Image credit: Screenshot of Pacific Northwest Seismic Network website by Cujo359
Out along U.S. Highway 2 in western Washington is a pair of little towns called Gold Bar and May Creek. Neither is much more than a wide spot in the road - there are a couple of restaurants and a gas station, and a grocery store, if I remember correctly.
But something weird has been going on out there recently, as the Pacific Northwest Seismic Network (PNSN) noted today in its blog:
Though the residents of Gold Bar may not have noticed, tiny earthquakes have been popping off fairly regularly just a few kilometers east of town (and 11 kilometers below it) over the past few months. A total of about 300 quakes have occurred here since October, though only 100 of these were big enough to make it into the PNSN catalog, the rest were too small to show up on enough seismic stations.
Tremors Under Gold Bar
I'd noticed this before, since I check the PNSN earthquakes site fairly often. Hardly a week has gone by in the last few months without one of those little quakes. They really are small quakes, too, most less than 2.0 magnitude, so almost no one living there has noticed them. The screenshot at the top of the article is one that I took of it back in January.
There is no volcano in that area that I know of. The quakes bear a superficial resemblance to those that occur around Mt. St. Helens and Mt. Rainier, but those quakes don't occur around such a small area for so long.
There's also an old mine in the area, according to the OpenCycleMaps data, but the tremors were west of it, and I doubt that mine went 11 kilometers (roughly six miles) beneath the surface, so subsidence or any other form of mine collapse seemed an unlikely explanation, too.
I was about to ask the PNSN folks what they thought, but it turns out they don't seem to know, either:
[graphic from original article. Click on it to see it full size.]
When we look at patterns of event occurrence using this more complete dataset (below), we see that these little quakes have been popping off nearly continuously since October, with some sporadic bursts of activity, particularly in December.
A high resolution relocation of the events relative to each other using a method called “double difference relocation” shows that the earthquakes fall more or less on a oval-shaped patch ~1.5 km tall and ~0.8 km wide that dips steeply, about 70°, toward the southwest. There is no obvious migration in the locations over time to suggest fluid migration, sometimes a cause of swarm-like seismicity, so what is it? Perhaps Hollywood has some answers?
Tremors Under Gold Bar
What they seem to be describing is some sort of steep slippery slope about six miles underground, but nothing seems to be headed anywhere.
If they're looking for answers in Hollywood, I'd say no one has any real idea yet.
Whatever it is, it looks like it will be a mystery for a while longer.