HR: 09:15h
AN: G21A-04 INVITED     [Abstracts]
TI: Holocene deceleration of the San Andreas fault zone in San Bernardino and implications for the eastern California shear zone rate debate
AU: * Bennett, R A
EM: rab@geo.arizona.edu
AF: University of Arizona, 1040 E 4th St, Tucson, AZ 85721 United States
AU: Lavier, L
EM: luc@utig.ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Road, #600, Austin, TX 78759 United States
AU: Anderson, M L
EM: anderson@geo.arizona.edu
AF: University of Arizona, 1040 E 4th St, Tucson, AZ 85721 United States
AU: Matti, J
EM: jmatti@swfo.arizona.edu
AF: United States Geological Survey, 20 N. Park Avenue, Tucson, AZ 85719 United States
AU: Powell, R E
EM: rpowell@swfo.arizona.edu
AF: United States Geological Survey, 20 N. Park Avenue, Tucson, AZ 85719 United States
AB: New geodetic inferences for the rate of strain accumulation on the San Andreas fault associated with tectonic loading are ~20 mm/yr slower than observed Holocene surface displacement rates in the San Bernardino area, south of the fault's intersection with the San Jacinto fault zone, and north of its intersection with the eastern California shear zone (ECSZ). This displacement rate "anomaly" is significantly larger than can be easily explained by locking depth errors or earthquake cycle effects not accounted for in geodesy-constrained models for elastic loading rate. Using available time-averaged fault displacement-rates for the San Andreas and San Jacinto fault zones, we estimate instantaneous time-variable displacement rates on the San Andreas-San Jacinto-ECSZ fault zones, assuming that these fault zones form a closed system in the latitude band along which the fault zones overlap with one another and share in the accommodation of steady Pacific-North America relative plate motion. We find that the Holocene decrease in San Andreas loading rate can be compensated by a rapid increase in loading/displacement rate within the ECSZ over the past ~5 kyrs, independent of, but consistent with geodetic and geologic constraints derived from the ECSZ itself. Based on this model, we suggest that reported differences between fast contemporary strain rates observed on faults of the ECSZ using geodesy and slow rates inferred from Quaternary geology and Holocene paleoseismology (i.e., the ECSZ rate debate) may be explained by rapid changes in the pattern and rates of strain accumulation associated with fault loading largely unrelated to postseismic stress relaxation. If so, displacement rate data sets from Holocene geology and present-day geodesy could potentially provide important new constraints on the rheology of the lower crust and upper mantle representing lithospheric behavior on time-scales of thousands of years. Moreover, the results underscore that disagreement between geodetic and geologic fault displacement rates may reflect changes in strain accumulation rates associated with far-field elastic loading and thus earthquake potential, and not just transients.
DE: 1206 Crustal movements--interplate (8155)
DE: 1208 Crustal movements--intraplate (8110)
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
SC: Geodesy [G]
MN: 2005 Joint Assembly