HR: 0800h
AN: T51C-0684    [Abstracts]
TI: Fault Zone Structure at Seismogenic Depth from Seismic Guided Waves
AU: * Wu, J
EM: wujd@vt.edu
AF: Virginia Polytechnic Institute and State University, Department of Geosciences, Blacksburg, VA 24061, United States
AU: Hole, J A
EM: hole@vt.edu
AF: Virginia Polytechnic Institute and State University, Department of Geosciences, Blacksburg, VA 24061, United States
AU: Snoke, J A
EM: snoke@vt.edu
AF: Virginia Polytechnic Institute and State University, Department of Geosciences, Blacksburg, VA 24061, United States
AB: The low-velocity damage zone of major faults can act as a seismic waveguide. The fault-zone guided waves provide a potential method to measure fault-zone properties in situ at depth. Recently, there has been debate over the depth extent of the observed seismic waveguide and whether fault properties can be constrained at seismogenic depth. To answer those questions, we generated elastic finite-difference synthetic seismograms for a uniform-thickness fault-zone model that includes a realistic increase in seismic velocity with depth inside and outside the fault. Two primary results have been discovered in this model: 1) Earthquakes both inside and outside of the fault zone generate strong guided waves within the near-surface part of the depth-varying fault zone, regardless of whether the waveguide is terminated at shallow depth or continues beyond the depth of the earthquakes. This result differs from previous synthetic studies for a homogeneous fault, where earthquakes off the fault do not generate guided waves. 2) Guided-wave velocity dispersion changes with depth. Near-surface fault structure traps/guides waves at low frequencies that are not efficiently trapped (travel as body waves) at greater depth. Deep fault structure can only be derived from analyses of data at higher frequencies than the guided waves that dominate at the surface. In order to determine deep fault structures, we measured the frequency-dependent group arrival times at two receivers closely spaced along the propagation path to calculate the local dispersion curve. The results closely matched the synthetic fault-zone properties between the two receivers. Similarly, a pair of closely spaced earthquakes and a single receiver can be used to derive the fault- zone dispersion between the earthquakes. This method was used to correctly reproduce the synthetic fault-zone structure at seismogenic depth. We will apply this method to existing earthquake data from the EarthScope- SAFOD borehole seismic station to attempt to constrain deep San Andreas Fault structure.
DE: 0935 Seismic methods (3025, 7294)
DE: 7200 SEISMOLOGY
DE: 7299 General or miscellaneous
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting