Time-Dependent Properties of Rocks and Its Implications on Earthquake Prediction

Abstract

Strain-rate dependence of various properties associated with fracture of brittle rocks are briefly reviewed. Fairly simple scaling laws on the strain-rate dependence of failure strength, dilatant strain, elastic-wave velocity variation, acoustic emission activity were obtained. The acoustic emission rate generally increases exponentially with the applied stress, σ, and its level is proportional to the strain rate. At a threshold stress, σ_AE, the acoustic emission rate is accelerated and deviates from the linear trend of the (log AE versus log σ) curve. The threshold stress becomes closer to the failure stress, Y, as the strain rate decreases. If we extrapolate the observed strain-rate dependence of Δσ( = ∣σ_AE – Y∣) to a strain rate of 10⁻¹⁴/sec, Δσ becomes 10⁻² to 10⁻³ bar. The low stress-difference Δσ means that the AE activity before a macro-fracture of a rock begins only when a stress level approaches within a few ten millibars from the failure stress. This observation suggests that foreshock activity of a natural earthquake may be influenced significantly by a small stress fluctuation such as earth tide or atmospheric pressure variation, and may provide a physical basis of many reported correlations of seismic activity or volcanic eruption with tidal stress.