Propagating episodic creep and the aseismic slip behavior of the Calaveras Fault north of Hollister, California

Abstract

A detailed kinematic study of fault slip occurring from the surface to a depth of about 7 km on the Calaveras fault north of Hollister was conducted during the summer of 1977. The observations coincided with a period of propagating episodic fault creep activity sensed along the fault trace. Data used in the investigation consist of creepmeter records, near‐field strainmeter observations, and high‐resolution geodetic measurements, all collected contemporaneously over a period of 4 months. Detailed descriptions and analyses of the creepmeter and geodetic data have been presented elsewhere. The near‐field strain measurements are here reported in detail, and their analysis draws upon the previous two data sets for support. The strainmeter observations are most sensitive to slip occurring in the upper 2 km; hence the emphasis of the paper is placed upon the role of propagating episodic creep in the broad‐scale behavior of the fault. The results suggest that propagating episodic fault creep as sensed along the fault trace is confined to the upper kilometer or so of the crust and represents the response of the surface layers to a longer‐term form of episodic aseismic slip occurring below. The mean form of the advancing rupture front within the upper kilometer is ostensibly the same as that indicated by records from the surface creepmeters. Evidence is presented, however, which suggests that propagating creep events may not always break the surface and may propagate at velocities much slower and at amplitudes significantly larger than those generally observed at the surface.