Geology

Breaking Early and Often

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Science  21 Nov 2008:
Vol. 322, Issue 5905, pp. 1165
DOI: 10.1126/science.322.5905.1165a

In 1857, a major earthquake ruptured the central part of the San Andreas fault from near Parkfield in the Carrizo Plain southward to just northeast of Los Angeles (a distance of about 300 km). Surface slip was up to 9 m. Excavations along this break have revealed evidence of several earlier major earthquakes; accurate dating of these events provides an indication of how frequent and regular such major quakes might be. Akciz et al. provide a series of new dates on a section through the fault at Bidart Fan, in the Carrizo Plain, that revise the history of previous quakes. Four previous events are recognized and resolved here, dating from about 1310. The intervals between the quakes range from as short as about 80 years to as long as 200 years; the average recurrence interval is 137 years. This interval is shorter than previously thought and now longer than the time since the 1857 quake. Although the revised dates imply a more regular rupture of the fault, it is not yet clear if the previous major quakes were like that in 1857, or even greater, or ruptured to the north, rather than the south. — BH

J. Geophys. Res. 10.1029/2007JB005285 (2008).

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