Plate-tectonic theory explains earthquakes at plate boundaries but not those in continental interiors, where large earthquakes often occur in unexpected places. We illustrate this difference using a 2000-year record from North China, which shows migration of large earthquakes between fault systems spread over a large region such that no large earthquakes rupture the same fault segment twice. However, the spatial migration of these earthquakes is not entirely random, because the seismic energy releases between fault systems are complementary, indicating that these systems are mechanically coupled. We propose a simple conceptual model for intracontinental earthquakes, in which slow tectonic loading in midcontinents is accommodated collectively by a complex system of interacting faults, each of which can be active for a short period after long dormancy. The resulting large earthquakes are episodic and spatially migrating, in contrast to the more regular spatiotemporal patterns of interplate earthquakes.
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