Figure 13.38 shows site velocities estimated using the GAMIT/GLOBK software package for the 1994-2004 period relative to stable North America, as defined by a set of 20 fiducial stations. We are currently processing the data from the most recent surveys that will provide new estimates of velocites for sites between the southern and northern profiles shown. Most of the velocities are derived from data spanning 8-10 years, whereas those with the largest error ellipses include data from only a 4 year span. The easternmost stations exhibit motions typically associated with Sierran-Great Valley block (ORLA: 12.5 mm/yr NW). The westernmost sites are moving close to the Pacific plate rate (PTAR: 45.9 mm/yr NW). Fault-normal contraction is observed east of the Ma'acama fault, in the region of the Coast Ranges near the Central Valley where similar contraction has been observed elsewhere (e.g., Murray and Segall, 2001).

Also shown in Figure 13.38 are velocities predicted by angular velocity-fault backslip modeling techniques (e.g., Murray and Segall, 2001) to account for both far-field plate motions and interseismic strain accumulation. We are developing a 3D fault model and applying the same modeling approach that we used in our BAVU study of the San Francisco Bay area (d'Allesio et al., 2005). Preliminary results show that the agreement between observed and predicted velocities is typically less than the 2 mm/yr level. Misfits are larger in a few areas close to faults, such as along the central Ma'acama and near the MTJ, that should be decreased with further refinement of the fault geometry. Total deformation across the San Andreas fault system is 38 mm/yr, in agreement with previous studies, but deep slip is concentrated on the Ma'acama fault (24 mm/yr) and on the Bartlett Springs fault (10 mm/yr), with only 4 mm/yr on the San Andreas. We are currently investigating this result, which is due in part to the high-degree of correlation between the slip rates on the 3 faults, and will test methods for adding geologic and other information using Bayesian techniques, which should reduce the correlations on slip rates and provide better resolution on other parameters such as locking depths.

We are also working with D. Agnew (UCSD), R. King (MIT), and Z-K. Shen (UCLA) to combine these results with the BAVU, SCEC Crustal Motion Model (CMM 3.0), and other studies to provide an integrated California-wide velocity field. The preliminary velocity field is shown in Figure 20. and includes over 2000 stations. Additional stations will be included from the Cape Mendocino triple junction region. Results from this study will be used to develop a state-wide deformation and fault slip model that will be incorporated into a hazard assessement project by the Working Group on California Earthquake Probabilities under the auspicies of the USGS. Preliminary results of the WGCEP project are expected in mid-2006.

Figure 13.39: Velocities of sites in California relative to stable North America from a combination of velocities from this study, BAVU, the SCEC Crustal Motion Map, and other studies.
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