HR: 17:45h
AN: G32C-08    [PDF]
TI: Results From Long-Base Strainmeters in California: Implications for the PBO
AU: Wyatt, F K
EM: fwyatt@ucsd.edu
AF: IGPP/SIO, University of California, La Jolla, CA 92014 United States
AU: * Agnew, D C
EM: dagnew@ucsd.edu
AF: IGPP/SIO, University of California, La Jolla, CA 92014 United States
AB: We have been making continuous measurements of strain using long-base instruments at various locations in southern California for some time: since 1974 at Pi\~non Flat, near the San Jacinto and San Andreas faults; since 1994 at Durmid Hill, very close to the southern termination of the San Andreas; and for the past year in Verdugo Canyon, at the northern edge of the Los Angeles basin. The first lesson learned from these measurements is that, even over baselines of hundreds of meters, the measurements must be made between points carefully anchored to depth; if this is done, the strain record can provide both very low noise at high frequencies (better than GPS by factors of 100 or more at periods of a week) and also long-term noise low enough to reliably measure secular strain accumulation. While the measurements have been made in only a few locations, these are in areas of active faulting, with secular deformation rates as high as in most areas along the plate boundary. We have observed coseismic signals from a number of earthquakes, and postseismic signals from a few: most notably a two-year signal from the 1992 Landers shock, and shorter-term deformations from the 1987 Superstition Hills and 1999 Hector Mine events. We have not observed any preseismic anomalies. The instrument at Durmid Hill has observed a number of short-term strain changes, and anomalous coseismic deformation from the Hector Mine earthquake; we attribute these effects to shallow creep on the San Andreas fault nearby. These high-quality data suggest that along the strike-slip boundary (and excepting creeping sections of fault zones) ``strain events'' are rare: rather, steady strain accumulation is punctuated only by occasional seismic events and relatively modest postseismic deformation. This picture is consistent with other data now available; installation by the PBO of many more instruments along the plate boundary will further test this idea.
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2003 Fall Meeting