POSTSEISMIC DEFORMATION FOLLOWING THE 1989 (M = 7.1) LOMA-PRIETA, CALIFORNIA, EARTHQUAKE

Postseismic deformation along a 90-km profile bisecting the projected surface trace of the coseismic rupture of the 1989 Loma Prieta earthqu ake has been monitored by frequent GPS surveys for 3.3 years following the earthquake. In addition to the expected deformation associated wi th secular strain accumulation on the San Andreas and Calaveras faults , deformation associated with postseismic readjustment has been detect ed. Most of that deformation can be attributed to 1.5 m right-lateral and 0.9 m reverse postseismic slip on a 5-km-wide downdip extension of the Loma Prieta rupture. In addition, there seems to be a 0.1 m posts eismic collapse of the Loma Prieta rupture zone in the direction perpe ndicular to the plane of the rupture. The fault-normal (N48-degrees-E) surface displacements plotted as a function of time exhibit a curvatu re suggesting a relaxation time of about 1.4 years. Similar plots of t he fault-parallel (N42-degrees-W) displacement components do not exhib it significant curvature. Presumably, the deformation shown in those p lots is dominated by secular strain accumulation along the San Andreas and Calaveras faults rather than postseismic relaxation.