Earthquake swarm off coast prompts research
By Joel Gallob Of the News-Times

A swarm of thousands of small earthquakes that began on Saturday, Feb. 27 has prompted the quick design and chartering of a research project that sending the R/V "Thompson" to the far western edge of the Juan de Fuca plate, about 300 miles off the Northwest coast.

Some 20 scientists from the National Oceanic and Atmospheric Administration, Oregon State University, left from Seattle on Saturday, and will stay at the area of the quakes until this coming Friday.

The quakes in the swarm were generally between 2.0 and 4.0 in intensity, and the swarm ended this past Saturday. None were felt on land, and none threatened to produce a tsunami, according to Robert Dziak, an oceanographer with Oregon State University and NOAA stationed at OSU's Hatfield Marine Science Center in Newport.

The quakes were located at the far side of the Juan de Fuca tectonic plate, the piece of sub-ocean crust that is slowly diving under the larger North American plate. A site that is feared someday will produce a major earthquake and tsunami is at the other, closer end of the Juan de Fuca plate, at the diving edge known as the Cascadia Subduction Zone. The subduction zone is roughly 50 to 100 miles out from the Northwest coast; the ridge that defines the plate's far edge is as much as 300 miles out.

The recent quake swarm went unfelt in Oregon and Washington, even though it included a few larger quakes, from 4.4 to 4.8 in magnitude - about the same magnitude as two quakes this past summer felt across much of coastal Oregon. Because of the greater distance of these more recent earthquakes they past unnoticed except by the scientists' detection equipment.

"The earthquakes in this swarm are associated with seafloor spreading," said Dziak. "We suspect that magma pushed up into the crust and the lava may have broken the surface." Or, he added, it may not have reached the surface.

Either way, it may have generated a hydrothermal plume - an in-the-water geyser that erupts out of the quake-ruptured seafloor rock into the ocean. If one was created, it would have been rich in the rare microorganisms, that survive on the chemistry and/or heat of sub-seafloor regions heated by the magma.

Dziak is the lead researcher for a team operating the Sound Surveillance System, or SOSUS, out of the Hatfield Marine Science Center. This system of hydrophones located on the ocean floor was originally used during the Cold War to monitor submarines in the north Pacific, but the U.S. Navy has allowed NOAA to make use of them and their data.

During the first 36 hours, SOSUS detected nearly 1,500 small quakes. On Wednesday, the swarm continued with 10 to 30 events per hour. Earthquake activity continued on Thursday with between four and 45 events per hour.

"We sent the ship to the area with equipment to take up sea water from different depths and test for conductivity, temperature, density. It's basically a wire with bottles around it, which we fill at different depths," Dziak said.

Even though any hydrothermal plume will have been finished a few days before the vessel gets to their location, Dziak said, "If a plume was released, it should be detectable; it should not have dissipated that fast."

The lava, he said, is probably "your typical mid-ocean ridge basalts. I'm interested in seeing what happens when something like this happens on the seafloor," Dziak continued.

"Are lavas released, or did they stay beneath the seafloor surface? Will the quakes cause cracks that release a plume? Are there faults propagated along the seafloor?" While the researchers will not see any actual propagating of cracks, they will be able to tell, by remote cameras, if there are recently created fissures and cracks.

All that may give new information about "the character of the quakes," and, perhaps, "how they relate to the eruption" of a plume.

In addition, there could be a large amount of rare microorganisms "entrained in the plume," Dziak said. They come from a unique place, the sub-seafloor biosphere.

The microorganisms live inside the water in cracks and pores of the rock, and stay largely dormant for long time periods. "When the rocks crack and the fluid is released, they become active," he said, invigorated by the chemicals in the magma-driven water, the heat, or both.

Some geologists believe rising pressures on one edge of a tectonic plate can produce a build-up of stress that may eventually prompt a quake at the other end. But Dziak says he disagrees with that.

Even though the Juan de Fuca plate may appear on maps as a single triangular-shaped mass (as large as Oregon and Washington), pressure from rising magma at one side is unlikely, Dziak said, to prompt quakes at the other side. "It's not a one-to-one thing. It doesn't seem to correlate to sudden movements at the other end. The pressure may be taken up by faults in the middle of the plate. Also, some places have slow quakes, ground deformation events that do not snap. Vancouver Island has moved west and then come back again, moved west and then come back again," he said.

Shortly after the swarm ended, Dziak reported, there were two magnitude five quakes south of the swarm, on the edge of another, smaller plate, the Gorda Plate, west of Coos Bay.

When asked if these two were related to the swarm, Dziak replied "no." But it is possible there are other undersea geological links that we as yet dimly perceive.

Earthquakes send out pressure waves that travel through the Earth. "If there are big waves from a quake and they strike a place with a lot of fluid or hydrothermal activity, that could produce quakes in those environments. The 1992 Landers earthquake, on the San Andreas fault in Southern California, produced an increase in earthquakes at all the geothermal sites in the western U.S., at Yellowstone, at Mono Lake in California, at the geysers in another location in central California. That fluid makes the faults slip easier," he said, "and we have seen that sort of fluid in the seafloor."

While most of the scientists on the expedition are from the Northwest - with the Pacific Marine Environmental Laboratories in Newport and Seattle - Jim Cowan, the chief scientist, is from the University of Hawaii. His participation is sponsored by National Science Foundation. Also underwriting the exploration is the NOAA Ocean Exploration Plan and the National Science Foundation.

"It went really quickly," Dziak said of the effort that organized the expedition. "We've done this a few times in the past, so the process was streamlined. And the University of Washington ship, the "Thompson," was at dock. We called the National Science Foundation and the Ocean Exploration program and they said yes in a couple of days. It is one of the most rapid response efforts we've put together."