In Situ Measurements of the Mechanical Properties of Active Fault Zones

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Agnew, Duncan CarrEAR-0001116This proposal requests funds to support the continued operation of a portable laser strainmeter adjacent to the southern San Andreas fault and co-located with a fully anchored laser strainmeter. The investigator had planned to pull out the portable instrument in the fall of 1999, but is requesting funds to continue to operate the instrument because over the last six months a number of unusual strain events have been observed. First, there has been observed several aseismic but rapid strain changes which are unlike the previous behavior of the strain record since 1994. Second, there has been observed the postseismic response to the Mw 7.1 Hector Mine earthquake of 17 October 1999. This earthquake triggered a swarm of earthquakes adjacent to the southern termination of the San Andreas fault, near the strainmeters, leading to genuine concern over the possibility of accelerating deformation in the area. Given the current level of activity, it is very likely that continued operation of this instrument will detect further such events. Understanding better where, and perhaps how, these events occur should provide important information on fault mechanics in general and about the behavior of the San Andreas fault at its southern end: one end of that segment of the San Andreas which has the longest elapsed time since the last large earthquake.