Beginning in late March of 2008, a series of earthquakes shook the ocean floor off the Oregon coast. Over the next two months, researchers would observe some 1600 quakes. The first intense period, or swarm, lasted about ten days. That was followed by three more clusters. Most of the quakes were measured between magnitude 3.0 and 4.0. Some topped magnitude 5.0.
Had you been onshore at the time, you wouldn’t have noticed a thing. The quakes were too deep and too far away to be felt on land. But even though they were harmless to people, the quakes were strange enough to make international news.
Scientists at Oregon State University now think they’ve figure out what happened.
Earthquake swarms off the Oregon coast are not that unusual by themselves. There have been eight recorded swarms over the past dozen years. But the first swarm of last Spring was different because it happened inside the Juan de Fuca plate. Most quake activity in this area happens along the plate boundary.
The Earthquake Detectives
When the first swarm was detected, OSU dispatched the research vessel Wecoma to the quake area to take water samples and look for signs of volcanic activity on the ocean floor. Swarms are often caused by volcanic activity, so in a sense the scientists were looking for the “usual suspect”.
Then in September of that year, other scientists returned on the research vessel Melville. Using a multi-beam sonar, they remapped the ocean bottom. And that’s when they discovered a new system of earthquake faults. OSU researcher Susan Merle was aboard the trip and recalls seeing a 20-meter (or about 60 feet) displacement of the seafloor.
“That’s a pretty big fault,” says OSU Marine Geologist Bob Dziak. He says it shows that the Juan de Fuca plate is “being squeezed” by the Pacific plate to the west and the continental plate to the east. “It isn’t clear if the swarms that occurred in 2008 represent normal stress release within the plate, or if they are from deformation related to the Cascadia Subduction Zone. We simply don’t yet know.”
As For Those Other Swarms
The following swarms occured along plate boundaries, areas that are more likely to see quake activity. This follow up activity was, in some ways, even more intense than the first swarm. In an area called the Gorda Ridge, scientists recorded more than 1000 quakes in just five days.
One of the questions the scientists are trying to answer is, are the two events related?
Dziak seems to think they are. ”But,” he says, “we don’t yet completely understand how they are related and what triggers the sequence. But it is interesting that the stress release within the plate could trigger swarms of earthquakes on the plate boundaries.”
