Velocity contrast along the Calaveras fault from analysis of fault zone head waves generated by repeating earthquakes

Abstract

We systematically investigate the velocity contrast along the Calaveras fault that ruptured during the 1984 Morgan Hill earthquake using fault zone head waves (FZHW) that refract along the fault interface. We stack waveforms in 353 sets of repeating clusters, and align the peaks or troughs of the direct P waves assuming right‐lateral strike‐slip focal mechanisms. The obtained velocity contrasts are 2–3% and 12–14% NW and SE of station CCO, respectively. The FZHW and the fault plane outlined by the relocated seismicity SE of CCO are more complicated than those NW of CCO. The results can be explained by a relatively simple and sharp fault interface in the NW, and a complicated fault structure with a presence of a low‐velocity zone in the SE. The along‐strike variations in the strength of the velocity contrast are consistent with surface geological mapping and recent 3D tomography studies in this region.