HR: 16:30h
AN: T54A-03    [Abstracts]
TI: Constructing a Model of a Dipping Southern San Andreas Fault, Southern California
AU: * Scheirer, D
EM: dscheirer@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Fuis, G
EM: fuis@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Langenheim, V
EM: zulanger@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AB: The San Andreas Fault (SAF) in southern California is predominantly non-vertical, based on potential field observations, deep seismic imaging, and earthquake focal-mechanisms, aftershocks, and microseismicity. Dip values of the southern SAF vary systematically along its length. Between the Carrizo Plain and Lake Hughes (~180 km distance), dips are consistently to the southwest and range between 55° and 80°; between Lake Hughes and Wrightwood (~70 km), dips are essentially vertical; between Wrightwood and the Salton Sea (~220 km), dips are consistently to the northeast and vary between 37° and 75°. In the structurally complex San Gorgonio Pass area, the dip of one of the strands of the SAF in the uppermost 1-2 km may be as low as 10°. In all, 13 sites that are unequally-spaced along the southern SAF provide well-constrained observations of fault dip. Uncertainties of the fault dip observations are less than ±10°, and in some cases less than ±5°, although the different types of observations have different sensitivities to age and depth of features indicating dip. Gravity and magnetic models are sensitive to the structures that result from millions of years of strike-slip motion at shallow and intermediate crustal levels (<10-15 km); earthquake studies reflect motions at seismogenic depths since the start of the seismic catalogs (<50 years); deep seismic imaging is sensitive to the integrated history throughout the crust. Notwithstanding the different sources of observations, a self-consistent model of the fault surface may be constructed by interpolating between points with well-defined dips and assembling with three-dimensional modeling software. The three-dimensional form of this fault surface is crudely reminiscent of a propeller. The steepest sections (>75°) of the southern SAF correspond to locations where the fault trace is straightest. Where the SAF trace bends, dips are generally <75°. The variation of dip and strike of the SAF implies that strike-slip motion along the fault surface must be accompanied by changes in topography and that partial accommodation of long-term plate motion by adjacent faults may be important. Calculation of shaking potential for scenario major earthquakes on the southern SAF and calculation of geodetic deformation from buried slip on the SAF may be inaccurate unless the non-vertical dip of the fault is taken into account.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting