The observable postseismic transients decay with time with most of the motion occurring within the first year after the earthquake. The horizontal and vertical postseismic motions, at stations where they are significant, can be fit fairly well by an exponential time dependence, with a characteristic decay time in the range 0.3-1.4 years. Both the horizontal and vertical motions can be fit equally well (or perhaps slightly better) by a logarithmic function (Marone et al., 1991). Exponential decay characterizes relaxation of a Maxwell viscoelastic solid, while fault afterslip may be expected to obey a logarithmic law. These results imply that we are unable to distinguish between afterslip and relaxation mechanisms. The models presented in this paper suggest that both processes may have occurred following the earthquake.
While the lower crustal viscosity is not very strongly constrained by geodesy, pre earthquake strain profiles favor a relatively cold and stiff lower crust in this region with a relaxation time of the order of a few hundred years. Such a long characteristic relaxation time is inadequate to explain the kind of short-term transient motions observed after the Northridge earthquake. In addition to predicting the wrong time dependence, lower crustal viscoelastic relaxation produces horizontal motions that are in the same sense as the observed motions, but the vertical motions are in the opposite sense, particularly over the fault plane. The observations indicate uplift over the fault plane following the earthquake while viscoelastic models predict downward motion for that region.
The rate of decay for the postseismic motion can be fit by a logarithmic function indicative of fault afterslip. The initial coseismic slip rate for the thickness-average region undergoing afterslip is 174 mm/day which is within the range of that observed for the Superstition Hills earthquake. The friction rate parameter is about 0.002 which matches laboratory values for poorly consolidated materials (C. Marone, written communication, June 1998).
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