“Ghost Transient” Corrections to the Southern California GPS Velocity Field from San Andreas Fault Seismic Cycle Models

Abstract

Abstract Seismic cycle models of the northern and southern San Andreas fault (SAF) were developed to estimate corrections for viscoelastic transients embedded in the present-day Global Positioning System (GPS) velocity field. These models incorporate a viscous shear zone embedded in either a layered or a 3D viscoelastic structure. In the southern SAF model, the 3D effective viscosity structure is based on the Southern California Earthquake Center (SCEC) community rheology model (CRM) and the SCEC community thermal model. Viscoelastic materials are represented with Burgers or Maxwell viscoelastic rheologies, and ranges of rheological parameters and earthquake chronologies are explored. For the northern and southern SAF models, surface velocity perturbations or “ghost transients” (GTs) of up to 3.5–6 mm/yr are obtained for a wide range of conditions. In all cases, the GT velocities are consistent with left-lateral rather than right-lateral motion across the SAF, and subtracting them from the present-day GPS velocity field increases the apparent SAF offset rate. For the southern SAF model, GT velocities are fairly insensitive to the timing of large SAF earthquakes prior to 1857, but they are sensitive to rheology (i.e., temperatures and strain rates assumed for computing CRM effective viscosities). For the northern SAF model, GT velocities are sensitive to both rheology (mantle, lower crust and shear zone effective viscosities, and Burgers Body parameters Δb and RB) and mean earthquake recurrence interval. For the reference northern and southern SAF models provided to U.S. Geological Survey National Seismic Hazard Mapping Project deformation modelers, the maximum GT velocities are 3.73 and 4.84 mm/yr, respectively.