Geodetic Studies in the San Francisco Bay area

Mark Murray

Subsections

• Strain accumulation rates in the San Francisco Bay area 1972-1989
• Kinematics of the Pacific-North America plate boundary zone, northern California

Strain accumulation rates in the San Francisco Bay area 1972-1989

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 M$\sim$6 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 $58^{\circ}$E and the 2 axis N $32^{\circ }$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 $\varepsilon_{11} = 9.2 \pm 7.4$, $\varepsilon_{12} = -160.7 \pm 4.6$, and $\varepsilon_{22} = 8.2 \pm 6.2$ 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 $32^{\circ }$W (Figure 27.1). The net increase in the 12,225 km² area of the trilateration network is only 212 $\pm$ 110 m²/yr, which arises from almost equal extensions in the N $32^{\circ }$W and N $58^{\circ}$E directions. Within the networks the strain rates vary from polygon to polygon. The N $58^{\circ}$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 $58^{\circ}$E extension rate were zero or negative. Significant areal dilatation rates are observed in almost 1/3 of the individual polygons and the N $32^{\circ }$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.

  

Figure 27.1: The average strain rate $\varepsilon _{12}$ (right-lateral shear across a vertical plane striking N $32^{\circ }$W) in the San Francisco Bay area within each of the 32 polygonal regions, in nanostrain/yr (boldface type indicates rates that differ from 0 at the 95% confidence level).

  

Figure 27.2: The average strain rate $\varepsilon _{11}$ (fault-normal extension) in the San Francisco Bay area (conventions as in Figure 27.1). The fact that $\varepsilon _{11}$ tends to be positive (extension) in the entire network suggests that its source is not the strike-slip mechanisms of the dominant faults.

  

Figure 27.3: The average strain rate $\varepsilon _{22}$ (fault-parallel extension) contoured using a third-degree polynomial approximation. Faults shown by sinuous white lines. The east Bay tends to be more positive and the southwest Bay tends to be more negative. This may be due to the systematic differences in orientation between the faults located in each region and the average N $32^{\circ }$W trend (the east Bay faults tend to be oriented more easterly, and west Bay faults tend to be more westerly).

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

We measured motions of 54 sites in an east-west transect across California at 38- $40^{\circ}$ 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 $40 \pm 1$ 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.

  

Figure 27.4: Velocities of GPS sites in the Coast Ranges and western Great Valley, relative to Point Reyes NCMN on the Point Reyes peninsula ( $38^{\circ }$N, $123^{\circ }$W) with 95% confidence regions. Virtually all velocities are parallel to the San Andreas fault system (sinuous NW-trending lines, principally comprised from west to east of the San Andreas, Ma'acama, and Bartlett Springs faults). The NOAM and SNGV arrows show predicted NUVEL-1A North America and Sierra Nevada-Great Valley motions (Don Argus, personal communication, 1998).

  

Figure 27.5: Optimal model and confidence regions for slip rates, showing correlations between the San Andreas (SAF), Ma'acama (MF), and Bartlett Springs (BSF) faults. The star indicates the optimal model determined by random cost optimization methods. Contours are 50% (solid) and 95% (dashed) confidence regions determined by bootstrapping (resampling with replacement) methods. Note the strong correlations between slip rates on adjacent faults, SAF and MF, and MF and BSF.

  

Figure 27.6: Ukiah (top) and Willits (bottom) profiles, showing fault-parallel and fault-normal velocities along with predictions of the optimal model. Fault-normal velocities were not modeled but are shown for reference. While total fault-parallel slip is $40 \pm 1$ mm/yr, only $3 \pm 2$ mm/yr of fault-normal shortening is observed between the Great Valley and Pacific Coast.

The Berkeley Seismological Laboratory, 202 McCone Hall, UC Berkeley, Berkeley CA 94720
Questions and comments to www@seismo.berkeley.edu
Copyright 1999, The Regents of the University of California.