Peninsula rocks out every 14 months

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Sequim Gazette

There are earthquakes — and then there are earthquakes.


In recent weeks the world has witnessed the devastating impact of the 9.0 earthquake that left thousands dead or missing in Japan.


And then there are tiny earthquakes, like the 2.0 that occurred in Sequim last week.


Less well known are “Episodic Tremor and Slip” (ETS) earthquakes. One of these, measuring about 6.5 on the Richter scale, rocks the entire Olympic Peninsula every 14 months or so.


Until very recently, no one was aware of these events.


That’s because an ETS moves very, very slowly. As a result, extraordinarily sensitive equipment is required even to register one of these ETS earthquakes.


In one sense, an ETS is exactly the same as any other earthquake — it involves one tectonic plate moving over another. But an ordinary earthquake takes seconds, maybe minutes. An ETS, on the other hand, usually takes a few weeks.


The peninsula ETSs aren’t insignificant — if the same amount of action took place quickly, the china in your home definitely would rattle.


Instead the only thing that happens — the only way to register the event, including its duration and magnitude — is the formation of tiny waves, called “seiche,” in two long pipes at the Sequim airport and at two other locations on the peninsula.


Andy Sallee, who runs the airport, says the ETS-detecting apparatus consists of little more than “pipes with water and wire.”


A computer in the back room of the airport office gathers the collected data and forwards it to Central Washington University where it’s integrated with other data to aid in the pursuit of a number of research objectives.


Sallee says the airport receives no funding for participating, “but it’s harmless and a neat thing for science.


“Earthquakes can be a big deal here,” Sallee said. “We were glad to contribute.”

Measure by measure

The device, called a “tiltmeter,” consists of two long pipes half filled with water and affixed at nearly a right angle to each other. Where they meet, the pipes hold sensors that measure the changes in the water level, using a solid pier monument as a reference. It’s one of three similar units that make up the “Pacific Northwest Geodetic Array” and together plot out the frequency, duration and strength of each ETS.


Rex Flake, the project’s managing geologist, says these devices are very different from the standard seismometers that most laymen are familiar with. Those measure “high frequency waves,” he said, including the 2.0 earthquake recently noted in Sequim and the 9.0 in Japan.


While the tiltmeter registers those earthquakes, the scientists running the project remove those high-frequency anomalies to better chart the low-frequency waves.


Flake compared tremors to the electromagnetic spectrum. “Our eyes only see a certain length of the spectrum. This instrument lets us see what seismometers can’t see,” he said.


The research is useful for a number of reasons, including the ongoing effort to improve Global Satellite Positioning data. Flake says GPS is great for lateral positioning, but “terrible” for height. The data also is useful when combined with other data, including that produced by seismographs, in determining why and when earthquakes occur. This “helps us understand these plates to better understand stress accumulation and to get a better handle on the mega-thrust dynamics,” Flake said. The eventual goal of all this work — which Flake admits is far off — is the ability to predict earthquakes.


The ETSs that occur on the peninsula also interest science because unlike most tectonic activities, they occur on a fairly regular basis — every 14 months or so.

Highly sensitive information

The tiltmeter measures just that — the tilt of the pipe. “One end goes up and the other goes down,” Flake said.


Flake noted the “earth is elastic,” and as such is subject to constant influences — even the rumbling of a big truck. Flake and his colleagues look for “static displacement” — permanent movements of the land. That’s “when the earth doesn’t bounce back,” Flake said.


The equipment is astonishingly sensitive, measuring changes in the angle of tilt of less than one-half of a millionth of a degree.


Flake provided an illustration, saying “if you took a straight-edge ruler and laid it from San Francisco to New York, then raised the New York end by one-half of a millionth of a degree, it would rise by the thickness of a nickel.”


Reach Mark Couhig at



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