Geodetic Predictions of a strike‐slip fault model: Implications for intermediate‐ and short‐term earthquake prediction

Abstract

Mechanical models of faulting during an earthquake cycle now exist that are based on laboratory‐derived constitutive laws for rock friction. Such models allow calculation of geodetic changes as a function of depth, distance from the fault, and time and are useful for gaining insight on the magnitude and timing of geodetic changes that might occur prior to an earthquake. We have used the Tse and Rice (1986) model of a crustal strike‐slip fault to study the sensitivity of surface deformation to changes in the values of the friction constitutive parameters and the amount of dynamic overshoot during an earthquake. Models were examined with values of the characteristic decay distance Dc in the constitutive law ranging from 5 to 80 mm, with near‐surface values of the steady state velocity dependence a−b of −0.0015 and −0.003, and with either zero or 30% dynamic overshoot. The time variations of the predicted displacement, velocity, strain, and strain rate distributions are strongly affected by changes in Dc and a−b, and only slightly affected by the amount of dynamic overshoot. The larger values of Dc and the less negative value of a−b (i.e., large ratios of Dc to b−a) produce larger and earlier precursory signals. The most useful signals for predicting the model earthquakes are strain changes near the fault, because for nearly all model parameters studied, the predicted strain changes are considerably larger than the detectability limits of existing borehole strain meters in the presence of earth noise. Detectable changes occur a few minutes to a month prior to the earthquake, depending on parameter values. Premonitory increases in velocity also occur; a pattern of larger velocities at distances a few kilometers away from the fault trace is a diagnostic indication of accelerating slip at depth that would be useful for predictive purposes. Unfortunately, these velocity changes are detectable with creep meters and two‐color laser electronic distance measuring devices only for the largest ratios of Dc to b−a. In agreement with recent measurements for several earthquakes, the predicted amount of preseismic moment release is <0.5% of the earthquake moment for most simulations, and the postseismic moment release is a significant fraction of the earthquake moment. This two‐dimensional model suggests that earthquakes of magnitude 6 or greater on strike‐slip faults that cut the Earth's surface could be predicted using existing instrumentation, unless the ratio of Dc to b−a for natural faults is equal to or smaller than the smallest values investigated by Tse and Rice (1986). Study of three‐dimensional models is needed, since premonitory signals will be smaller if along‐strike variation is considered.