It's got to be nerve-wracking to be a seismologist monitoring the Cascadia fault. Running perilously close to the major cities of Seattle, Vancouver, Portland, and several other populations centers in the Pacific Northwest, it's easily the most dangerous fault in North America. Lined with state-of-the art sensors, scientists can watch its every move.
And move it does. Every 15 months or so, it slips — ever so slightly — in an event scientists call "Episodic Tremor and Slip" or ETS. It's just about equivalent to seeing the rise and fall of a sleeping dragon's chest, and hoping with every breath that it doesn't wake up.
In this recent simulation, the Washington State Department of Transportation showed how Seattle's waterfront highway and docks would be devastated by a magnitude 7.0 earthquake:
Trouble is, scientists expect the Cascadia megathrust could pop off a magnitude 9.0 quake, which would be 1,000 times more powerful than the 7.0. That pancaked freeway was pretty scary to watch, but when the big one hits, it might be the least of the city's problems.
ETS is a key part of understanding the mighty Cascadia monster, and since it was discovered several years ago, scientists have learned a lot about it. For one, they know it's deep, occurring between 30 and 45 kilometers down in the crust. Closer to the surface the fault stays locked. So every time seismometers record an ETS at depth, stress is loading up on the locked portion of the fault just a little more, raising the risk that the big one could come at any moment.
Second, tremors during an ETS event shimmy up the fault from south to north. Why this happens has been kind of a mystery, but scientists writing in the journal Geophysical Research Letters think they've figured out this quirk of the giant fault. Each tremor transfers stress a little further north along the fault, causing a chain reaction of tremors, like a zipper unzipping.
The researchers note that this pattern of chain reaction is very similar to how big, damaging "fast" earthquakes go from small to enormous in the span of a few seconds.
Of course, no one knows what Cascadia's next big quake is going to look like, or if it will mimic the patterns the researchers are seeing in ETS tremors.
But these observations could be seen as a sort of slow-motion enactment of the real thing — something startling to behold, but nothing to worry about. It's just the dragon rolling over in its sleep.