Evolving towards a critical point: A review of accelerating seismic moment/energy release prior to large and great earthquakes

There is growing evidence that some proportion of large and great earthquak es are preceded by a period of accelerating seismic activity of moderate-si zed earthquakes. These moderate earthquakes occur during the years to decad es prior to the occurrence of the large or great event and over a region la rger than its rupture zone. The size of the region in which these moderate earthquakes occur scales with the size of the ensuing mainshock, at least i n continental regions. A number of numerical simulation studies of faults a nd fault systems also exhibit similar behavior. The combined observational and simulation evidence suggests that the period of increased moment releas e in moderate earthquakes signals the establishment of long wavelength corr elations in the regional stress field. The central hypothesis in the critic al point model for regional seismicity is that it is only during these time periods that a region of the earth's crust is truly in or near a "self-org anized critical" (SOC) state, such that small earthquakes are capable of ca scading into much larger events. The occurrence of a large or great earthqu ake appears to dissipate a sufficient proportion of the accumulated regiona l strain to destroy these long wavelength stress correlations and bring the region out of a SOC state. Continued tectonic strain accumulation and stre ss transfer during smaller earthquakes eventually re-establishes the long w avelength stress correlations that allow for the occurrence of larger event s. These increases in activity occur over longer periods and larger regions than quiescence, which is usually observed within the rupture zone of a co ming large event. The two phenomena appear to have different physical bases and are not incompatible with one another.