Predicting time-to-failure in rock extrapolated from secondary creep

Stress relaxation experiments are reported that culminate in energetic failure in rocks analogous to the loading cycle and subsequent localization or reactivation on brittle faults embedded in an elastic medium. Universally, rapid primary deformation arrests and transitions into a long secondary deformation phase that ultimately accelerates to catastrophic rupture. Primary deformation (u) conforms to Andrade's law as du/dt ∼ (t_c - t ₀)^θ with a standard exponent of 2/3. In the long, and readily observable, secondary phase, the samples both deform and analogously shed load near linearly in time. This stress relaxation rate exhibits a robust power law dependency with time-to-rupture and exhibits the same 2/3 power law exponent observed in the primary phase. Similarly, the brittle strain energy released in the tertiary collapse scales with a normalized secondary stress relaxation rate. Together, these observations suggest a way to predict both the timing of rupture and its energetics from the observed stress (or strain) rate during the secondary relaxation stage.