A reinterpretation of the precursory seismic b-value anomaly from fracture mechanics
A new model is presented which can explain the major temporal fluctuations in seismic b-value in terms of the underlying physical processes of time-varying applied stress and crack growth under conditions of constant strain rate. The model predicts two minima in the b-value, separated by a temporary maximum of inflexion point; a corollary being that a single broad maximum would be expected in scattering attenuation. These fluctuations in b-value are consistent with reported ‘intermediate-term’ and ‘short-term’ earthquake precursors separated by a period of seismic quiescence. We present preliminary results from controlled laboratory experiments and report recent field observations both of which are consistent with the predicted form, and in particular exhibit the distinctive, second short-term anomaly which reaches a value bc = 0.5 during the critical phase.