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Nearly frictionless faulting by unclamping in long-term interaction models

¹ U.S. Geological Survey, MS-999, 345 Middlefield Road, Menlo Park, California 94025, USA

In defiance of direct rock-friction observations, some transform faults appear to slide with little resistance. In this paper finite element models are used to show how strain energy is minimized by interacting faults that can cause long-term reduction in fault-normal stresses (unclamping). A model fault contained within a sheared elastic medium concentrates stress at its end points with increasing slip. If accommodating structures free up the ends, then the fault responds by rotating, lengthening, and unclamping. This concept is illustrated by a comparison between simple strike-slip faulting and a mid-ocean-ridge model with the same total transform length; calculations show that the more complex system unclamps the transforms and operates at lower energy. In another example, the overlapping San Andreas fault system in the San Francisco Bay region is modeled; this system is complicated by junctions and stepovers. A finite element model indicates that the normal stress along parts of the faults could be reduced to hydrostatic levels after 60–100 k.y. of system-wide slip. If this process occurs in the earth, then parts of major transform fault zones could appear nearly frictionless.

Key Words: friction • faulting • San Andreas • transforms