Changing Views of the San Andreas Fault

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Science  26 Feb 2010:
Vol. 327, Issue 5969, pp. 1089-1090
DOI: 10.1126/science.1186770

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The magnitude 7.0 earthquake that struck Haiti on 12 January 2010 is a reminder of the devastation caused by large earthquakes. Because recurrence of large (M 7–8) earthquakes is rare, on the order of centuries, studying the past behavior of a fault guides future expectations. Paleoseismologists examine the stratigraphic and geomorphic history of deposits and landforms along a fault for evidence of past ruptures. Such observations provide information on when earthquakes happened, what parts of the fault failed, and the size of the earthquakes. The collected geologic data form the backbone of probabilistic seismic hazard analyses (1) used by the insurance and engineering industries and are increasingly used to explore models of lithosphere rheology and fault interaction (2, 3). Because of sparse data, however, inferences about patterns of strain accumulation and release are a common occurrence. On pages 1119 and 1117 of this issue, Zielke et al. (4) and Grant Ludwig et al. (5) present data and interpretations providing an exciting new view that questions fault behavior models that have been applied to the south central San Andreas Fault for decades, highlighting the value of revisiting old problems with new techniques.