GEOLOGY: Tolerant to a Fault

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Science  21 Dec 2001:
Vol. 294, Issue 5551, pp. 2431b
DOI: 10.1126/science.294.5551.2431b

The San Andreas fault is the boundary where the Pacific plate slides horizontally past the North American plate. Although most of the motion is parallel to the fault (or strike-slip), in several places, including the prominent bends in the fault trace, some of the motion is directed across the fault and has led to the formation of prominent mountain ranges. This convergence across parts of the fault may also influence where it becomes locked; rupture at these sites could then give rise to large earthquakes.

Argus and Gordon apply several years of measurements recorded by the Global Positioning System to investigate how the motion of the North American and Pacific plates is reflected in the dynamics of the fault and to test the relation between convergence across the fault and earthquake hazards and topography. The data imply that the fault is slipping at an average of 39 ± 2 millimeters per year. An analysis based on the size of mountains along the fault and the amount of convergence seen implies that the motion between the two plates changed about 6 to 8 million years ago, and it also finds that not all of the locked sections of the fault occur where convergence is high.—BH

Geol. Soc. Am. Bull. 113, 1580 (2001).

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