OREGON STATE UNIVERSITY

Earthquake swarm off coast prompts research cruise

03/04/2005

NEWPORT, Ore. - A large earthquake "swarm" that began last Saturday (Feb. 27) has resulted in thousands of small earthquakes off the Oregon coast during the last several days and prompted an investigation by a multi-agency research team that includes scientists from Oregon State University and the National Oceanic and Atmospheric Administration (NOAA).

The research team will leave Seattle Saturday morning aboard the R/V Thompson en route to a site on the Juan de Fuca Ridge northwest of Astoria called the Endeavor segment.

"These earthquake swarms are associated with seafloor spreading," said Robert P. Dziak, an oceanographer with Oregon State University and NOAA stationed at OSU's Hatfield Marine Science Center in Newport. "We suspect what happened was that magma pushed up into the crust and the lava may have broken the surface.

"The swarm was strong enough that we decided to send out a ship to investigate," he added. "The quakes seem to have subsided, but we hope that a hydrothermal plume is still out there in the water over the ridge."

Dziak said that these earthquakes are generally small and no threat to create a tsunami, although the Cascadia Subduction Zone off the Northwest coast is similar to the Indian Ocean terrain that produced a powerful 9.0 quake and subsequent tsunami that devastated portions of southeast Asia in December.

These are much smaller quakes that generally range from 2.0 to 4.0 in intensity and can occur in swarms during seafloor spreading events. A few larger quakes did occur this week that were measured in the 4.4 to 4.8 range, however, which also piqued the scientists' interest.

Dziak is the lead researcher for a team operating the Sound Surveillance System, or SOSUS, out of the OSU Hatfield Marine Science Center. This system of hydrophones located on the ocean floor - originally used during the Cold War to monitor submarine activity in the northern Pacific Ocean - began recording an intense earthquake swarm on Feb. 27.

During the first 36 hours of the swarm, SOSUS detected nearly 1,500 small quakes. On Wednesday, the swarm continued with 10 to 30 events an hour. Earthquake activity continued Thursday at a "moderate pace," Dziak said, with between four and 45 events an hour.

The swarm is similar to past seafloor spreading events that took place at Endeavor in 1999, and at the Middle Valley site of Juan de Fuca Ridge in 2001, said Dziak, who added that the movement of tectonic plates triggers the seismic activity.

"In the last 10 years, we've recorded seven of these swarms," Dziak said. "The plate doesn't move in a continuous manner and some parts move faster than others. Generally, movement occurs when magma is injected into the ocean crust and pushes the plates apart. When it does, these swarms occur and sometimes lava breaks through onto the seafloor.

"Usually, the plate moves at about the rate a fingernail might grow - say three centimeters a year," he added. "But when these swarms take place, the movement may be more like a meter in a two-week period."

The research team aboard the R/V Thompson hopes to study and measure the plume that likely formed if lava broke through the crust, Dziak said. The measurements will help scientists learn more about seafloor spreading, the composition of the lava that came to the surface, and the effects on the surrounding environment.

Researchers aboard the vessel include principal investigators Ed Baker, with the NOAA/Pacific Marine Environmental Laboratory, and Jim Cowen, the University of Hawaii. Several other researchers will join them, including three scientists from the Hatfield Marine Science Center: Bill Chadwick and Joe Haxel, who have dual appointments with OSU and NOAA; and Shannon Ristau of NOAA.

A website describing the earthquake activity is at: http://www.pmel.noaa.gov/vents/acoustics/seismicity/nepac/endeav0205.html