Inferring Segment Strength Contrasts and Boundaries along Low-Friction Faults Using Surface Offset Data, with an Example from the Cholame-Carrizo Segment Boundary along the San Andreas Fault, Southern California

Rupture segmentation arises from changes in fault geometry and strength. We use boundary element models of frictionless strike-slip fault segments to quantify how fault geometry and strength change earthquake surface offset distributions. Using these relationships between fault geometry, strength, and surface offsets, we can infer fault strength from the surface offsets in cases where the fault geometry can be independently constrained. This article includes normalized plots of the surface offset distribution expected from rupture along low-friction fault segments with strength contrasts of 1/4, 1/3, 1/2, 1, 2, 3, and 4 for a range of fault segment geometries. These plots may be used with offset data to constrain the strength of two coplanar, adjacent fault segments. This analysis is applied to the Cholame and Carrizo segments of the San Andreas Fault. The available surface offset data suggest that the offset increases where the fault deepens; in addition, the observed offset gradient at the segment boundary requires a 2/3–1/4 strength ratio of the Cholame to the Carrizo segment.