Triggered earthquakes and the 1811-1812 New Madrid, central United States, earthquake sequence

The 1811-1812 New Madrid, central United States, earthquake sequence included at least three events with magnitudes estimated at well above M 7.0. I discuss evidence that the sequence also produced at least three substantial triggered events well outside the New Madrid Seismic Zone, most likely in the vicinity of Cincinnati, Ohio. The largest of these events is estimated to have a magnitude in the low to mid M 5 range. Events of this size are large enough to cause damage, especially in regions with low levels of preparedness. Remotely triggered earthquakes have been observed in tectonically active regions in recent years, but not previously in stable continental regions. The results of this study suggest, however, that potentially damaging triggered earthquakes may be common following large mainshocks in stable continental regions. Thus, in areas of low seismic activity such as central/ eastern North America, the hazard associated with localized source zones might be more far reaching than previously recognized. The results also provide additional evidence that intraplate crust is critically stressed, such that small stress changes are especially effective at triggering earthquakes.