Mysterious "Swarm" of Quakes Strikes Oregon Waters

Richard A. Lovett
for National Geographic News
April 16, 2008
This weekend scientists will take to the water to try to puzzle out the cause of a "swarm" of mysterious earthquakes that has shaken the seafloor near Oregon in recent weeks.

About 600 earthquakes have been recorded in a small region about 190 nautical miles (350 kilometers) offshore from Yachats, said Robert Dziak, a geophysicist with Oregon State University and the National Oceanic and Atmospheric Administration (NOAA) in Newport, Oregon.

Most of the temblors were small, about magnitude 2 or 3, although a few were magnitude 4 or 5.

The earthquakes pose no threat to coastal residents, Dziak and other scientists say. But they are intriguing because they're occurring in a zone in which earthquake activity is not expected.

The area, which measures about 30 nautical miles (55 kilometers) across, is part of the Juan de Fuca tectonic plate, whose southern edge grinds against the Pacific Plate in a boundary similar to California's San Andreas Fault.

(Related: "San Andreas Fault May Be Rare Quake 'Superhighway'" [August 16, 2007].)

That boundary is a zone of frequent earthquakes, Dziak said. But the recent earthquakes have been occurring 30 nautical miles north, far enough away that the fault may not be involved.

"Another odd aspect is that they are showing a swarm-like behavior," Dziak said.

"Typically, on a fault, you have a main shock and then smaller aftershocks," he added. But a swarm comprises many small, similar-size earthquakes.

Possible Volcano

Earthquake swarms normally indicate volcanic activity. But they could represent stresses being released in an unusual manner in the middle section of the Juan de Fuca plate.

"Plates are by definition supposed to be rigid in their interior," said Robert Embley of NOAA's Pacific Marine Environment Laboratory in Newport, Oregon. "But that's not perfect, especially when they are small in area."

(See a map of the world's tectonic plates.)

The three-day research cruise, scheduled to begin on Friday, may solve some of these puzzles.

The scientists say they may find lava oozing out onto the seafloor or hot water percolating up from magma-heated undersea hot springs. They could also come across colder water squeezed out of the underlying crust by tectonic forces.

"The people going out on the ship are going to be lowering instruments down near the seafloor to look for plumes in the water above the area where the earthquakes have been," said Bill Chadwick, a geologist at Oregon State University and NOAA.

"[They will be] looking for temperature anomalies or cloudy water [and] taking samples which can be analyzed in the laboratory for chemical indicators of either magmatic or crustal fluids coming out of the sea floor. Hopefully one or all of those will give us some hints as to what's going on."

Cold War Relic

All of this, the scientists say, is an example of how much we still have to learn about ocean tectonics.

To begin with, Embley said, the area isn't even well mapped.

"We don't really know what the topography looks like out there," he said. "Our good maps are just along the plate boundary, where 98 percent of the [normal] seismic activity occurs. So when things like this happen, it's hard to determine just what kinds of structures it might be occurring on."

This is one of the few sections of the ocean where such an earthquake swarm could even have been detected at all.

Too small to be studied by distant, land-based seismometers, the earthquakes were found by underwater microphones in a network called SOSUS, built by the U.S. Navy during the Cold War to monitor Soviet submarines.

Scientists have had access to the hydrophone data since 1991.

"We've detected several volcanic events that we've gone out and done studies on," Embley said. "It's been very successful, in that we've been able to study things we never would have known were happening otherwise.

"Elsewhere, the ocean floor is basically unmonitored," he added. "We can usually only see larger events detected by land[-based] seismometers, but not little ones."

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