Role of pore fluids in the generation of seismic precursors to shear fracture

A SYSTEMATIC study of temporal changes in seismic b-values (defined as the log-linear slope of the earthquake frequency–magnitude distribution) has shown that large earthquakes are often preceded by an intermediate-term increase in b, followed by a decrease in the months to weeks before the earthquake1. The onset of the b-value increase can precede earthquake occurrence by as much as 7 years. A recently proposed fracture mechanics model of the earthquake source2 explains these temporal fluctuations in b in terms of the underlying physical processes of time-varying applied stress and crack growth. The model predicts two minima in b, separated by a short-lived maximum. Here we report the results of controlled laboratory deformation experiments, done in simulated upper-crustal conditions on both air-dried and water-saturated rock specimens. As found in previous experiments3–5, shear fracture in dry specimens is characterized by a decline in b during anelastic deformation to a single minimum reached just before failure. But in water-saturated specimens, when pore-fluid volume is kept constant by servo-control we also observe a second, intermediate-term b-value minimum, so reproducing the double b-value anomaly predicted by the model2.