HR: 0800h
AN: G51B-0834    [Abstracts]
TI: Long-Term Creep-Rate Changes and Their Causes
AU: * Bokelmann, G H
EM: goetz@alumni.princeton.edu
AF: University of Montpellier, Laboratoire de Tectonophysique, Place Eugene Bataillon CC049, 34095 Montpellier, Cedex 05 France
AU: Kovach, R L
EM: kov@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305-2215 United States
AB: We discuss measurements of the long-term creep rate from the San Juan Bautista section of the San Andreas fault. These observations of aseismic slip have a length of approximately half a century making it the longest creep record ever measured along an active fault zone. The data indicate transient deformation on all measured scales ranging between days and tens of years, namely from creep events till long-term transients. We will focus on the systematic changes in long-term creep rate over time scales of tens of years. In principle, such variations may be caused by changes in either the resistive forces acting within the fault zone or by variations in plate-tectonic driving forces. We test a recent suggestion for a transient in plate-tectonic driving forces due to the earthquakes in the Aleutians in the 1950s and 60s that should show up in the creep observations, in principle. We suggest that the most likely explanation for most of the long-term changes is elastic reloading of the adjacent locked portion of the San Andreas fault produced by large earthquakes and possibly aseismic strain release that occurs before the earthquakes. There are clear changes in creep-rate associated with the Loma Prieta earthquake, and several moderate-size earthquakes in the area. The higher creep-rate after Loma Prieta is due a reloading of the adjacent locked Loma Prieta section, and it may take several decades before the creep-rate approaches the background rate. Perhaps the most interesting feature of the creep-rate changes is that they begin before the earthquakes. We can only speculate whether this is a general feature of larger earthquakes in the area. If it is indeed true it would have important relevance to the question of earthquake nucleation and it would constrain models of earthquake behaviour.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8150 Plate boundary: general (3040)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Geodesy [G]
MN: Fall Meeting 2005