Large earthquakes are always followed by aftershocks — a series of smaller but still potentially damaging quakes produced as the ground readjusts. But how long does it take for the aftershocks to die out? A new study suggests some areas can experience aftershocks decades or even centuries after the original earthquake. From a report:

In earthquake-prone areas it is hard to tell the difference between aftershocks and ordinary background seismicity. But recognising aftershocks is an important part of assessing a region’s disaster risk. To understand how long aftershocks can persist, researchers turned to the stable continental interior of North America, where earthquakes are uncommon. Using statistical analysis they assessed the timing and clustering of quakes that followed three large magnitude 6.5 to 8 historical earthquakes: one near south-east Quebec in Canada in 1663; a trio of quakes around the Missouri-Kentucky border from 1811 to 1812; and an earthquake in Charleston in South Carolina in 1886. Their results, published in Journal of Geophysical Research: Solid Earth, suggest that the Quebec quake in 1663 has likely shaken itself out, but to their surprise nearly a third of modern quakes in the Missouri-Kentucky area were most likely to be aftershocks from the 1811-12 event, and about 16% of recent quakes in the Charleston region are probably aftershocks from the 1886 quake.