After a large earthquake, the regional hazard of yet more large earthquakes is increased but the global hazard is not, suggests a study online this week in Nature Geoscience. In the vicinity of an earthquake, the risk of strong subsequent shaking is raised after the event, but beyond a threshold distance only small follow-up earthquakes seem to be triggered.

Tom Parsons and Aaron Velasco assembled a 30-year catalogue of all earthquakes greater than magnitude 5 that, according to relative timing, could have been triggered by a preceding shock of magnitude 7 or larger. They find that the increase in seismic hazard associated with large earthquakes is confined to a radius around the main quake’s epicenter of about two to three times the length of its rupture.