Microseismic properties of a homogeneous sandstone during fault nucleation and frictional sliding

Abstract

SUMMARY The formation and evolution of faulting in three initially intact, oil-saturated specimens of Clashach sandstone is examined under conditions of constant strain rate loading at three different confining pressures, simulating the effect of tectonic loading at different depths in the Earth’s upper crust. After a fault is formed the specimens are slid for a time, and then the initial confining pressure is increased to simulate the long-term recovery of strength expected in the Earth. The differential stress u and natural acoustic emissions (AE) are measured during the three separate phases of fault nucleation, sliding and strengthening. At the end of each individual phase the fluid permeability is measured by a pulse-decay technique at constant stress. The AE are interpreted using a mean field theory for damage evolution which calculates a mean crack length (c) from the seismic event rate N and the b-value, and a mean energy release rate (G) from u and (c). (c) and (G) both increase non-linearly in the nucleation stage, and are associated with large fluctuations in the scaling exponent b. In contrast, the b value remains almost constant at a value near unity in the sliding phase, even when the fault is mechanically strengthened. Small fluctuations in ( G ) observed during sliding may be associated with the breaking of individual asperities. The relative stability of b, combined with a relatively constant sliding stress, implies that frictional sliding has all the hallmarks of a self-organized critical (SOC) phenomenon. In contrast, the fresh fracture of intact rock shows a continuum of states with power-law scaling at a variety of b values, where b is negatively correlated to (G). During the fault nucleation stage the implied subcritical crack growth index n’ obtained from the AE data shows a distinct break of slope during the failure of the intact specimens, from n’ 2, consistent with a theoretical transition from stable damage to dynamic instability. A possible mechanism is the transition from local mechanisms dominated by dilatant hardening (negative feedback) to microcrack coalescence (positive feedback). Interpreted in this way, the results imply that the organized coalescence of microcracks to form a fault occurs before the ultimate strength (peak stress) is reached. This emphasizes the fact that local mechanisms of softening or hardening do not always scale simply to the macroscopic rheology in a heterogeneous medium. The fluid permeability reduces steadily throughout all phases of deformation, testament to the self-sealing nature of deformation in porous sedimentary rocks.