Quakes stress seismic threat

A string of minor earthquakes in Spokane and northeast Washington in recent months has renewed concern about seismic hazards facing the region east of the Cascade Mountains, including North Idaho.

New research underscores a long-held suspicion by scientists that large areas of the Inland Northwest are vulnerable to damaging earthquakes even though seismic activity here is relatively low compared to the Pacific coast. Those same scientists are proposing new high-tech studies to assess the potential hazards.

The latest quakes in north Spokane and in southern Stevens County over the past two months were so small that none was reported as being felt. They turned up on an improved network of seismographs covering the Western U.S., including a station at a farm in Colbert north of Spokane.

A “swarm” of minor earthquakes in 2001 and 2002 rattled north Spokane and the downtown area from depths so close to the surface that seismologists warned that a quake of one magnitude stronger could cause serious damage, particularly to the city’s stock of older, non-reinforced brick and masonry buildings.

Similar quakes have occurred periodically in Spokane, while larger quakes have been documented in the region since 1872. A magnitude 5.5 quake occurred in 1942 north of Coeur d’Alene, an event that shows the potential for stronger tremors in the region.

“Spokane was lucky you didn’t get a big earthquake, say a (magnitude) 5,” Craig Weaver, seismologist with the U.S. Geological Survey, said about the 2001-2002 earthquake swarm.

“I think this is a more important issue than Spokane might realize,” he said.

Spokane had been thought to be a low-hazard area until a 3.7 tremor awakened the city to its earthquake potential on June 25, 2001. It was the first of more than 50 earthquakes recorded over the next 10 months, the largest of which reached magnitude 4.0.

A one-point increase in magnitude on the scale is actually a 10-fold increase in amplitude, so a 5.0 quake is ten times larger than a 4.0 quake.

“The fact that it was right there in town was pretty dramatic,” said University of Washington seismology professor Steve Malone. “What does that mean for the future?”

Scientists really don’t know, but they would like to get a better idea of the region’s earthquake features hidden along the ground.

New technologies such as aerial laser mapping and global positioning are helping scientists better understand earth movements.

Historically in the Inland Northwest, seismic activity has been concentrated in three or four areas: Walla Walla and Milton-Freewater, Ore., where Columbia Basin geology bumps into the Blue Mountains; south-central Washington where a “fold and thrust belt” of rock layers occurs; and in the Lake Chelan area where newer volcanic basalt rock meets older types of rock at the north edge of the fold-and-thrust belt.

Spokane is also located at the northern edge of the volcanic basalts, making it a boundary area where earthquake hazards are typically found.

On the North Side of Spokane, four earthquakes in September at magnitudes of 1.5 to 2.3 occurred in a cluster along the Little Spokane River. In addition, a cluster of three similarly small quakes occurred earlier this month in southern Stevens County.

Spokane’s historic record includes numerous newspaper reports of shaking felt by residents over the years, as well as minor quakes spotted on a seismograph operated by Jesuit priests at the former scholasticate at Mount St. Michael northeast of Hillyard. The reports mostly from the North Side and downtown came in the 1940s, ‘50s and ‘60s, and there was one minor earthquake about eight miles south of downtown in 1995.

High-tech laser mapping done in recent years in the Puget Sound area has uncovered major earthquake fault lines extending in an east-west direction toward Eastern Washington.

The so-called South Whidbey Island Fault has been traced from Victoria, B.C., to North Bend, making it a major fault capable of producing a 7.5 magnitude quake, Weaver said. A fault through the Tacoma area along the White River was traced into the crest of the Cascades and is believed to be connected with the Yakima Fold Belt to the east.

The USGS is planning aerial laser mapping in the Toppenish Ridge and Yakima areas in an effort to trace the Tacoma fault line eastward.

That type of mapping in recent years has consistently revealed unexpected features, such as the South Whidbey Island Fault.

Scientists increasingly believe that seismic features throughout Washington are interrelated.

“We need to figure out how to put all of Washington into a more complete earthquake and tectonic model,” Weaver said.

In Spokane, the U.S. Soil and Conservation Service is planning the first high-resolution laser mapping of the urban area, about 400 square miles around the city, Weaver said.

In addition, scientists would like to have an aerial magnetic survey, which can point to fault lines as well. Then, they would undertake field trenching across any faults that might be discovered through mapping or surveys.

Scientists have surmised that the northward movement of the Pacific plate along the West Coast is putting pressure on the rock layers of the Columbia Basin, which translates into periodic earthquakes in the Columbia Basin, but not necessarily in Spokane where other forces on the North American plate may be at work, Malone said.

Comparisons of global positioning data over the past eight to 10 years confirms that north-south compression in the basin, or a squeezing of the earth, is occurring at about twice the rate previously suspected – 1 to 2 millimeters a year. Eventually, the pressure is released through earthquakes.

“The amount we don’t know compared to what we do know makes Eastern Washington a target for more research in coming years,” said Bill Steele, seismologist with the Pacific Northwest Seismic Network at UW.