Abstract
Dilatancy, the opening of cracks in rock under applied shear stress, has been proposed as a mechanism to explain seismic velocity anomalies prior to earthquakes. In recent years many studies have been made in search of precursory changes in Vp/Vs, but positive results have been extremely limited. Since dilatancy induced velocity anomalies require undersaturation of rock pores, we look carefully at porosity and fluid compressibility within the crust in an effort to determine the conditions that are most favorable for induced velocity anomalies. Simple porosity-temperature-pore pressure models of the crust are examined. Various dilatant strain distributions are imposed on the models and in. every case the most significant potential for velocity anomalies occurs within the upper 3 to 5km of the crust. Low porosity and high temperature at depth have the net effect of giving smaller velocity anomalies than in near surface regions. Actual crustal porosity, pore pressure and strain distributions prior to an earthquake are not well known. Nevertheless, our results suggest that searches for velocity anomalies should be concentrated in the upper few kilometers of the crust where the most significant anomalies are favored to occur.
