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Kinematics of the Pacific–North America Plate Boundary Zone, Northern California
Jeffrey T. Freymueller1, Mark H. Murray2, Paul Segall, and David Castillo3
Department of Geophysics
1 Presently at Geophysical Institute, University of Alaska, Fairbanks
2 Also at Seismological Laboratory, University of California, Berkeley
3 Presently at Department of Geology and Geophysics, University of Adelaide, Australia
Submitted to Journal of Geophysical Research, April 21, 1998.
Revised November 4, 1998.
Jeffrey T. Freymueller Mark H. Murray
Geophysical Institute Department of Geophysics
University of Alaska, Fairbanks Stanford University
Fairbanks, AK 99775 Stanford, CA 94305
Tel 907-474-7286 Fax 907-474-5618 Tel 650-723-9594 Fax 650-725-7344
Paul Segall David C. Castillo
Department of Geophysics Department of Geology and Geophysics
Stanford University University of Adelaide
Stanford, CA 94305 Adelaide
Tel 650-725-7241 Fax 650-725-7344 AUSTRALIA
We measured motions of 54 sites in an east-west transect across California at 38°-40° north by Global Positioning System (GPS) observations over a four year span to study the plate boundary zone in northern California. 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 (68.6% confidence intervals from a non-linear inversion are indicated by superscripts and subscripts). 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. Our best 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 Andreas , Ma’acama , Bartlett Springs . The data are fit best by models in which the San Andreas fault is locked to km, the Ma’acama fault locked to 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 Coast Ranges, except possibly 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 associated with right-lateral strike slip on the San Andreas fault system. This is consistent with models of the Pacific–North America plate boundary zone in which the relative plate motion is partitioned into two domains, one strike-slip and one dominantly extensional, separated by the elastically-deforming Sierra Nevada-Great Valley block. No right-lateral strain is observed on the eastern edge of the Sierra Nevada, a possible northern extension of the Eastern California Shear Zone, nor is any appreciable extension across the Honey Lake Fault, the westernmost extensional fault clearly associated with the Basin and Range at this latitude.
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