Correlation of strain and velocity during dilatancy

Abstract

PRESENT earthquake mechanism models are predicated on the hypothesis of the formation of a dilatant zone in the region of the proposed break and the time history of physical rock properties that are a function of crack porosity. For example, field observations have shown drops in compressional wave velocity in earthquake areas prior to events. The anomaly persists for a time dependent on the source dimensions of the impending shock, and returns to normal prior to the event^1–4. Velocity measurements made during laboratory fracture tests have shown a decrease in compressional wave velocity above approximately 80% of the fracture stress^5,6. The effect is most pronounced normal to the direction of greatest compression. Similarly, as the fracture stress is approached the inelastic volumetric strain is the greatest⁷, indicating a definite correlation between velocity and inelastic volumetric strain.