Scientists Find Cause of 2008 Earthquake Swarm

JEFF BARNARD,
AP Environmental Writer

Scientists mapping the seafloor off Oregon found a surprising sight — 20-foot vertical upheavals they had never known existed in the middle of the Juan de Fuca Plate, where some odd swarms of earthquakes were detected last year.

Oregon State University marine geologist Robert Dziak said in a statement Monday that the discovery about 140 miles off Newport, Ore., suggests the Juan de Fuca Plate is still actively converging with North America in the Cascadia Subduction Zone.

The subduction zone, where the earth's crust beneath the Pacific slides under North America, is widely expected to someday produce a huge and destructive earthquake and tsunami.

Shortly after the quakes that started in March 2008, OSU scientists went out in a research vessel and took water samples to test for volcanic activity.

Then last September, another research vessel mapping the seafloor with multi-beam sonar returned to the area, and discovered the upheavals, known as scarps.

Jochen Braunmiller, a research associate at OSU's College of Ocean and Atmospheric Sciences, said the earthquakes that started in March 2008 were not big enough to produce the scarps, which had some sediment on them suggesting they were not new. Onshore instruments measured most of the 1,600 quakes at between magnitude 3.0 and 4.0, with a few exceeding magnitude 5.0. None of the quakes were felt on shore.

What was odd about the swarm, detected with a network of underwater microphones originally installed to hear submarines, was that it was located in the interior of the Juan de Fuca Plate, not on the edges, Braunmiller said.

A few weeks after the initial swarm in the Juan de Fuca Plate, more earthquakes occurred along the Blanco Transform Fault, a boundary between plates, and a few weeks later another swarm was detected along the North Gorda Ridge, Braunmiller said.

The quakes occurred in a short period of time across a large area, raising questions about whether they were related, or a coincidence, Braunmiller said.

Dziak added that they do not know what triggered the quakes, and it is not clear if they represent a normal release of stress within the plate, or something else.