Earth science: lasting earthquake legacy

Earthquakes occur within continental tectonic plates as well as at plate boundaries. Do clusters of such mid-plate events constitute zones of continuing hazard, or are they aftershocks of long-past earthquakes?

Early on the morning of 16 December 1811, an earthquake of about magnitude 7 shook the centre of the United States around a small town on the Mississippi called New Madrid. By 7 February 1812, it had triggered three more shocks of similar magnitude. The earthquakes broke a set of faults along the Arkansas, Missouri and Tennessee state boundaries, apparently reactivating an ancient rift in the interior of a continental tectonic plate.

On 31 August 1886, a magnitude-7 shock struck Charleston, South Carolina; low-level activity continues there today. One view of seismic hazard is that large earthquakes will return to New Madrid and Charleston at intervals of about 500 years. With expected ground motions that would be stronger than average, that prospect produces estimates of earthquake hazard that rival those at the plate boundaries marked by the San Andreas fault and Cascadia subduction zone. The result is two large 'bull's-eyes' on the US National Seismic Hazard Maps — which, for example, influence regional building codes and perceptions of public safety.