Some violent earthquakes that have occurred unexpectedly in places with no recent record of tremors may be the aftershocks of quakes that took place decades or centuries ago, scientists have discovered.
The finding could explain the many unexpected earthquakes that hit the centre of continental shelves, such as the disastrous quake in Sichuan, in the heart of China, that killed at least 68,000 people and injured up to 400,000 more, in 2008.
At 7.9 on the Richter scale, it was one of the deadliest quakes in history.
Earthquakes usually occur at the boundary of two or more tectonic plates but can also occur hundreds of kilometres from a fault line. These earthquakes may be the result of delayed aftershocks rather than background seismic activity, scientists believe.
A study that tested how tectonic faults work has found that the further away an earthquake is from such faultlines, the more likely it could be the delayed aftershock of a previous earthquake. Mian Liu, professor of geological sciences at the University of Missouri-Columbia, said scientists have tried to predict the occurrence of larger earthquakes by looking at the frequency of smaller ones, which is why the Sichuan earthquake was a surprise.
"Until now, we've mostly tried to tell where large earthquakes will happen by looking at where small ones do," Mr Liu said.
A magnitude 7 earthquake that occurred in 1811 near a town on the Mississippi is still causing aftershocks that can be felt in the American Midwest.
"A number of us had suspected this because many of the earthquakes we see today in the Midwest have patterns that look like aftershocks. They happen on the faults we think caused the big earthquakes in 1811 and 1812, and they've been getting smaller with time," Liu said.
The research may help to predict when and where an earthquake is likely, said Professor Seth Stein at Northwestern University.
"Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modelling to look for places where the earth is storing up energy for a large future earthquake," Stein said.