A new GPS velocity field for the Pacific Plate – Part 2: implications for fault slip rates in western California

Abstract

Lower and upper bounds for present deformation rates across faults in central California between the San Andreas Fault and Pacific coast are estimated from a new Global Positioning System (GPS) velocity field for central, western California in light of geodetic evidence presented in a companion paper for slow, but significant deformation within the Pacific Plate between young seafloor in the eastern Pacific and older seafloor elsewhere on the plate. Transects of the GPS velocity field across the San Andreas Fault between Parkfield and San Juan Buatista, where fault slip is dominated by creep and the velocity field thus reveals the off-fault deformation, show that GPS sites in westernmost California move approximately parallel to the fault at an average rate of 3.4 ± 0.4 mm yr−1 relative to the older interior of the Pacific Plate, but only 1.8 ± 0.6 mm yr−1 if the Pacific Plate frame of reference is corrected for deformation within the plate. Modelled interseismic elastic deformation from the weakly coupled creeping segment of the San Andreas Fault is an order-of-magnitude too small to explain the southeastward motions of coastal sites in western California. Similarly, models that maximize residual viscoelastic deformation from the 1857 Fort Tejon and 1906 San Francisco earthquakes mismatch both the rates and directions of GPS site motions in central California relative to the Pacific Plate. Neither thus explains the site motions southwest of the San Andreas fault, indicating that the site motions measure deformation across faults and folds outboard of the San Andreas Fault. The non-zero site velocities thus constitute strong evidence for active folding and faulting outboard from the creeping segment of the San Andreas Fault and suggest limits of 0–2 mm yr−1 for the Rinconada Fault slip rate and 1.8 ± 0.6 to 3.4 ± 0.4 mm yr−1 for the slip rates integrated across near-coastal faults such as the Hosgri, San Gregorio and San Simeon faults.