Earthquake Cycles on the San Andreas Fault System in Northern California

Abstract

The 24 August, 2014, M = 6.0 South Napa earthquake was the first earthquake to significantly affect the populous San Francisco Bay region since the 1989 Loma Prieta earthquake. The Napa earthquake has resulted in a new interest in earthquake risk on the San Andreas Fault (SAF) system north of San Francisco. The deformation in this region is dominantly right lateral shear between the rigid Pacific Plate and the rigid Sierra-Nevada-Central-Valley Plate. GPS observations are well approximated by a uniform shear strain across this 100 km wide zone. This zone is recognized to be a “slab window” with a relatively thin lithosphere. In this paper, we hypothesize that this lithosphere is composed of a 12 km thick brittle elastic upper lithosphere and a 15 km thick viscoplastic lower lithosphere. We attribute the observed near uniform surface strain to flow in the viscoplastic zone. Deformation in the brittle upper lithosphere results in seismicity. A substantial fraction, about 20 mm year⁻¹, of the 32 mm year⁻¹ of the right lateral deformation takes place on the SAF. However, the SAF is not parallel to the motion of the bounding rigid plates. This geometrical incompatibility requires distributed deformation across the 100 km wide zone. A fraction of this distributed deformation takes place on three relatively well defined strike-slip fault zones to the east of the SAF as well as on other faults. We suggest that when a large earthquake occurs, the localized stress concentrations are relaxed by flow in the viscoplastic zone. We give a detailed description of this process during and following the 1906 earthquake and show the process is consistent with observations. We also relate our tectonic model to the present distribution of seismicity in our study area.