Abstract

A mechanical basis for the occurrence of curvature or sinuosity on strike-slip faults is postulated from finite-element simulation of an earthquake sequence on the Parkfield, California, segment of the San Andreas fault. Using discontinuity fields from measured displacements associated with the M ≅ 5.3 earthquakes of June 1966, motions along 16 km of ground breakage are modeled to investigate the displacement fields and stresses during faulting. An originally rectilinear fault trace, subjected to a variable single-shear couple in terms of displacements, is curved into the extensional quadrant during the simulation. Displacement magnitudes normal to the fault trace have magnitudes similar to the relative discontinuity vector. Calculated moments Mb, = 3.2 × 106 dyne-cm cause the curvature. Shear stress drops of Δ τ = 10.8 × 106 dyne/cm2 occur in the fault zone.

It is postulated that successive earthquakes at the same place would enhance this initial sinuosity by repeated motion along weakened sections of the fault. The amount of curvature that can occur is a function of material properties, the magnitude of displacements during the earthquake(s), and the bending moment, which would eventually become large enough in rock masses adjacent to the fault to “lock” the broken segment. Younger rectilinear fault traces would occur with further motion, explaining certain aspects of the origin of braided fault traces and bedrock slices.

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