Earthquakes in eastern China and the eastern United States occur far away from known plate boundaries. M ≥ 8 and 6 < M < 8 events occurred in both areas throughout known history, but the frequency of occurrence of earthquakes was apparently higher in eastern China than in the eastern U.S. Eastern China is composed of many geologically different blocks, which are often bounded by active faults. In comparison the eastern U.S. geology is less heterogeneous. Whereas seismicity in eastern China is frequently, although not always, dearly related to neotectonic activities along the boundaries of “rigid block”, most of the seismogenic structures associated with eastern U.S. events are often overlain by thick recent sediments or have not yet been clearly deciphered; two “rigid blocks” in the eastern U.S. with recognizable seismogenic structures on all sides are tentatively identified. Judging from the episodicity of earthquake occurrence in eastern China in the last 2000 years, we do not yet have enough time to assess the true recurrence rate of earthquakes in the eastern U.S (with a historical record spanning only 400 years). If some eastern U.S. earthquakes occur along boundaries of blocks as they do in eastern China, areas next to the historically active areas in the eastern U.S. may also be sites of significant future seismicity.

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