Interesting interview with retired University of Washington seismologist, Steve Malone, about volcanoes and earthquakes.
Here are some excerpts I found intriguing. First, here's the definition of a term used frequently in the interview:
Episodic Tremor and Slip (ETS)
Episodic Tremor and Slip (ETS) is the name given to a process that occurs deep below the Earth's surface, along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow sliding, one plate over the other, of a few centimetres over a period of several weeks, accompanied by energetic seismic noise, called tremor. Tremor is distinctly different from the seismic signals generated by earthquakes.
http://thesunbreak.com/2010/05/13/the-northwest-is-earthquake-country-everywhere-you-look
...
As it turned out, we ended up talking across the breadth of Malone's career, covering volcanic earthquakes, megathrust quakes, and the mysterious Episodic Tremor and Slip (ETS). His interest in seismology developed in graduate school; he'd studied physics as undergraduate, but was not "keen on modern physics" and gravitated toward geology, then seismology. At first he enjoyed the outdoors aspects of the work, "particularly in the volcano world," but as his research progressed, he ended up in the office running computer models.
...
"The ETS we have here are fairly regular, every 14 and a half months or so. The next one is scheduled--if you can call it that--for sometime mid-summer, maybe mid-August. We've got some experiments we're planning to gear up and get extra equipment in the ground for. There's some extra equipment out there now, running in background mode, but when the ETS arrives we'll double the equipment, trying to look ever closer at it."
...
"Whether earthquakes are due to these large-scale tectonic forces, plate motions, or whether they're due to magma rising within the crust, it's rock breaking," emphasized Malone. "They look a little different on volcanoes, and usually you don't have such big earthquakes--the faults, or weaknesses, are concentrated in the volcano itself, but in a few cases you can get moderate-sized earthquakes." It's the why of the rock-breaking that provides material for a lifetime of study.
As a student of Pacific Northwest volcanic history, he rolled his eyes a little at Iceland's Eyjafjallajokull volcano. "Oceanic volcanoes are going off all the time, and Iceland is nothing more than an oceanic volcano which happens to be where the ocean is very shallow, like, non-existent. But it's part of the oceanic rift system, which goes down through the Atlantic, more or less in the middle. We have one off our coast here, called the Juan de Fuca Ridge, 400 or 500 kilometers off our coast, same thing: There's eruptions going on out there fairly frequently."
For Malone, the big tectonophysics excitement centers on sneaky episodes of tremor and slip (ETS), which have only been discovered in the last ten years or so: "The slip is slow, it takes place over the course of days to weeks, rather than an earthquake. An earthquake takes place over the course of seconds, tens of seconds, maybe a few minutes for the really biggest." Besides deformation in the ground, measurable via GPS, seismographic listening devices pick up an accompanying tremor that has scientists puzzled. What they do know is that slow or not, something major is going on.
The instances of ETS that the Pacific Northwest sees are mostly thanks to a major oceanic plate, the Juan de Fuca Plate, subducting under North America. Except, as Malone put it, "It's stuck. It's stuck along the Cascadia Fault Zone, which is just off our coast. Suddenly it's going to let loose, and that's when you get these megathrust earthquakes, magnitude 8 to 9 events." In the meantime, if you put out GPS instruments around western Washington, you can watch them move toward the east or northeast--those on the coast more than inland, Malone told me, as the earth is compressed in the collision between plates.
But approximately every 14 months, your instruments will move back toward the ocean. "Maybe over the course of 14 months it moves four-and-a-half centimeters toward the northeast, but then it moves back again four or five millimeters over the course of a couple weeks," said Malone.
What fascinates seismologists is that this kind of ETS event, in its slow-motion way, unleashes energy equivalent to a 6.5 magnitude quake or greater. It just does it with such nuance that no one noticed until recently. "During the height of one of these ETS events, you may have tremor going on for 24 hours a day," Malone said. "It's probably been going on for decades, centuries, but it's only in the last decade, since about 1999, where it was for seen in Japan and reported in the literature, that we started looking for it here" (because of our subduction zone quirks*).
