The Oregon earthquake swarm

I live in western Washington, an area not as well-known as California for being earthquake-prone but which is nevertheless earthquake territory. That's because we're right on the edge of the North American tectonic plate, which runs almost directly up the western coast of North America. When you think about the fault lines created by this boundary, you might think first of arguably the most famous fault line in the US – the San Andreas fault. The San Andreas fault stretches approximately 800 miles up the coast of California; here, the passage of the North American plate (which moves southeast) and the Pacific plate (which moves northwest) generates the most memorable earthquakes of the region. This includes the devastating 1906 San Francisco quake, which was an estimated 7.8 on the Richter scale and responsible for some 3000 deaths.

Now, the San Andreas does not extend up into Washington state. However, being at the edge of the North American tectonic plate means that we also have fault lines capable of generating substantial earthquakes in the area. For us, however, the danger comes primarily from where the North American plate meets the Juan de Fuca plate. The Juan de Fuca plate is very small, extending from the southern border of Oregon to British Columbia, Canada; it was once part of a much large plate (the Farallon plate) that has largely subducted (meaning sunk) underneath the North American plate. There are three remnants of the Farallon plate still in existence - the Juan de Fuca plate off of the Washington coast, the Cocos plate off of Central America, and the Nazca plate along the western edge of South America. Subduction of the Juan de Fuca plate is responsible for the formation of the Cascade mountains which includes two volcanoes that you might be familiar with - Mount St. Helens and Mount Rainier.

The last major earthquake to occur off a fault from the Juan de Fuca plate was around 1700, with a magnitude of somewhere around 9.0 on the Richter scale. However, in the last 2 weeks, scientists have detected a rash of earthquakes off the Oregon coast – over 600 of them, up to a magnitude of 5.4. But here’s the odd thing – these earthquakes do not appear to be coming from any of the fault lines from the Juan de Fuca plate. Instead, they are centered in the middle of the plate – about 40 miles from its edge.

There are actually several odd things about these earthquakes. First, of course, is that they do not correspond to a fault line. But second, they do not follow the typical earthquake swarm pattern of a major shock followed by steadily decreasing aftershocks. It has been a steady stream of earthquakes of mostly equal size. That means this is unlikely to be caused by a fault internal to the plate itself. In fact, scientists really have no firm idea what’s causing these tremors to occur.

One of the major researchers studying this swarm is Dr. Robert Dziak, marine geologist from Oregon State University. Though he says that nothing like this has been detected in this region ever since monitoring has begun, he has a couple of suggestions as to what’s causing the swarm. It’s possible that a new fault is opening in the middle of the Juann de Fuca plate. Or it could be that the entire plate is under stress, being squeezed by the plates around it, which could cause it to crumple a little in the middle. Or there could be new volcano activity in the area immediately beneath this spot, injecting new magma into the middle of the plate and pushing the plate much faster than it has previously moved.

I should mention that these earthquakes do not appear to be any danger to the inhabitants of the Oregon coast. The quakes are too far away, too deep and too small to even be felt on the mainland. They are detectable by a system of hydrophones set up on the ocean floor – which were actually originally set up to detect submarine activity off the Pacific coast during the cold war. But they’ve been put to a more peaceful use as of late.