Abstract
Three-component borehole seismic profiling of the recently active
Oroville, California, normal fault and microearthquake event recording
with a near-fault three-component borehole seismometer on the San
Andreas fault at Parkfield, California, have shown numerous instances
of pronounced dispersive wave trains following the shear wave arrivals.
These wave trains are interpreted as fault zone-trapped seismic modes.
Parkfield earthquakes exciting trapped modes have been located as
deep as 10 kilometers, as shallow as 4 kilometers, and extend 12
kilometers along the fault on either side of the recording station.
Selected Oroville and Parkfield wave forms are modeled as the fundamental
and first higher trapped SH modes of a narrow low-velocity layer
at the fault. Modeling results suggest that the Oroville fault zone
is 18 meters wide at depth and has a shear wave velocity of 1 kilometer
per second, whereas at Parkfield, the fault gouge is 100 to 150 meters
wide and has a shear wave velocity of 1.1 to 1.8 kilometers per second.
These low-velocity layers are probably the rupture planes on which
earthquakes occur. 10.1126/science.249.4970.763
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