by MARK ST.J. COUHIG
Sequim Gazette
A 6.4 magnitude earthquake that struck western British Columbia in early September made headlines around the world.
But when another earthquake, one possibly measuring higher on the Moment Magnitude Scale, struck in Sequim last August, no one wrote about it. In fact, only a very few people even noticed it.
There’s a good reason for that: while the B.C. quake rattled the dishes, the Sequim event produced a slow tremor that lasted for about a month. It was only measurable by very delicate instruments called seismometers.
The recent Sequim event was captured by an “array of arrays” stationed on the peninsula by seismologists with the University of Washington’s Earth and Space Science department, which includes the Pacific Northwest Seismic Network.
In cooperation with an alphabet soup of additional agencies, the seismology group at the University of Washington has been running an experiment on the peninsula measuring and locating these so-called “deep tremors” since mid-June 2009.
Recently they wrapped up the experiment and removed the equipment, which included a number of boxes, each containing a data logger, a GPS antenna and a seismometer.
Unusual activity
The peninsula is interesting to seismologists because it undergoes regular occurrences of what is called “Episodic Tremor and Slip” (ETS). Like an ordinary earthquake, an ETS results from two tectonic plates moving against each other, but an ETS usually lasts for a few weeks, rather than one or a few big jolts occurring within a few seconds.
The peninsula’s seismic activity is particularly interesting because for the most part these ETS events occur with a striking regularity — every 14 months.
The peninsula is just one of two places in the world where they are studied extensively — the other is southern Japan.
The array of arrays on the peninsula was created by a team from the University of Washington led by Ken Creager, a professor in the university’s Department of Earth and Space Sciences. Abhijit Ghosh, a research associate and then-doctoral candidate at the University of Washington, helped assemble the array of arrays to provide new insights into the origin of these deep tremors, research he incorporated into his Ph.D. thesis. To determine the source of these tremors, Ghosh devised a new technique, which he calls “multi-beam backprojection,” that provides a means of triangulating on the source, both laterally and vertically.
Ghosh said the array of arrays allowed him to determine to a fair degree of certainty the source of the tremors. Surprisingly, it lies about 20 miles below Sequim.
Ghosh said determining the source of the tremor was an integral part of the recent experiment, one that was very difficult to solve. “A garden-variety earthquake is like a bang,” he said, “and standard methods exist to determine its location.”
But a tremor associated with an ETS, he explained, is like a continuous chatter. Researchers have been trying for the past several years to come up with innovative methods to locate its source precisely.
Raising more questions
The research conducted by Ghosh and his colleagues hasn’t answered all of the questions regarding these events.
As Ghosh’s research animations show, the tremors tend to move about.
The obvious question, “Why do they move?” produced a laugh from Ghosh and from Steve Malone, the now-retired director of the Pacific Northwest Seismic Network. Malone continues to keep his hand in, including working with Ghosh on the peninsula deep tremor project.
The answer, they said, is “We don’t fully understand them.”
“We think what happens is they propagate slowly,” Malone said. They theorize that as the less sticky areas move, creating the slow tremors, they create additional stress nearby that causes further tremors in areas that also were unstable.
That’s not the only question they’re seeking to answer.
Malone noted the latest ETS began in July, cutting short its anticipated 14-month nap by about five months. It ended in early September.
He shrugged. “Mother Nature is just that way,” he said.
(To watch a visual representation of the tremors of a single day — May 7, 2008 — go to vimeo.com/29153665.)
Predicting the Big One
The researchers also are trying to determine the relationship, if any, the tremors have to larger earthquakes, particularly what they call “the big one.”
These “big ones” occur every 200 to 700 years along the Cascadia Fault, Malone explained, causing a magnitude 9 earthquake.
The last big one occurred on Jan. 26, 1700. Scientists know about it because it is part of the oral tradition of the local Native American tribes and because it created a significant tsunami in Japan where written records were kept. “They called it an ‘orphan tsunami,’” Malone explained, “because they didn’t know where it came from.”
The recent research indicates the ETS events are somehow related to the big ones, but that doesn’t mean Sequim will be at the big one’s epicenter. “It will rip from Northern California to Southern B.C.,” Malone said.
The two plates even now are sliding across each other, but it’s not a uniform process. To the west, they’re stuck pretty firmly. To the east, they are sliding silently, probably because the lower plate is softened by the higher temperature at depth, Malone said. Below Sequim they are in almost constant motion, but the movement isn’t going altogether smoothly.
It’s the movement below Sequim that is adding stress to the plates to the west. Or so they hypothesize: “The ETS may give us some idea of when the big one is coming,” Malone said. “We want to know the relationship between the two.”
Reach Mark Couhig at mcouhig@sequimgazette.com.
