Where is the real transform boundary in California?

Abstract

The zone of highest geodetically defined strain rate in California does not everywhere coincide with the surface trace of the San Andreas Fault (SAF). To determine whether this reflects the pattern of long‐term, permanent deformation, we analyzed the velocity field on swaths across the transform, located so as to avoid intersections among the major fault strands. Slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model, representing the interseismic strain accumulation. Our slip rates compare well with current geologic estimates, which suggests that the present‐day velocity field is representative of long‐term motions. We find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the SAF and has an overall trend closer to the relative plate motion vector than the SAF. Most sections of the SAF take up less than half of the total slip rate, and slip is transferred from one part of the system to another in a way that suggests that the SAF should not be considered as a unique locator of the plate boundary. Up to half of the total displacement takes place on faults outside the high strain rate zone, distributed over several hundred kilometers on either side. Our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere, around which stress and strain rate decrease on a length scale controlled by the length of the transform.