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The extent to which ongoing seismicity in intraplate regions represents long-lived aftershock activity is unclear. We examined historical and instrumental seismicity in the New Madrid central U.S. region to determine whether present-day seismicity is composed predominantly of aftershocks of the 1811–1812 earthquake sequence. High aftershock productivity is required both to match the observation of multiple mainshocks and to explain the modern level of activity as aftershocks; synthetic sequences consistent with these observations substantially overpredict the number of events of magnitude ≥ 6 that were observed in the past 200 years. Our results imply that ongoing background seismicity in the New Madrid region is driven by ongoing strain accrual processes and that, despite low deformation rates, seismic activity in the zone is not decaying with time.
Rolling Under New Madrid
During 1811–1812, the New Madrid Seismic Zone experienced a sequence of three large intraplate earthquakes and at least one comparably sized aftershock. There have been no earthquakes of similar magnitudes since then. Using a combination of historical data dating back to the original large events and an epidemic-type aftershock sequence model, Page and Hough (p. 762, published online 23 January) found that the current low seismicity is not part of an aftershock sequence. Instead, despite low observable deformation rates, there is ongoing accumulation of strain, leaving the potential for large earthquakes in the region.