Geology

A Stronger Fault

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Science  25 Feb 2000:
Vol. 287, Issue 5457, pp. 1365
DOI: 10.1126/science.287.5457.1365d

The San Andreas fault is perhaps the most thoroughly studied fault in the world, in part because it fails in large earthquake events. Such release of stress in large earthquakes suggests that the crust should be strong. Nonetheless, the San Andreas has been thought to be an anomalously weak fault, in part because several studies failed to find any excess heat released along the fault, as might be expected if the fault zone were strong such that friction would be high during faulting. Also, the main tectonic stresses were inferred to be close to perpendicular to the fault, which imply that the fault slipped at low stress.

Scholz has now assembled a variety of arguments that the fault is actually strong, as would be consistent with most laboratory measurements of rock failure and other known faults. He argues that stresses near the fault are rotated toward low angles and thus high along the fault and that ground water flow, driven by the high topography along the fault, is redistributing the heat produced by high friction so broadly that there is no sharp heat flow anomaly. This model would imply that the permeability of the upper crust along the fault is higher than inferred in the original heat flow models, but consistent with values predicted or measured elsewhere.—BH

Geology28, 163 (2000).

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