Slip distribution of the 1985 Central Chile earthquake

Abstract

Geodetic elevation changes record the deformation associated with the M s = 7.8, March 3, 1985, Central Chile earthquake. By summing elemental point sources in a half-space and employing a gradient technique with positivity constraints, we resolve the slip distribution on the dislocation surface. Most of the fault slip is concentrated on two patches at a depth between 30 and 40 km on a plane dipping 18° E. The patch containing the maximum slip is oriented parallel to the coast and is closely located to the reported hypocenter of the mainshock. The overall region of maximum slip (slip >- 2 m) is more than 140 km long and oriented N-S. The dislocation model obtained from the static field is in remarkable agreement with the mainshock location and aftershock distribution, moment release pattern from analysis of body and surface waves, and location and fault geometry of the main aftershock. The total moment derived from the dislocation model for the coseismic period is 1.6 ×10 28 dyn cm. Comparison of coastal uplift predicted by the model and observations associated with the previous large event in the region (August, 1906, M T = 8.4) suggests a possible variable rupture mode of the region. Estimations of the future crustal readjustment due to restressing and viscoelastic relaxation indicate a general subsidence over a broader area than the coseismically uplifted region. However, the maximum amplitude of the predicted postseismic elevation change is less than 10% of the maximum coseismic uplift. The proximity of the two patches of maximum slip to the aftershock areas of intermediate size thrust earthquakes in the region since 1971 suggests that these compressional events may represent slip prior to failure in the “weaker” updip region of the main asperities.