Geophysics

Trading Places

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Science  26 Nov 2004:
Vol. 306, Issue 5701, pp. 1439
DOI: 10.1126/science.306.5701.1439b

The southernmost segment of the San Andreas fault extends from the Salton Sea, bends around the San Bernardino and San Gabriel mountains, and intersects with the San Jacinto fault at its northern edge. Bennett et al. have found that the faults have been taking turns at sliding along to accommodate plate boundary motions. The San Andreas fault had an average displacement rate of 35 mm/year around 1.5 million years ago (Ma), then decreased to 9 mm/year at about 90 thousand years ago (ka), whereas the San Jacinto fault had almost no displacement at 1.5 Ma and then increased to 26 mm/year at 90 ka.

The big switch occurred at about the time that the bend in the San Andreas fault formed and suggests that the bend created a barrier to slip that was taken up by the San Jacinto fault. Today, the rates have switched back again, so that the San Andreas displacement rate is 27 mm/year whereas the San Jacinto rate is 8 mm/year. These rates do not correlate with seismic activity; the San Jacinto fault has had multiple moderate-to-large-magnitude historic events whereas the southern San Andreas segment has had no large-magnitude events. This could mean that the southernmost San Andreas only ruptures in large-magnitude earthquakes regardless of its average displacement rate. — LR

Geology 32, 961 (2004).

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