SOME earthquakes can leave a legacy of aftershocks that last for centuries.

Low-level seismic rumbles appear to foreshadow many quakes. Yet not always: the 2008 Sichuan quake in China (pictured) came out of the blue. These rumbles may not be precursors but aftershocks – readjustments at a fault following a larger event, in some cases centuries earlier.

Seth Stein of Northwestern University in Evanston, Illinois, and colleagues analysed the rate of fault slip in various tectonic settings. At plate boundaries, motion rapidly “reloads” a fault with new stress and changes conditions there, so tremors that can be clearly identified as …