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

• Published in: Nature (London) 1992-09, Vol.359 (6392), p.228-230
• Main Authors: Sammonds, P. R, Meredith, P. G, Main, I. G
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Earthquakes; Earthquakes, seismology; Engineering and environment geology. Geothermics; Exact sciences and technology; Humanities and Social Sciences; Internal geophysics; letter; multidisciplinary; Natural hazards: prediction, damages, etc; Plate tectonics; Science; Science (multidisciplinary); Seismic activity
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/359228a0

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 earthquake 1 . 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 source 2 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 experiments 3–5 , 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 model 2 .