Why was the Sumatra quake so large?

The most puzzling question about today's 8.6 earthquake off the coast of Indonesia is how it got to be so big.

Most large earthquakes that occur in oceans take place at subduction zones, where one tectonic plate is thrust beneath another. The epicentre of today's quake, however, was a vertical crack through the ocean crust 400 kilometres west of Sumatra. This is some distance from the Sundra trench, the nearest subduction zone (see map). Vertical cracks generally don't generate this much energy.

"It is a remarkable event, scientifically," says geophysicist John McCloskey at the University of Ulster in Coleraine, UK. McCloskey is an expert in the seismology of the Indian Ocean. Together with colleagues, he is currently running calculations to understand what led to the unusual event.

The vertical orientation of the crack also explains why the quake did not generate a tsunami.

Low energy

Vertical cracks like the one that ripped earlier today form at mid-ocean ridges, where oceanic crust is created. As newly formed crust moves away from the ridge, it develops a series of cracks, or "transform faults", that run perpendicular to the ridge. These faults typically become inactive in older oceanic crust that is no longer at the ridge. This morning's quake happened because one of them became active again.

Earthquakes on transform faults tend to be smaller than the great megathrust events generated, especially compared to megathrust events in subduction zones. That's because faults in subduction zones extend diagonally through the brittle crust and so travel greater distances before running into the viscous mantle lower crust beneath. Vertical transform faults take a shorter route to the mantle lower crust (see diagram).

As a result, the energies released when vertical faults slip are generally smaller than subduction quakes.

"I would have to check this but I don't believe I've ever heard of an 8.7 vertical quake," says McCloskey. "We are still looking at very preliminary data but if everything stays the way it is looking now, this is an amazingly large event for this mechanism."

Non-tsunamigenic

Vertical cracks also tend not to generate tsunamis. In subduction zones, earthquakes happen when mounting pressure causes one plate to suddenly pop up vertically. Last year's Tohoku quake occurred in a subduction zone and lifted the seafloor by 10 to 20 metres. In 2004, the 26 December Sumatran earthquake lifted trillions of tonnes of water by around 5 metres when the Sundra subduction fault ruptured.

Earthquakes on vertical cracks occur because two chunks of crust grind past each other horizontally, with little to no vertical movement. They do not tend to lift much water.

Earlier today a buoy in the Bay of Bengal was reported to have sensed a 30 centimetre wave. That's exactly what you would expect after an earthquake on a vertical crack, says McCloskey. The crack that ruptured may have been on a slight diagonal, generating a small vertical motion, or it could have shifted some underwater topography structure like a seamount. That would have generated a small wave, but nothing lethal.

Why so big?

So why was the earthquake so large? McCloskey and his colleagues are working on a few theories. Although the modelled epicentre of this morning's earthquake is not in a subduction zone, it is close to one: the Sundra trench. Faults rip over large areas, so it's possible that the Sundra trench megathrust shifted as well during the event.

It is also possible that the vertical fault ruptured backwards in the opposite direcation into the Indian Ocean over hundreds of kilometres. The longer the rupture, the larger the potential energy released.

Either way, although the event is of great scientific interest preliminary data suggests it is unlikely to have significant human repercussions. A backwards rupture would be perfectly safe, says McCloskey. There are no islands in that direction.

Even if the event did involve the Sundra trench, he says that is not necessarily cause for concern. The huge quantities of energy released by the 2004 quake and another that followed in 2005 mean the area "would be reasonably relaxed", he says.

McCloskey and his colleagues are currently running calculations to distinguish between these and other possibilities.

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