ROCK FRICTION AND ITS IMPLICATIONS FOR EARTHQUAKE PREDICTION EXAMINEDVIA MODELS OF PARKFIELD EARTHQUAKES

The friction of rocks in the laboratory is a function of time, velocit y of sliding, and displacement. Al though the processes responsible fo r these dependencies are unknown, constitutive equations have been dev eloped that do a reasonable job of describing the laboratory behavior. These constitutive laws have been used to create a model of earthquak es at Parkfield, CA, by using boundary conditions appropriate for the section of the fault that slips in magnitude 6 earthquakes every 20-30 years. The behavior of this model prior to the earthquakes is investi gated to determine whether or not the model earthquakes could be predi cted in the real world by using realistic instruments and instrument L ocations, Premonitory slip does occur in the model, but it is relative ly restricted in time and space and detecting it from the surface may be difficult. The magnitude of the strain rate at the earth's surface due to this accelerating slip seems lower than the detectability limit of instruments in the presence of earth noise. Although not specifica lly modeled, microseismicity related to the accelerating creep and to creep events in the model should be detectable. In fact the logarithm of the moment rate on the hypocentral cell of the fault due to slip in creases Linearly with minus the logarithm of the time to the earthquak e. This could conceivably be used to determine when the earthquake was going to occur. An unresolved question is whether this pattern of acc elerating slip could be recognized from the microseismicity, given the discrete nature of seismic events. Nevertheless, the model results su ggest that the most likely solution to earthquake prediction is to loo k for a pattern of acceleration in microseismicity and thereby identif y the microearthquakes as foreshocks.