Solomons Quake 1st Seen to Jump Tectonic Barrier

Richard A. Lovett
for National Geographic News

April 9, 2008

A deadly earthquake and tsunami that swept the Solomon Islands last year gained part of its power by breaking through what should have been a major geologic barrier, scientists say.

The April 1, 2007, quake struck in a section of the South Pacific where two tectonic plates, the Australian plate and the Woodlark plate, slide underneath a third, the Pacific plate.

The two plates that are sliding under, or subducting, are moving at slightly different rates and in slightly different directions from each other.

This means there's a fault between them that such a rupture would not ordinarily be expected to cross, said Fred Taylor, a geologist at the University of Texas at Austin and lead author of a new study of the earthquake.

Patterns in the 2007 event's aftershocks and the way the tsunami hit suggested that the rupture had indeed crossed the fault.

In a new study, Taylor's team used coral reefs to confirm the find, marking the first time that an earthquake has been directly observed to propagate across a plate boundary.

High and Dry

Vertical motions in earthquakes can raise coral reefs above sea level, where they dry out and die.

Taylor's team found such high-and-dry reefs on both sides of the Australia-Woodlark plate boundary, near the tsunami-hit islands of Simbo, Mono, and Ranongga (Ganongga). (See photos of the Solomons' reefs after the quake.)

This proved that both plates had been involved in the Solomons earthquake.

The power of an earthquake is related to the length of the rupture zone. The fault between the plates should have stopped the quake when it would have been smaller and presumably less deadly.

But crossing to the second plate allowed the quake to build up power and reach magnitude 8.1, creating a huge tsunami that killed at least 52 people.

Taylor and colleagues describe their work in the March 30 online issue of the journal Nature Geoscience.

Emile Okal is a geophysicist at Northwestern University in Chicago who was not involved in the study. He agrees with Taylor's conclusion.

"Indeed, what was considered to be a barrier was jumped," Okal said.

But he doesn't see this as a surprising find. Two decades ago researchers who reconstructed a 1932 earthquake in Mexico reached a similar conclusion.

"Ever since, we've looked at these triple junctions as vulnerable. We know that these are barriers that can be jumped," he said.

Lead author Taylor agrees that the Solomon Island's earthquake probably wasn't the only one of its kind.

Offshore from Washington, Oregon, and Northern California, the Cascadia subduction zone is divided into three major segments and several minor ones.

Geological evidence indicates that all of these segments had major earthquakes about 300 years ago.

"[The] paleoseismology doesn't prove it was a single earthquake," Taylor said. "It just proves that they all ruptured at about the same time."

But in 1700 Japan was hit by a tsunami that was probably caused by an earthquake in the Cascadia zone.

"Given the size of the tsunami, people infer that all of those segments ruptured at once," Taylor said.

"So we should be aware of the possibility that a lot of these segments will gang up on us at once and all break."

Such an event could cause a much more devastating earthquake than many researchers had thought possible in the Cascadia zone.

Young and Shallow

By contrast, the same study backed up theories about how deep earthquakes should be within the Australia-Woodlark subduction zone.

The location of the elevated reefs indicated that the 2007 quake was extremely shallow, Taylor said, involving only sections of the fault from the surface to a depth of 9 miles (15 kilometers).

This matches theories that relatively young, warm subducting plates can form shallow quakes.

"The younger the down-going plate, the shallower the rupture zone should be," he said.

That's because younger plates have not yet fully cooled from the lava that formed them.

Warmth means that the plates' constituents, most notably clay minerals, can get stuck as they move, building up tension that causes earthquakes.