Abstract
The time-varying deformation field within a fault zone, particularly
at depths where earthquakes occur, is important for understanding
fault behaviour and its relation to earthquake occurrence. But detection
of this temporal variation has been extremely difficult, although
laboratory studies have long suggested that certain structural changes,
such as the properties of crustal fractures, should be seismically
detectable. Here we present evidence that such structural changes
are indeed observable. In particular, we find a systematic temporal
variation in the seismograms of repeat microearthquakes that occurred
on the Parkfield segment of the San Andreas fault over the decade
1987-97. Our analysis reveals a change of the order of 10 m in the
location of scatterers which plausibly lie within the fault zone
at a depth of \~3 km. The motion of the scatterers is coincident,
in space and time, with the onset of a well documented aseismic transient
(deformation event). We speculate that this structural change is
the result of a stress-induced redistribution of fluids in fluid-filled
fractures caused by the transient event.
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