HR:     0830h
AN:     G71G-06
TI:     Broad Scale Deformation Across the Pacific-North America 
        Boundary in Northern California
AU:     *Prescott, W 
EM:     wprescott@usgs.gov
AF:     US Geological Survey,
        345 Middlefield Road, MS/977 Menlo Park, CA 94025-3591  United States
AU:     Baxter, R 
EM: 
AF:     University of California Berkeley, CA   
AU:     Burgmann, R 
EM: 
AF:     University of California Berkeley, CA   
AU:     Burgmann, R 
EM: 
AF:     University of California Davis, CA   
AU:     Murray, M 
EM: 
AF:     University of California Berkeley, CA   
AU:     Murray, M 
EM: 
AF:     Stanford University Stanford, CA   
AU:     Romanowicz, B 
EM: 
AF:     University of California Berkeley, CA   
AU:     Savage, J 
EM: 
AF:     US Geological Survey,
        345 Middlefield Road, MS/977 Menlo Park, CA 94025-3591  United States
AU:     Svarz, J 
EM: 
AF:     US Geological Survey,
        345 Middlefield Road, MS/977 Menlo Park, CA 94025-3591  United States
AU:     Wendt, K 
EM: 
AF:     US Geological Survey,
        345 Middlefield Road, MS/977 Menlo Park, CA 94025-3591  United States

AB:     We have estimated velocity vectors for nine stations in
        Northern California using continuous Global Positioning
        System data from the Bay Area Research Deformation network.
        The western-most station (Farallon Islands off San
        Francisco) has a velocity of 47.5\pm0.6 mm/yr,
        N$35.4\pm0.7\deg$W. All velocities are relative to
        stable North America, realized through several International
        GPS Service stations on North America. The Farallon velocity agrees
        well in direction with NUVEL models and is intermediate
        between NUVEL NNR1 (48.7 mm/yr, N$33.8\deg$W) and NNR1A
        (46.6 mm/yr, N$33.8\deg$W) velocities. Deformation
        associated with the plate boundary faults of the San Andreas
        system spans a zone about 70 km wide at the latitude of San
        Francisco, and accounts for 38 mm/yr of NW motion. The
        Sierra Nevada-Great Valley microplate has a velocity of 10
        mm/year directed approximately N$50\deg$W. The data from the
        BARD network suggest a SNGV-NA pole at $45\pm1\deg$,
        W$110\pm1\deg$, $-0.51\pm0.06\deg$/ma.  This contrasts with
        the pole found by Argus and Gordon [1991], $32\pm1\deg$,
        W$128\deg$, $-0.61\pm0.06\deg$/ma. However, the difference
        may not be significant, since over this area the vectors are
        nearly parallel and not very sensitive to the pole location.
        There is some weak evidence to suggest that the Sierra
        Nevada and Great Valley are moving independently with
        different poles. Two stations in the Great Valley have much
        more northward directed velocities than either the Sierra
        Nevada or coastal stations. If this evidence is accepted,
        then the SN-NA pole is N$47\pm1\deg$, W$107\pm1\deg$,
        $-0.60\pm0.1\deg$/ma, while the GV-NA pole is N$39\pm2\deg$,
        W$145\pm3\deg$, $0.24\pm0.06\deg$/ma. There is no evidence
        in support of significant motion normal to the small-circle
        plate boundary direction, although the data are inadequate
        to rule out small rates of such motion (say 1 mm/yr or less).
DE:     8150  Plate boundary--general (3040)
DE:     8158  Plate motions--present and recent (3040)
DE:     7209  Earthquake dynamics and mechanics
DE:     7230  Seismicity and seismotectonics
DE:     3040  Plate tectonics (8150, 8155, 8157, 8158)
SC:     G
MN:     1998 Fall Meeting