from http://www.unknowncountry.com/news/?id=4456
Earthquake Swarm Off Oregon Coast
07-Mar-2005
A swarm that has now reached over 3,500 small earthquakes began last weekend off the Oregon coast, but officials insist that they do not pose any tsunami threat, even though part of the affected ocean floor is similar to the area in the Indian Ocean area that produced the magnitude 9 quake that caused last December’s huge tsunami in southeast Asia. The small quakes off the Oregon coast range from a magnitude of 2 to 4.
NOAA scientists think the quakes are caused by an underwater volcano which is about to erupt. A NOAA team plans to dive down to investigate and snap photos of the lava welling up from the seafloor, where the Juan de Fuca plate is located. This has been called a “tectonic time bomb,” because it is capable of producing earthquakes and tsunamis that could equal the disaster in Indonesia, although scientists don’t expect that to happen as a result of these small quakes.
Our story was provided to us by our reporter in Seattle, but we have received numerous requests for more information about this story. To read another news story about this, click here.
April 12
By JEFF BARNARD, Associated Press Writer
GRANTS PASS, Ore. (AP) – Scientists listening to underwater microphones have detected an unusual swarm of earthquakes off central Oregon, something that often happens before a volcanic eruption – except there are no volcanoes in the area.
Scientists don’t know exactly what the earthquakes mean, but they could be the result of molten rock rumbling away from the recognized earthquake faults off Oregon, said Robert Dziak, a geophysicist for the National Oceanic and Atmospheric Administration and Oregon State University.
There have been more than 600 quakes over the past 10 days in a basin 150 miles southwest of Newport. The biggest was magnitude 5.4, and two others were more than magnitude 5.0, OSU reported.
On the hydrophones, the quakes sound like low thunder and are unlike anything scientists have heard in 17 years of listening, Dziak said. Some of the quakes have also been detected by earthquake instruments on land.
The hydrophones are left over from a network the Navy used to listen for submarines during the Cold War. They routinely detect passing ships, earthquakes on the ocean bottom and whales calling to one another.
Scientists hope to send out an OSU research ship to take water samples, looking for evidence that sediment has been stirred up and chemicals that would indicate magma is moving up through the Juan de Fuca Plate, Dziak said.
The quakes have not followed the typical pattern of a major shock followed by a series of diminishing aftershocks, and few have been strong enough to be felt on shore.
The Earth’s crust is made up of plates that rest on molten rock, which are rubbing together. When the molten rock, or magma, erupts through the crust, it creates volcanoes.
That can happen in the middle of a plate. When the plates lurch against each other, they create earthquakes along the edges.
In this case, the Juan de Fuca Plate is a small piece of crust being crushed between the Pacific Plate and North America, Dziak said.
Latest Earthquake Swarm off Central Oregon Coast Puzzles Scientists
Published on Apr 12, 2008 – 7:50:52 AM
By: Oregon State University
NEWPORT, Ore. April 11, 2008 — 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.
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.”
Quakes off Oregon coast shake up a mystery
It’s not the Big One, it’s a bunch of small ones, and geologists plan to send a ship to find out why
Saturday, April 12, 2008
MICHAEL MILSTEIN
The Oregonian Staff
Geologists at Oregon State University are preparing to divert a research ship to investigate an unusual offshore swarm of earthquakes — including three of magnitude 5 or larger — about 170 miles southwest of Newport.
The earthquakes started about 10 days ago and are continuing, although they appeared to taper off slightly Thursday night, said Robert Dziak, an OSU marine geologist in Newport. Instruments have recorded more than 600 tremors, many of them very small.
The quakes are puzzling because they are not occurring along the edge of the tectonic plates that make up Earth’s crust, where geologists are used to seeing seismic activity. Instead, they’re centered in the Juan de Fuca Plate, a span of crust off the northwest coast, about 40 miles from the plate’s edge.
The largest earthquake was a magnitude 5.4 tremor Monday. A magnitude 5 quake hit Thursday.
The tremors have apparently been too small and too far offshore to cause damage. Dziak said he had heard reports that residents near Yachats had felt the quakes. But Jim Hawley, emergency services manager in Lincoln County, said no one had reported them to county authorities.
The swarm also is odd because it has not come in the form of a main shock followed by steadily decreasing tremors, known as aftershocks. That’s typically what geologists see when an earthquake occurs on a fault within one of the plates.
“This thing has been a steady stream of earthquakes through time,” Dziak said.
One explanation might be volcanic activity on the seafloor. But Dziak considers that unlikely because it’s most common along the edges of plates or around known volcanic “hot spots” such as Hawaii.
The tremors are striking in a basin between hills jutting 1,000 or more feet high from the seafloor. The area lies about a mile beneath the ocean’s surface, Dziak said.
He said it’s possible that the Juan de Fuca Plate, which is being squeezed by the plates around it, is under enough stress that it is showing the strain by crumpling. The tremors might be a sign of that.
OSU geologists hope to send either an OSU research vessel or another one operated by the National Oceanic and Atmospheric Administration to the area to investigate the tremors. The ships could lower research devices toward the seafloor and collect water samples in the area of the earthquakes.
Analysis of the water samples could identify chemical traces that would hint at volcanic activity or a newly formed fault, Dziak said. If a fault has recently opened, ocean water could pick up chemical signatures from the newly exposed earth.
Researchers are tracking the earthquakes through hydrophones, or underwater microphones, spread across the seafloor. The network is known as the Sound Surveillance System and was originally used during the Cold War to monitor submarines in the Pacific.
Michael Milstein: 503-294-7689; michaelmilstein@ news.oregonian.com
