Mid-continental earthquakes: Spatiotemporal occurrences, causes, and hazards

Large earthquakes in North China, Australia, Northwestern Europe, Central and Eastern United States, and other mid-continents show complex spatiotemporal patterns that do not fit existing earthquake models. Individual faults or fault zones tend to fall into long term (thousands of years or longer) dormancy after a cluster of ruptures, whereas large earthquakes seem to roam between widespread faults. These behaviors are characteristic of complex dynamic systems of interacting faults. In such systems, widespread faults collectively accommodate slow tectonic loading, and a major fault rupture both transfers stress to the neighboring faults and perturbs loading conditions on distant faults. Because of the slow tectonic loading, local stress variations from fault interaction or nontectonic processes, or changes of fault strength, could trigger min-continental earthquakes. The precise spatiotemporal occurrence of large mid-continental earthquakes may be unknowable, an intrinsic limitation of complex dynamic systems, but their qualitative system behavior may be understood by a system approach. This approach would render some commonly used concepts, such as seismic cycles, recurrence intervals, characteristic earthquakes, and seismic gaps inadequate or irrelevant in mid-continents, and calls for rethinking of the probability estimates based on these concepts. It requires a better understanding of fault interactions on multiple spatial and temporal scales rather than focusing solely on the balance of tectonic loading and yield strength of individual faults or fault segments. It also indicates the need for paleoseismic and geodetic studies extending beyond areas where recent large earthquakes occurred, and for hazard assessments to reduce the biasing influence of the most recent large earthquakes that tend to dominate the short and incomplete earthquake records.