Mega-earthquakes strike where fault lines are flat

The biggest earthquakes don't occur where plate boundaries are curved and locked together, spelling bad news for Mexico. Kate Ravilious reports.

The flat north section of the subduction zone sliding beneath Japan – the dark blue line near the top of this image – is a prime spot for earthquakes larger than magnitude-9 to strike, new research says.

When it comes to giant earthquakes, it’s the smooth, ramp-shaped fault lines you need to watch.

New work published in Science changes how seismologists understand which parts of the world are capable of producing a mega-quake – magnitude 8.5 or greater – and adds Mexico to the hit list, despite there being no historical evidence of mega-quakes there.

“This is potentially quite a significant change in our understanding of how thrust faults operate,” says Paul Somerville, a geoscientist at Macquarie University in Sydney, who wasn't involved in the study.

Around the rim of the Pacific Ocean, the ocean floor is forced under the continents it meets. It is along this diving tectonic plate, known as a subduction zone, that the world's largest earthquakes occur.

Seismologists assumed that mega-quakes only rattled young, fast-moving subduction zones, but the magnitude-9.3 Indian Ocean earthquake in 2004 (on a slowly moving plate) and the magnitude-9 Tohoku-Oki earthquake in Japan in 2011 (on a relatively old plate) completely overturned this theory.

Seismologists have since become wary of all subduction faults – and in particular any highly curved, locked regions where stress might build up.

But was this a fair assumption? Quentin Bletery from the University of Oregon and colleagues in the US and France wanted to find out, so they analysed whether the shape of a fault influenced the size of quake it could produce.

Using previously gathered seismic data, they calculated the curvature of each segment of fault around the Pacific Rim. They found mega-quakes struck only on relatively flat, smooth-moving sections of fault – a finding supported by their own fault models – not those strongly locked.

According to the new analysis, faults capable of producing mega-quakes run alongside Indonesia, Japan up to Kamchatka, the Aleutians to Alaska, Cascadia (along the north-west coast of North America), Central America and the entire coast of South America.

“Our results suggest that giant earthquakes are possible in Java, Peru and Mexico even though we don’t have historic evidence of mega-quakes in these regions,” Bletery says.

Meanwhile, good news for people living near sections of fault bordering the Solomon Islands, the Philippines, and between Santa Cruz and the Loyalty Islands in the western Pacific.

“The subduction fault is too highly curved in these locations [for such mega-quakes to strike],” Bletery explains.

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