Minor earthquake reminds residents of seismic risks
October 12, 2010
By Warren Kagarise
The ground beneath the Mirrormont neighborhood rattled late Sept. 3, but residents attributed the slight rumbling to everyday occurrences — not to the magnitude 2.6 earthquake shaking the community south of Issaquah.
“At first, I thought it was a large animal rattling my garage door,” Mirrormont resident Monique Blackwell said.
Jennifer Orr, another Mirrormont resident, said she thought a large tree had toppled nearby.
“It felt as though a large truck had hit something outside,” Ray Skoff recalled.
The temblor lasted only a handful of seconds, but long enough to cause speculation before residents realized the source: earthquake.
“It felt like wood and concrete had no solidity for a minute,” Karen Skoff said.
The earthquake followed tiny temblors in the Issaquah area — a magnitude 1.6 quake July 3 and a magnitude 1.1 quake June 9.
Tim Walsh, chief hazards geologist for the state Department of Natural Resources, said the micro-earthquakes do not mean a big roller is imminent.
“Worldwide, 10 to 15 to 20 percent of major earthquakes are preceded by foreshocks, that means that most of them aren’t,” he said. “You can never be sure whether these small earthquakes are precursors to something larger or not.”
Though residents across the Issaquah area — including homeowners along Issaquah-Hobart Road Southeast and Southeast May Valley Road — felt the Sept. 3 quake, the incident did not cause any damage or injuries.
“It ever so slightly raises the chances of an earthquake, but until it’s a magnitude 4 or 5, the change is just miniscule,” John Vidale, a University of Washington seismologist and director of the Pacific Northwest Seismic Network, said after the Mirrormont temblor.
Minor earthquakes tend to cause little damage in communities built to modern construction practices and stronger building codes.
“In areas that are suburban, you can probably shake them as much as you like and, probably, nothing will happen,” Vidale said.
Issaquah rests along the Seattle Fault, a shallow zone stretched along Interstate 90 from Puget Sound and east through lakes Washington and Sammamish. The city is also near the Rattlesnake Mountain and South Whidbey Island fault zones.
In a 2008 report, U.S. Geological Survey seismologist Craig Weaver estimated a magnitude 6.7 earthquake along the Seattle Fault could cause $33 billion in damage, hobble I-90 and other interstates, threaten drinking water supplies and cripple the region for years.
The study included a 1970s-era Issaquah shopping center to illustrate potential damage. Though the buildings continue to stand and roads remain usable, business suffers as shops close for inspections and repairs drag.
The state Department of Natural Resources released maps detailing the extension of the South Whidbey Island Fault to East King County last September.
“What we know is that they all seem to come together kind of a little bit southeast of Fall City,” Walsh said. “It’s all so munged up there that it’s hard to tell what’s really going on.”
Temblors could also trigger landslides and collapses of abandoned coalmines in the Issaquah Alps. In addition to the threat presented by ground shaking, liquefaction is also a concern in the city. Liquefaction occurs after loose soil becomes saturated, and loses strength and stability, due to ground shaking.
In February 2001, the Nisqually earthquake caused some liquefaction of the Issaquah Creek delta into Lake Sammamish.
The potential also exists for tsunamis in Lake Sammamish. Geologists believe the Seattle Fault runs through the southern portion of the lake. Or a quake could cause water to slosh around inside the lake basin and create a seiche, or standing wave.
Despite the inherent seismic risks of the region, Walsh said most Washington earthquakes do not release “more energy than a big truck going by,” and therefore do not present a threat.
“We get about 3,000 measured earthquakes a year,” he continued. “If you think about that, that’s eight a day.”
Warren Kagarise: 392-6434, ext. 234, or firstname.lastname@example.org. Comment at www.issaquahpress.com.