Long‐Lived Aftershocks in the New Madrid seismic Zone and the Rest of Stable North America

Abstract Seismicity in the New Madrid seismic zone and the rest of stable North America remains controversial. Some workers view present‐day earthquakes there as long‐lived aftershocks; others view them as background seismicity concentrated in lithospheric weak zones. Separating long‐lived aftershocks from background seismicity, however, is challenging. Here, we used the nearest‐neighbor (NN) method to identify long‐lived aftershocks. The NN method calculates the distances between pairs of earthquakes in a space‐time‐magnitude domain. If the distances are too close to be expected for independent background events that follow the Gutenberg‐Richter law and Poisson distribution, the pairs are taken as clustered events (i.e., the later one in a pair is an aftershock of the earlier one). Our results suggest that, depending on the size and location of the 1811–1812 New Madrid mainshocks, 10.7%–65.0% of the M ≥ 2.5 earthquakes in the New Marid region between 1980 and 2016 may be long‐lived aftershocks. Similarly, considerable present‐day earthquakes in South Carolina are long‐lived aftershocks of the 1886 Charleston earthquake, but the contemporary seismicity in Charlevoix, Québec is predominately background seismicity. These results suggest that present‐day seismicity in stable North America, and perhaps other stable continents, includes both background seismicity and long‐lived aftershocks. Distinguishing aftershocks from background seismicity can be important for hazard assessment.