Maps of the strain accumulation rate in the San Francisco Bay area have been constructed from trilateration observations extending from about 1972 until the Loma Prieta earthquake in late 1989. The observations were corrected to remove offsets imposed by shallow fault creep and by four M6 earthquakes that occurred in the Bay area during that time interval. The Bay area was divided into 32 contiguous polygons (Figure 27.1), and the uniform (in both space and time) strain rates that best explain the changes in the corrected distances within each polygon were calculated. In a coordinate system with the 1 axis directed N E and the 2 axis N W (perpendicular and parallel to the local tangent to the small circle drawn about the Pacific-Sierra Nevada pole of rotation) the average of these 32 strain rates (each weighted by the area of the polygon) are , , and nanostrain/yr, where extension is reckoned positive and quoted uncertainties are standard deviations. As expected from the Pacific-Sierra Nevada relative plate motion, the overall strain rate is predominantly right-lateral shear across a vertical plane striking N W (Figure 27.1). The net increase in the 12,225 km2 area of the trilateration network is only 212 110 m2/yr, which arises from almost equal extensions in the N W and N E directions. Within the networks the strain rates vary from polygon to polygon. The N E extension rate is positive in 22 of the 32 polygons (Figure 27.2), a proportion that is significantly larger than would be expected by chance if the N E extension rate were zero or negative. Significant areal dilatation rates are observed in almost 1/3 of the individual polygons and the N W extension rates tend to be negative to the west of the Hayward-Rodgers Creek fault trend and positive east of it (Figure 27.3). The pre-1989 strain accumulation across the eventual site of the Loma Prieta rupture involves fault normal contraction as well as right-lateral shear, consistent with the rupture mechanism.
Savage, J. C., R. W. Simpson, and M. H. Murray, Strain accumulation rates in the San Francisco Bay area, 1972-1989, J. Geophys. Res., 103, 18,039-18,051, 1998.
We measured motions of 54 sites in an east-west transect across California at 38- north by Global Positioning System (GPS) observations over a four year span to study the plate boundary zone in northern California (Figure 27.4). GPS velocities from this network place tight constraints on the total slip rate on the San Andreas fault system, which we estimate to be mm/yr. Slip rates on the individual faults are determined less precisely due to the high correlations between estimated slip rates and locking depths, and between slip rates on adjacent faults (Figure 27.5). Our best fitting model fits the fault-parallel velocities with a normalized rms of 1.0185, and the following estimated fault slip rates (all in mm/yr, with 68.6% confidence intervals): San Andreas 17.4+2.5-3.1, Ma'acama 13.9+4.1-2.8, Bartlett Springs 8.2+2.1-1.9 (Figure 27.6). The data are fit best by models in which the San Andreas fault is locked to 14.9+12.5-7.1 km, the Ma'acama fault locked to 13.4+7.4-4.8 km except for shallow creep in the upper 5 km, and the Bartlett Springs fault may be creeping at all depths. Our estimated slip rate on the San Andreas fault is lower than all geologic estimates, although the 95% confidence interval overlaps the range of geologic estimates. Our estimate of the Ma'acama fault slip rate is greater than slip rate estimates for the Hayward or Rodgers Creek faults, its continuation to the south. The Ma'acama fault most likely poses a significant seismic hazard, as it has a high slip rate and a slip deficit large enough to generate a magnitude 7 earthquake today since there have been no significant earthquakes on the fault in the historical record. The shallow creep observed on the Ma'acama fault relieves only a fraction of the tectonic stress. We find little or no geodetic evidence for contraction across the Coast Ranges, except possibly at western edge of Great Valley where 1-3 mm/yr of shortening is permitted by the data.
Freymueller, J. T., M. H. Murray, P. Segall, and D. Castillo, Kinematics of the Pacific-North America plate boundary zone, northern California, J. Geophys. Res., submitted, 1998.