V p /V s anomalies in dilatant rock samples

Summary In a series of triaxial experiments we have measuredV _p ,V _s and volumetric strain simultaneously in dilating dry and saturated rocks. For the first time these data permit quantitative comparison of seismic velocities or their ratio and dilatant volumetric strain. In air-dry samplesV _p /V _s decreases by a few per cent at strains of 10^–3; in saturated materials with high pore pressure,V _p /V _s increases by a comparable amount. Decreases in seismic velocity ratio are difficult to generate in initially saturated rocks even with low pore pressures and at strain rates of 10^–4/sec. A liquid-vapor transition will not produce a significant drop inV _p /V _s . If dilatancy and fluid flow are responsible for seismic travel time anomalies prior to earthquakes, our results suggest that such anomalies will occur only in regions where pore fluid source to sink dimensions are of the order of 10 km or more, or in regions where the rocks are not saturated to begin with.