Small earthquakes shift views

For decades, the "doomsday" fault off the coast of Washington and Oregon has appeared to be locked tight, building up stress that scientists say will one day be unleashed in a monster earthquake and tsunami.

Now researchers from Oregon State University (OSU) have discovered that portions of the fault have actually been slipping, leading to a series of small earthquakes over the past several years — including one Thursday.

The finding doesn't mean a major quake is more — or less — likely to strike anytime soon. It does provide a new window through which to study the fault and understand what it might do next.

"We may not fully understand what it means, but if we can figure out why the fault is creaking here, we may be a step closer to forecasting earthquakes," said OSU geophysicist Anne Tréhu, lead author of a study in the journal Geology, published by the Geological Society of America.

The researchers also found some of the small quakes off the coast were preceded six months earlier by "silent" earthquakes deeper underground, which some experts believe might trigger surface quakes.

But the link isn't clear, and more study is needed, Tréhu said.

She and her colleagues used detailed new geological data to pin down the origins of three clusters of earthquakes off the Oregon coast in 2004 and 2007. The two largest quakes, of magnitude 4.8 and 4.9, were felt on the mainland but caused no damage.

The quakes clustered in two spots on the Cascadia subduction zone, the 600-mile-long fault where the ocean floor is being forced under the continental plate. Earlier analyses had suggested the quakes occurred beneath the boundary between the two plates, but Tréhu's data show that the quakes occurred right where the plates meet, an area long thought to be stuck.

No other quakes in modern history have been traced to the "locked" section of the fault.

"The Cascadia subduction zone has been frustrating to study because it's so quiet," Tréhu said. "It's like somebody who never talks to you."

The fault has spoken loudly before, though, with as many as 23 major earthquakes spread over the past 10,000 years, according to OSU scientist Chris Goldfinger. The most recent, estimated at magnitude 9, set off a tsunami that destroyed villages in Japan in 1700.

The tsunami that killed more than 225,000 people around the Indian Ocean three years ago was triggered by an earthquake of similar size, on a similar fault.

"When you come to Cascadia, you know this thing is going to rip one of these years," said Tim Melbourne, a geophysicist at Central Washington University who was not part of the new study. "And we've got millions of people living along the coast."

Melbourne was among the first to recognize the existence of silent earthquakes, which occur regularly along the Cascadia subduction zone, but much deeper underground than conventional earthquakes.

Also called "episodic tremor and slip," silent quakes don't shake the ground in a noticeable way but create tiny tremors that can persist for days. The theory is that they occur when geologic plates slip past each other at great depths, where the rock is more plastic than near the surface.

But when the plates move deep underground, that puts more stress on the portion of the fault near the surface that is most likely to rupture in a major subduction-zone quake. That's why scientists think silent quakes may someday help predict the real thing, Melbourne said: "There's every reason to think they're connected at some level."

U.S. and Canadian scientists and emergency-management officials will meet in Victoria, B.C., next month to discuss silent quakes and whether they may be harbingers of more destructive events, Tréhu said.

Melbourne manages a regional network of GPS sensors that measure gradual land motions caused by the movement of geologic plates. Those measurements can detect silent quakes and estimate the amount of strain building up in geologic faults — which could have predictive value.

Now that the OSU group has shown that earthquakes are occurring in the Cascadia subduction zone, scientists will also be able to zero in on those segments of the plate that appear to be slipping.

"It's creaking in these two spots, and it behooves us to listen and figure out why," Tréhu said.

She and her team have deployed 15 seismometers on the ocean floor and are setting up several on land to gather more detailed information the next time the fault slips.

On Thursday, after spending a snowy day in the mountains digging holes for the instruments, Tréhu checked the seismic network and saw a tiny quake had occurred near the others she studied.

"It's kind of intriguing that there was another event," she said. "It continues to be active, and we're listening."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com