Minor 2.8 earthquake rattles Tiger Mountain neighborhood
August 23, 2011
By Warren Kagarise
The ground beneath Tiger Mountain rumbled early Aug. 18, as a minor earthquake rattled the Mirrormont neighborhood.
Seismologists recorded a magnitude-2.8 earthquake just before 1 a.m. about a mile beneath the surface.
Such small earthquakes occur often. Washington experiences more than 1,000 tremors each year, although most temblors do not cause damage or even receive much notice from residents.
“Whenever there’s an earthquake it slightly raises the odds that we’ll see more earthquakes,” said John Vidale, Pacific Northwest Seismic Network director and a University of Washington seismologist. “We’re not exactly sure if that’s because earthquakes are a sign of things going on or if that’s because the earthquakes trigger other earthquakes.”
The initial report from the Pacific Northwest Seismic Network, based at the UW, pegged the Mirrormont temblor as magnitude 2.3 and deeper in the earth. The magnitude is a measure of earthquake size calculated from ground motion.
Pinpointing earthquakes’ magnitude and depth can present a challenge to seismologists, especially if no instruments to measure ground motion, such as seismometers, exist near the epicenter. The team recorded the Mirrormont earthquake using seismometers near Pine Lake and in Newcastle.
“Essentially, we don’t really know within a few-tenths of a magnitude how big things are, and it can depend on which way we’re measuring,” Vidale said. “It’s kind of an approximate number.”
The minor earthquake reminded residents about the seismic risks present along the Seattle Fault, a shallow fault stretched along Interstate 90 from Puget Sound and east through lakes Washington and Sammamish.
Issaquah is also near the Rattlesnake Mountain and South Whidbey Island fault zones.
Seismologists recorded a magnitude-2.6 earthquake in Mirrormont just before 10 p.m. Sept. 3, 2010. The temblor jolted residents — although some people thought a large tree toppled and caused the ground to shake.
“It’s probably not a coincidence that that area is active,” Vidale said. “It doesn’t mean much in terms of whether the next big fault will be there or somewhere else. There are persistent swarms of earthquakes in various places, so I’d guess it’s probably not a coincidence.”
Issaquah and the surrounding area experienced other micro-earthquakes last summer, a magnitude-1.6 temblor in July 2010 and a magnitude-1.1 tremor in June 2010.
The last major earthquake to occur in the Puget Sound region came a decade ago during the magnitude-6.8 Nisqually earthquake.
Scientists at the Pacific Northwest Seismic Network and the U.S. Geological Survey continue to develop a regional early warning system for earthquakes.
“One of our current efforts is to detect earthquakes, interpret them and send out the information fast enough to get the information to people before the shaking arrives,” Vidale said. “It’s not going to work for earthquakes five kilometers away where you only have half of a second to work with.”
If a temblor occurs along the Washington coast, however, seismologists could use the moments before the shaking starts in Greater Seattle to alert people to the coming seismic activity.
But efforts to predict the location, magnitude and timing of tremors remain beyond seismologists’ grasp.
“We would love to predict earthquakes, but that’s just not working at all,” Vidale said. “The only clue is that earthquakes tend to follow an earthquake. That’s not very useful at all.”
Warren Kagarise: 392-6434, ext. 234, or firstname.lastname@example.org. Comment at www.issaquahpress.com.