Continuous measurements of crustal deformation for the 1992 Landers earthquake sequence

Abstract

Abstract We describe, and attempt to interpret, continuous measurements of strains and tilts made at Pinon Flat Observatory (PFO) before, during, and after the Landers and Joshua Tree earthquake sequences. These data show substantial transient deformation following the Landers mainshock, with a total amplitude of several percent of the co-seismic deformation, and a decay time of at least several days. Comparing data from the many types of instruments at PFO allows us to infer possible sources for this deformation. The immediate postseismic transient was nearly the same size on three long-base strainmeters, suggesting either broadscale deformation or local motion near one part of the observatory. The latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source. Possible mechanisms for broad-scale deformation include postseismic fault slip, time-dependent creep in near-surface rocks, and elastic or thermal responses to water-table changes. The first two agree best with the observations from PFO, but if postseismic fault slip is the source, it must have been distributed differently than the co-seismic slip, and may have included faults other than those that ruptured seismically. If one of the other mechanisms is the main source, the PFO data imply that the postseismic-slip must have been very much smaller than the seismic slip, perhaps 2% or less. No significant preseismic deformation was observed, at a level of 2 × 10 −3 of the co-seismic deformation, for the days to minutes before the earthquake.