Kaneda modeled the Morgan Hill segment of the Calaveras fault as a mixture of creeping regions, especially at depth, with localized locked regions. The heterogeneous character of this fault segment allowed stress to accumulate at a much higher rate than normal, so that the fault failed only 5 years after the San Francisco earthquake. This high stress rate also may help to explain the relatively short recurrence rate on this segment of the Calaveras, which experienced another earthquake of magnitude greater than 6 in 1984. — LR

Geophys. Res. Lett. 28, 2261 (2001).

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