Kinematics of the Pacific North America plate boundary zone, northern California

We measured motions of 54 sites in an east-west transect across northern Ca lifornia at 38 degrees-40 degrees north by Global Positioning System (GPS) observations over a 4 year. We estimate the total slip rate on the San Andr eas fault system to be 39.6(-0.6)(+1.5) mm/yr (68.6% upper and lower confid ence intervals from a nonlinear inversion are indicated by superscripts and subscripts), Slip rates on the individual faults are determined less preci sely due to the high correlations between the estimated parameters. Our bes t fitting model fits the fault-parallel velocities with a mean square error of 1.04 and the following estimated fault slip rates (all in mm/yr): San A ndreas 17.4(-3.1)(+2.5), Ma'acama 13.9(-2.8)(+4.1), and Bartlett Springs 8. 2(-1.9)(+2.1). The data are fit best by models in which the San Andreas fau lt is locked to 14.9(-7.1)(+12.5) km, the Ma'acama fault locked to 13.4(-4. 8)(+7.4) km except for shallow creep in the upper 5 km, and the Bartlett Sp rings fault creeping at all depths. The Ma'acama fault most likely poses a significant seismic hazard, as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake. We find l ittle evidence for contraction across Coast Ranges, except at western edge of Great Valley where 1-3 mm/yr of shortening is permitted by the data. No strain is observed within the Great Valley or Sierra Nevada except that ass ociated with right-lateral strike slip on the San Andreas fault system. Thi s is consistent with models of the Pacific-North America plate boundary zon e in which the relative plate motion is partitioned into two domains, one s trike-slip and one dominantly extensional, separated by the elastically def orming Sierra Nevada-Great Valley block.