POSTSEISMIC STRAIN FOLLOWING THE 1989 LOMA-PRIETA EARTHQUAKE FROM GPSAND LEVELING MEASUREMENTS

Postseismic deformation in the 5 years following the 1989 Loma Prieta earthquake has been measured with the Global Positioning System and pr ecise leveling. Postearthquake velocities at distances greater than si milar to 20 km from the coseismic rupture are not significantly differ ent from those observed in the 20 years prior to the earthquake. Howev er, velocities at stations within similar to 20 km of the rupture exce ed preearthquake rates and exhibit unanticipated contraction normal to the strike of the San Andreas fault system. A combination of forward modeling and nonlinear optimization suggests that the observed postsei smic deformations were caused by aseismic oblique reverse slip averagi ng 2.9 cm/yr on the San Andreas fault and/or the Loma Prieta rupture z one and 2.4 cm/yr reverse slip along a buried fault within the Foothil ls thrust belt. The best fitting sources of postseismic deformation ar e all located at depths of less than 15 km. We find no evidence for ac celerated flow or shear below the Loma Prieta rupture in the first 5 y ears following the earthquake. The inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture.