Hi Roger. The following in part is from a report on aseismic transient (deformation event) by Niu F, Silver PG, Nadeau RM, McEvilly TV.

"Here we present evidence that such structural changes are indeed observable. In particular, we find a systematic temporal variation in the seismograms of repeat micro earthquakes 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 approximately 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."

Talking with some of the personnel at USGS I learned that there was a drop in the magnetics on the southwest side of the San Andreas Fault, while at the same time there was an increase in magnetics on the northeast side of the fault. This was in the Parkfield area and took place in 1993. I don't think it was ever determined if the change in magnetics cause the event or the event cause the change in the magnetics. However this was a couple of years ago that I learned this and a lot could have changed between then and now. Take Care…Don in creepy town

• Aseismic Transient