Two New Quake Faults Found in Seattle

A federal scientist has come up with more evidence that Washington state is major earthquake country.

Samuel Johnson, a research geologist with the U.S. Geological Survey in Colorado, says he has discovered two new faults under Puget Sound capable of generating earthquakes of magnitude 6.5 or greater.

And another previously known fault that runs under Seattle--the Seattle Fault--is moving faster than scientists had estimated, Johnson said. That means this fault could be more dangerous than previously believed.

Scientists can’t predict with precision when the next quake will occur. But Johnson said the new findings mean the probability of a major earthquake hitting the region in, say, the next 50 years, should go up.

“I think it increases the risk, maybe significantly,” he said.

Johnson presented his findings at a news conference Feb. 11 at the University of Washington, where he once taught, and later at a meeting of the Northwest Geological Society.

Johnson’s research is preliminary and hasn’t been published yet. That means scientists haven’t had a chance to review his data.

Bill Steele, seismology lab coordinator at the University of Washington, called Johnson’s reported findings “exciting.” He said the more information scientists have, the better they can predict the probability of a major quake occurring in a certain region within a specific time frame.

Johnson said he began finding evidence of the two new faults in 1995. That’s when he and other scientists sailed Puget Sound in a 120-foot boat, the Robert Gray, and fired sound from an air gun at the top layer of rock under the sound.

By mapping the returning echoes, he found that the east-west Seattle Fault that runs under Seattle and across Puget Sound is interrupted by two new north-south faults. These two faults, which run parallel to each other and are several kilometers apart, extend beneath Puget Sound from the southern tip of Whidbey Island to Vashon Island and possibly all the way to Tacoma, Johnson said.

The two newly discovered faults are “strike-slip” faults, producing quakes in which two blocks of earth move horizontally. In contrast, the Seattle Fault is a “thrust” fault, producing quakes that result from vertical motion of two adjacent blocks.

The magnitude 3.5 quake that struck the Seattle area Feb. 10 is believed to have originated from the Seattle Fault. That fault is thought to have generated a devastating earthquake about 1,100 years ago that jacked up land masses, spawned a huge tidal wave and sent forests sliding to the bottom of Lake Washington.

“Beyond the Seattle Fault there is now a new potential for major earthquakes,” Johnson said.

“I think we have to acknowledge the fact that beyond the Seattle Fault, there is now an independent earthquake source in the Puget Sound area. These structures could rupture and generate earthquakes just as the Seattle Fault could rupture and generate earthquakes,” he said.

Many small quakes have been recorded along these new faults in the last 25 years, Johnson added. He said researchers will do more surveys this summer to try to map these faults more thoroughly.

More than 1,000 earthquakes are recorded in Washington each year, but only 15 to 20 of these temblors are strong enough to be felt. The last earthquake to cause widespread damage in the state occurred in 1965.