Shear-wave splitting in a critical crust: II - Compliant, calculable, controllable, fluid-rock interactions

This paper argues that the pervasive distributions of closely-spaced stress-aligned fluid-saturated microcracks in almost all rocks are a critical system close to fracture criticality and loss of shear strength. New evidence includes three examples in which observations and modelling directly imply non-linear interactive critical systems with some form of self-organised criticality (SOC). These are a direct calibration of anisotropic poro-elasticity (APE) by monitoring and modelling the response of a reservoir to a high-pressure injection. Monitoring and modelling velocity and attenuation dispersion in a rock physics laboratory. Monitoring the effect of the build-up of stress before earthquakes and volcanic eruptions, including the successful stress forecast of the time and magnitude of an M-L = 5 (M-S approximate to 6) earthquake in southwest Iceland. These new results from three very different fields strongly suggest that the earth's crust is a critical interactive non-linear system with self-organised criticality (SOC). Some effects are subtle and easily ignored. Others are so common and familiar that we have developed one-off explanations in terms of conventional deterministic physics to describe their behaviour and occurrence. We suggest that the identification of the sub-critical physical processes is one reason for the success of APE-modelling. Recognition of (crack) criticality leads to a new understanding of low-level (pre-fracturing) deformation that has massive implications for almost all dynamic processes in the crust. These include reservoir characterisation, hydrocarbon recovery, monitoring the progress of fluid-fluid fronts, and the build-up of stress before fracturing, faulting, and earthquakes, and the movement of magma before volcanic activity. The implications will be discussed and the arguments presented.