HR: 16:30h
AN: G14A-03    [Abstracts]
TI: Comparing Geodetic and Geologic Data from for the Creeping Segment of the San Andreas Fault, California
AU: * Titus, S J
EM: stitus@geology.wisc.edu
AF: University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Tikoff, B
EM: basil@geology.wisc.edu
AF: University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706 United States
AU: DeMets, C
EM: chuck@geology.wisc.edu
AF: University of Wisconsin, 1215 W. Dayton St., Madison, WI 53706 United States
AB: We present the results of geodetic and geologic measurements from the central creeping segment of the San Andreas fault. The 175-km creeping segment of the San Andreas fault, stretching from San Juan Bautista to Cholame, is characterized by aseismic slip and shallow microseismicity. The central, 55-km long segment has high, uniform creep rates. New differential GPS measurements, based on 30-year reoccupations of alignment arrays, yield a maximum right-lateral slip rate of 25 $\pm$ 1 mm/yr for the San Andreas fault. This slip rate is significantly slower than both an earlier slip rate estimate of 30 mm/yr and recent geodetic estimates of 39 $\pm$ 2 mm/yr of motion predicted between the Sierra Nevada - Great Valley block and the Pacific plate. New continuous GPS measurements between pairs of sites that flank the creeping segment at respective inter-site distances of 1 km and 70 km give relative fault-parallel slip rates of 28 $\pm$ 2 and 30 $\pm$ 2 mm/yr respectively for 17 months of observation. These observations indicate that right-lateral deformation rates increase with distance from the fault. Possible explanations for the cross-fault gradient observed in the geodetic data, and the 14 mm/yr deficit observed between the slip rate on the San Andreas fault and the predicted plate motion rate, are elastic strain accumulation along the creeping segment or significant distributed deformation on off-fault structures. Preliminary evidence from the Monterey Formation adjacent to the central creeping segment supports the latter model. Folds located away from the fault show evidence for hinge-parallel extension, consistent with fold rotation during progressive deformation in the borderlands. This type of deformation cannot be explained by a simple elastic strain accumulation model for the San Andreas fault or solely by strain/stress partitioning. Rather, the hinge-parallel extension of the folds indicates that some of the slip deficit is accommodated by permanent wrench deformation in the borderlands.
DE: 8005 Folds and folding
DE: 8010 Fractures and faults
DE: 8158 Plate motions--present and recent (3040)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting