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."

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