...
Capturing the slow movements of ETS requires the use of geodetic stations, which use GPS to measure the distances between stations to within a few millimeters over hundreds of kilometers. The University of Washington partners with Central Washington University for this part. "You put them in the ground and fix them in really solid so they're only going to move when that part of the earth moves," said Malone. "Then you remeasure them over months to many years, and you see deformation. You see the earth actually changing shape, at least that part." For the upcoming ETS experience in August, Malone and his cohorts at Pacific Northwest Seismic Network are setting up what they call an Array of Arrays.
Malone taking a measurement from a portable GPS device.
Currently they have 80 instruments (eight arrays with ten stations per array) placed over a one-kiometer area, which yields "pretty good" resolution of the tremor waves. But they plan to double their listening resolution to 20 stations this time around. The portable devices are about the size of a large beer can, Malone told me. "You can carry half dozen in your backpack." They're placed mostly on forest service land or private timber company land. While the ideal is flat topography, the researchers also have to avoid human habitation, which comes with all sorts of earth-shaking impacts of its own.
...
"We don't know whether the ETS is taking away some of the energy build-up, or maybe it's just concentrating it in a different place. Maybe you're more likely to have a quake during an ETS period. But at the same time, earthquakes on the subduction zone will rip the whole thing, from northern California to southern British Columbia. And we only see the ETS-- the one we're looking at anyway--in this part."
There are, in fact, other ETS events in Oregon and into northern California, but, Malone said, they seem to have a different schedule. "They don't occur at the same time as ours, and their regularity may be different. It may be 11 months, or 22 months. If you look around, someplace on the subduction zone one of these ETS is going on at least half the time, if not more. So who's to say it's not the one in California that could trigger the megathrust earthquake, as opposed to ours."
One of the reasons he's fascinated by ETS, admitted Malone, is that he has seen seismic signals on volcanoes that are very similar to ETS. "It's called volcanic tremor, and it's a more or less continuous, low-level shaking that has to do with the motion of fluids within the volcano: maybe magma, maybe water, maybe steam, maybe gases. So the generation of tremor in volcanoes may be able to give us a clue or a hint. They're really different--sometimes we call this non-volcanic tremor, but maybe there's something about the mechanism that has similarities to volcanoes…."
...
"What we think ETS represents is actual slip on part of the locked zone," he said. "It's stuck, but some place between the seriously stuck part and that part which is much deeper, which is always slipping a little bit, somewhere in there, there must be a transition zone. And this transition zone is stuck a lot of the time, but then for a few weeks it slides, or at least part of it slides. That is the slow slip. It doesn't take place like an earthquake--it doesn't take place rapidly, KABOOM! It takes place over a period of many days, maybe as much as three weeks.
...
Right now, you can watch for ETS online, at PNSN's interactive tremor map,
http://www.pnsn.org/tremor/ and I can also recommend their Recent Quakes page
http://www.pnsn.org/recenteqs/latest.htm. But don't let the slow-motion stuff fool you. Big and small shakes are a way of life in the Northwest. Each year, over 1000 earthquakes with magnitude 1.0 or greater occur in Washington and Oregon, says PNSN's site, which also shares this mind-bending factoid:
Vertical motion on the Seattle Fault and faults just to the north have caused the block of earth between faults H and I on the map to drop a total of more than 12,000 feet in the last 40 million years.
...
"There are some curiosities because there have been tremor bursts or periods of it--though it's still not well documented--in Alaska. In Alaska, it's the Pacific plate descending that is pretty old, not as old as in Japan, but still way older than here. Tremor has also been seen on the San Andreas Fault, but it's very difficult to spot there--it's quite weak and rare in comparison to these warm subduction zones. It's taking place in only a few places that people have been able to detect it…barely: the central part of the San Andreas Fault, central California, an area called Parkfield. Even there it's not occurring on the breaking part of the San Andreas, it's occurring on the part of the San Andreas that's really down deep, where it's warmer. The earthquakes themselves don't propagate that deep, but there must be some slippage going on, something between totally brittle and ductile deformation."
