Abstract
We used borehole seismic records of four repeating-earthquake clusters
and two explosions to investigate the coseismic changes of the scattering
wave field of the 28 September 2004 M 6 Parkfield earthquake. We
found systematic changes in P-wave and S-wave coda recorded from
repeating events that occurred before and immediately after the earthquake.
Applying the coda wave interferometry technique allowed us to determine
that the observed changes are caused by a localized change in the
scattering field. We further developed a technique based on decorrelation
indexes calculated from running time windows to locate migrating
scatterers. Synthetic tests showed that the technique is relatively
insensitive to changes in background velocity of the medium and source
location and thus can be applied to records of loosely colocated
clusters. We found a localized change of material property within
the fault zone at \~3 km depth beneath the Middle Mountain area.
The change is shown most clearly in the P-to-S scattering mode of
the active source data and both the P-to-S and S-to-S scattering
modes of the repeating earthquakes data, suggesting that the observed
scattering property change is a result of charge or discharge of
fluids in fractures caused by the 2004 Parkfield earthquake. The
same scatterer(s) was found to respond to the 1993 aseismic stress/strain
transient event, acting like an in situ stress-strain meter at seismogenic
depth. Four-dimensional time-lapse imaging of the scattering wave
field thus provides an effective way to monitor the subsurface stress-strain
field.
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