The world's biggest earthquakes can happen in clusters if the geological geometry is right.

Great quakes of magnitude 8.5 and above occur where one plate of Earth's crust dives beneath another. If the width of plate overlap is particularly wide — more than around 120 kilometres — then even a single huge earthquake cannot relieve all the stress that builds up between the two plates, says a team led by Robert Herrendörfer of ETH Zurich in Switzerland.

The team's modelling suggests that only a series of huge quakes, culminating in a final big one, can overcome the wide plate overlap and relieve seismic stress. This 'supercycle' could explain larger-than-expected quakes, such as the 2011 Tohoku event in Japan. Supercycles might also occur in regions that have not previously been thought to be at risk, such as Alaska and the Antilles.

Nature Geosci. http://dx.doi.org/10.1038/ngeo2427 (2015)