Postseismic Strain Following the 1989 Loma Prieta Earthquake From GPS and Leveling Measurements

Journal of Geophysical Research, B, Solid Earth and Planets, v. 102, no.3 pp.4933-4955, March 10, 1997

Postseismic deformation in the five years following the 1989 Loma Prieta earthquake has been meas ured with GPS and precise leveling. Post-earthquake velocities at distances greater than ~20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake. However, velocities at stations within approximately 20 km of the rupture exceed pre-earthquake rates and exhibit unanticipated contraction normal to the strike of the San Andreas fault system. A combination of forward modeling and non-linear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2.9 cm/yr on the San Andreas fault and/or the Loma Prieta rupture zone, and 2.4 cm/yr reverse slip along a buried fault within the Foothills thrust belt. The best-fitting sources of post seismic deformation are all located at depths of less than 15 km . We find no evidence for accelerated flow or shear below the Loma Prieta rupture in the first five years following the earthquake. The inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture.

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