The 8.6-magnitude earthquake that shook the seafloor 400 kilometres off the west coast of Aceh in northern Sumatra on 11 April (see map) may not have been a killer, but its surprising power has seismologists rattled.

The quake, which lasted about 100 seconds, bears little resemblance to the catastrophic 9.1-magnitude quake that struck the same region in December 2004. Yesterday’s quake was marked predominantly by ‘strike-slip’ motion — meaning that the sections of Earth’s crust did not move vertically, with one piece overlapping another, but laterally. The absence of vertical motion accounts for why the quake did not generate a major tsunami. Nevertheless, this quake has generated some provocative scientific questions.

First, just how big was it?

The US Geological Survey (USGS) puts the epicentre of the quake at 22.9 kilometres below the sea floor. Estimates suggest there was motion along a 250–350-km stretch of fault. Geophysicist Chen Ji of the University of California in Santa Barbara puts the average slip between 15 and 20 metres, whereas Suleyman Nalbant, a geophysicist at the University of Ulster in Coleraine, UK, estimates the average slip to be around 4 metres — the same as that of the 1906 San Francisco earthquake. A more precise estimate will come with more analysis.

So it’s big — but what makes it unique?

“It is quite rare to have a strike-slip faulting at such a large magnitude,” says Nalbant. In fact, it’s possible that yesterday’s quake is the largest known earthquake of this type. (Although some say that an 8.6-magnitude earthquake in northeast India in 1950 could give yesterday’s quake a run for its money.)

The other significant factor is where it happened. “Our preconception is that earthquakes greater than 8.5 [magnitude] can only occur on the largest faults on the planet. Yesterday’s earthquake was what we’d call a ‘plate-boundary related’ earthquake not on the major plate boundary fault, but adjacent to it,” says Susan Hough, a USGS geophysicist based in Pasadena, California.

Is this earthquake related to the 2004 earthquake in the same region?

Maybe. “In response to any movement in the brittle/strong crust, the hot and weak fluid that makes up the Earth’s upper mantle tries to catch up with that movement and flows at a faster rate than usual. This would transfer some strain/stress to the upper crust, which may promote earthquakes,” says Nalbant, who speculates that the stress could be higher in the area around the Australian–Indian plate and could have contributed to the 11 April quake. But there’s no way to tell for sure.

Could this earthquake produce more earthquakes in the Sumatran region?

Again, maybe. The 11 April quake has already produced aftershocks of magnitudes 6.5 and 8.2, and it could be part of a larger series of movements. “The Sumatra region has been in a particularly active period since the great 2004 Sumatra–Andamans earthquake. Whether the cluster is over, or whether there could be another (big) shoe left to drop, is impossible to say,” says Hough.

“It’s the idea that you have a big block of rocks that’s under stress, and somewhere within you have an earthquake. That will perturb the stress distribution,” explains Peter Molnar, a seismologist at the University of Colorado at Boulder. “In some places, you’ll enhance the stress, and raise the probability of an earthquake in that area. And, in other places you’ll decrease the stress, and lower the probability of an earthquake in that area.”

Does this change our understanding of earthquakes in this region?

“We’ve known that the three plates making up the Indian Ocean are really complicated for years,” says Seth Stein, a geoscientist at Northwestern University in Evanston, Illinois. “Everything shows you that the area is busted up and deforming, but we still don’t understand this on a fundamental level. This earthquake gives us a real opportunity to go in and study it.”

Hough also points out that “secondary faults are less well developed than major plate boundary faults, so the big earthquakes that occur on them might release more stress relative to the fault length.” How much energy it generated in relation to similar earthquakes will depend on further study.

Map credit: USGS