Episodic block motion and convergence along the Calaveras fault in Central California

Abstract

A geodetic array covering approximately 100 km 2 near Hollister, California has been monitored daily whenever possible since September 1975 using a multiwavelength distance-measuring (MWDM) instrument with a demonstrated precision of 1 part in 10 7. The MWDM array straddles the Calaveras fault, a portion of the transform fault system between the North American and Pacific tectonic plates. The deformation of the region typically occurs during episodes of significant movement, lasting several weeks, interspersed by periods of relative quiescence. An analysis of the MWDM data suggests that most of the observed crustal movements can be explained by rigid-block motion parallel to the strike of the Calaveras fault. However, there is a significant component of the motion that appears to be due to a convergence of the crustal blocks towards the Calaveras fault. This convergent trend, if it represents long-term crustal movement behavior, may be responsible for the small, elongated hills situated along the fault trace.