Testing self-organized criticality by induced seismicity

We examine the hypothesis proposed in recent years by several authors that the crust is in a self-organized critical (SOC) state. This hypothesis has been suggested on the basis of the observation of power law distributions, such as the Gutenberg-Richter law for earthquakes and the fault length distribution, and of the fractal geometry of sets of earthquake epicenters and of fault patterns. These self-similar properties are shared by simplified models of the crust exhibiting a spontaneous organization toward a critical point characterized by similar scale-invariant properties. The term "critical" is here used in the sense of phase transitions such as the Curie point in magnetism. The usefulness of a hypothesis is measured by its predictive and explanatory power outside the range of observations that have helped defined it. We thus explore how the SOC concept can help in understanding the observed earthquake clustering on relatively narrow fault domains and the phenomenon of induced seismicity. We review the major reported cases of induced seismicity in various parts of the world and find that both pore pressure changes (+/-Delta p) and mass transfers (+/-Delta m) leading to incremental deviatoric stresses of <1 MPa are sufficient to trigger seismic instabilities in the uppermost crust with magnitude ranging up to 7.0 in otherwise historically aseismic areas. Once triggered, stress variations of at least 1 order of magnitude less but still larger than the similar to 0.01 MPa tidal stress are enough to sustain seismic activity. We argue that these observations are in accord with the SOC hypothesis as they shaw that a significant fraction of the crust is not far from instability and can thus be made unstable by minute perturbations. This property is shared by simplified models of SOC. Not all perturbations, however, trigger seismic activity; this is also compatible with the SOC hypothesis which embodies naturally the existence of large heterogeneities in the stress field. The induced seismicity is found ro obey generally the Gutenberg-Richter law up to a magnitude cutoff which correlates well with the width of the local seismogenic bed, ranging in size from that of mine pillars for mining-induced seismicity to the thickness of brittle sedimentary beds in the vicinity of dams or depleted hydrocarbon reservoirs. In conclusion, the properties of induced seismicity and their rationalization in terms of the SOC concept provide further evidence that potential seismic hazards extend over a much larger area than that where earthquakes are frequent.