By Richard Lovett

A "failed" volcanic eruption caused a swarm of more than 30,000 earthquakes in a remote region of Saudi Arabia last year, a team of U.S. and Saudi scientists has found. The quakes show that plate boundaries can make their influence felt at far greater distances than researchers had supposed.

Magma rose from the bottom of the crust to within 2 kilometers of the surface, says team leader John Pallister, a volcanologist at the U.S. Geological Survey (USGS) Cascades Volcano Observatory in Vancouver, Wash. But rather than forming a conventional magma chamber, it forced its way through the rocks in a sheet, known as a dyke, many kilometers wide and as little as a meter thick.

The volcanic earthquakes caused by the magma's movement and the subsequent shattering of rocks are very different from those caused by tectonic movements. "They don't have a main shock and aftershock sequence," says Pallister. "Instead, there are many thousands of little earthquakes."

Many of the earthquakes measured 2 or less on the Richter scale, and so were detectable only by instruments. But one, on May 19, 2009, had a magnitude of 5.4 according to Saudi Geological Survey estimates, and 5.7 according to the USGS.

That was enough to open up an 8-kilometer-long fissure across the desert and to crack walls and foundations in a nearby town, forcing the evacuation of 40,000 people.

Near miss

The scientists' first job was to determine whether a volcanic eruption was imminent. They concluded that it wasn't. They also had to assess whether larger earthquakes could occur, but this is unlikely for volcanic earthquakes, Pallister says.

In the process, the Saudi authorities deployed an array of seismometers, and U.S .scientists used satellite-based laser altimetry to watch the surface as it bulged upward by as much as half a meter, measuring the uplift to an accuracy of 3 centimeters.

"It's just an amazing set of data," Pallister says. "It's one of the best examples anyone has ever seen of [monitoring] this type of deformation." The study is published this week in Nature Geoscience.

"We're documenting for the first time some aspects of this magma intrusion process," says Cynthia Ebinger, a geophysicist from the University of Rochester, N.Y., who has studied similar failed eruptions in East Africa.

The near-eruption occurred in an ancient lava field called Harrat Lunayyir in northwest Saudi Arabia. The area has seen many eruptions over the past 20 million years, but no volcanic activity in recorded history.

Now the region may be primed for future eruptions, failed or otherwise. "A pathway from the mantle nearly to the surface was established," says Pallister. "That's the type of thing that tends to get reactivated."

Safety implications

The volcanic activity in Harrat Lunayyir was of the type associated with the spreading of tectonic plates. But the main rift zone is almost 200 kilometers away, in the middle of the Red Sea. This, says Pallister, suggests that bits of hot mantle that are upwelling beneath the Red Sea have leaked out under Saudi Arabia.

For a rift zone to make its effects felt over such a long distance is unknown. "We're going to have to go back and revise textbook models to understand why we have active volcanism and active stretching in an area away from a plate boundary," says Gillian Foulger, a seismologist at Durham University, UK. "It shows that the simple picture of magmatism is unrealistic."

The finding also has implications for public safety. Most of Saudi Arabia's lava fields are in remote regions, but development near Medina is encroaching on a region known to have had a surface eruption in 1256.

And the Kenyan capital Nairobi lies within 100 kilometers of a volcanic region, says Ebinger. "We need a reassessment of volcanic hazards throughout East Africa and the Red Sea area," she adds.