POSTSEISMIC DEFORMATION FOLLOWING THE LANDERS EARTHQUAKE, CALIFORNIA,28 JUNE 1992

Accelerated strain followed the Landers and Big Bear earthquakes, retu rning to the normal rate only after a period of several months. We obs erved this strain throughout most of southern California using the Glo bal Positioning System (GPS). Three GPS receivers operating continuous ly in fixed positions at Pinyon Flat, Jet Propulsion Laboratory (Pasad ena), and Goldstone all recorded postseismic deformation in a relative sense. In addition, we established 16 sites where we deployed portabl e receivers occasionally over a period of about 6 months near the rupt ure zones of the earthquakes. Anomalous postseismic displacements rang ed from 55 mm near the epicenter to a few millimeters far from the fau lt. We modeled the displacements, using dislocation theory, as due to variable slip on the faults that were displaced at the times of the ea rthquakes. The model suggests that the postseismic strain released the equivalent of about 15% of the seismic moment of the mainshock. While the strain released from the upper 10 km is about the same as what ca n be explained by direct effects of aftershocks, the major contributio n of strain release comes from the lower layer, below 10-km depth. Sig nificant afterslip or viscous relaxation must have occurred below 10-k m depth to explain the observed deformation more than 100 km from the fault. One interpretation is that high stress on the margin of the co- seismic rupture zone drives the rupture to extend itself into unbroken rock below and along the initial rupture zone.