HR: 14:25h
AN: T53C-04 [Abstracts]
TI: High-Resolution Imaging of the San Andreas Fault from Fault-Zone Trapped Waves Recorded at the SAFOD Borehole Seismograph and Surface Array
AU: * Li, Y
EM: ygli@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089, United
States
AU: Malin, P E
EM: pmalin@duke.edu
AF: Dept. of Earth and Ocean Science, Duke University, Durham, NC 27708, United States
AU: Vidale, J E
EM:
AU: Cochran, E M
EM:
AB:
Highly damaged rocks along the San Andreas Fault at Parkfield create a low-velocity waveguide to trap seismic
waves. We recorded prominent fault-zone trapped waves (FZTWs) at the SAFOD mainhole seismograph at 3km
depth and the surface seismic array across the fault for microearthquakes. A systematic waveform analysis of
borehole and surface seismograms from several hundreds of local earthquakes and aftershocks of the 2004 M6
Parkfield earthquake allowed us to evaluate the variations in rock damage magnitude and extent on the SAF with
high-resolution. The FZTWs are characterized by relatively large amplitudes and dispersive wavetrains at 3-12 Hz
following S-waves. The duration time of dominant FZTWs after S-arrivals increase with the travel distance
between the source and receiver either along the fault strike or with the depth, showing the continuous low-
velocity waveguide existing on the SAF at Parkfield. We measured duration time of FZTWs after S-arrivals for the
events at different depths with raypath incidence angles from these events to the seismic station smaller than
30o from vertical. The wavetrain lengths of FZTWs measured at surface stations within the fault zone show a
progressively increasing trend from ~1.2 s to ~2.2 s as the event depths increase from 2.6 km to 11.7 km. These
measurements are confirmed by the data recorded at the SAFOD mainhole seismograph. In contrast, the
seismograms recorded at seismographs installed in the SAFOD pilot borehole ~1.8 km away from the SAF and
the surface stations deployed out of the fault zone show much brief wavetrains after body waves. These
observations indicate that the low-velocity waveguide formed by the damaged rock on the SAF likely extends
across seismogenic depths with prominent seismic velocity reduction at depths above ~7-8 km. The smaller
velocity reduction on the deeper portion of the fault damage zone is probably due to the larger confined stress at
greater depths. The surface array data show that the damage zone on the SAF is not laterally symmetric but
extends farther on the southwest side of the main fault trace. This could be due to rocks already weakened from
previous faulting. It could also be due to greater damage in the extensional quadrant near the propagating crack
tip of Parkfield earthquakes. We modeled these FZTWs using 3-D finite-difference methods. The models suggest
that, on average, the fault zone cross section consist of a composite of two nearly vertical layers, one a 30-40-m-
wide fault core, the other a surrounding ~150-250-m wide damage zone depending on the depth. The damage
zone velocities range between 70-80% of the fault zone wall rocks, while those of the core are even less, going as
low as 40-50% of the intact rock. The widths and velocity reductions of the fault core and damage zone at ~3-km
depth in our model are consistent with the direct measurements of fault-zone properties in the SAFOD mainhole
[Hickman et al., 2005] as well as the borehole observations of fault guided waves following P-waves [Ellsworth
and Malin, 2006]. We interpret the distinct low-velocity waveguide on the Parkfield SAF as being a zone of
accumulated damage from recurrent major earthquakes, including the 2004 M6 earthquake. This type of damage
varies with depth and also along the strike, and may relate to the on- and near-fault variations in stress and slip
distribution during earthquake rupture.
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting