Scientists at Oregon State University’s Hatfield Marine Science Center have recorded more than 600 earthquakes in the last 10 days off the central Oregon coast in an area not typically known for a high degree of seismic activity.

This earthquake “swarm” is unique, according to OSU marine geologist Robert Dziak, because it is occurring within the middle of the Juan de Fuca plate – away from the major, regional tectonic boundaries.

“In the 17 years we’ve been monitoring the ocean through hydrophone recordings, we’ve never seen a swarm of earthquakes in an area such as this,” Dziak said. “We’re not certain what it means. But we hope to have a ship divert to the site and take some water samples that may help us learn more.” The water samples may indicate whether the process causing the earthquakes is tectonic or hydrothermal, he added.

At least three of the earthquakes have been of a magnitude of 5.0 or higher, Dziak said, which also is unusual. On Monday (April 7), the largest event took place, which was a 5.4 quake. Seismic activity has continued through the week and a 5.0 tremor hit on Thursday. Numerous small quakes have continued in between the periodic larger events.

Few, if any, of these earthquakes would be felt on shore, Dziak said, because they originate offshore and deep within the ocean.

The earthquakes are located about 150 nautical miles southwest of Newport, Ore., in a basin between two subsurface “faulted” geologic features rising out of the deep abyssal sediments. The hill closest to the swarm location appears to be on a curved structure edging out in a northwestern direction from the Blanco Transform Fault toward the Juan de Fuca ridge, Dziak said.

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Analysis of seismic “decay” rates, which look at the decreasing intensity of the tremors as they radiate outward, suggest that the earthquakes are not the usual sequence of a primary event followed by a series of aftershocks, Dziak said.

“Some process going on down there is sustaining a high stress rate in the crust,” he pointed out.

Dziak and his colleagues are monitoring the earthquakes through a system of hydrophones located on the ocean floor. The network – called the Sound Surveillance System, or SOSUS – was used during the decades of the Cold War to monitor submarine activity in the northern Pacific Ocean. As the Cold War ebbed, these and other unique military assets were offered to civilian researchers performing environmental studies, Dziak said.

Hatfield Marine Science Center researchers also have created their own portable hydrophones, which Dziak has deployed in Antarctica to listen for seismic activity in that region. The sensitive hydrophones also have recorded a symphony of sounds revealing not only undersea earthquakes, but the movement of massive icebergs, and vocalizations of whales, penguins, elephant seals and other marine species.

This isn’t the first time the researchers have recorded earthquake swarms off the Oregon coast, Dziak said. In 2005, they recorded thousands of small quakes within a couple of weeks along the Juan de Fuca Ridge northwest of Astoria. Those earthquakes were smaller, he pointed out, and located along the tectonic plate boundary.

This is the eighth such swarm over the past dozen years, Dziak said, and the first seven were likely because of volcanic activity on the Juan de Fuca ridge. The plate doesn't move in a continuous manner and some parts move faster than others. Movement generally 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,” Dziak pointed out. “Usually, the plate moves at about the rate a fingernail might grow – say three centimeters a year. But when these swarms take place, the movement may be more like a meter in a two-week period."

But this eighth swarm may be different.

“The fact that it’s taking place in the middle of the plate, and not a boundary, is puzzling,” Dziak admitted. “It’s something worth keeping an eye on.”