New evidence of damaging quakes found in Eastern Washington

SPOKANE — New research has bolstered some scientists’ suspicion that large swaths of the Inland Northwest are vulnerable to damaging earthquakes.

Seismic activity in Eastern Washington and northern Idaho is relatively low compared with that on the Pacific coast, but scientists are proposing high-tech studies to assess the potential hazards.

Several recent earthquakes in north Spokane and southern Stevens County in northeast Washington were so small that no one reported feeling them. They turned up on an advanced network of seismographs covering the Western United States.

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

“I think this is a more important issue than Spokane might realize,” Craig Weaver, a seismologist with the U.S. Geological Survey, said about the 2001-2002 earthquake swarm.

Similar quakes have occurred periodically in Spokane, while larger ones have been documented across the region since 1872.

A magnitude-5.5 quake struck north of Coeur d’Alene, Idaho, in 1942, underscoring the potential for stronger tremors in the region.

Spokane had been considered a low-hazard area until a 3.7-magnitude tremor shook the city 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,” University of Washington seismology professor Steve Malone told the Spokesman-Review. “What does that mean for the future?”

Scientists aren’t sure, but they’re eager to get a better idea of what seismic hazards the region faces.

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

Historically, seismic activity in the Inland Northwest 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 sits at the northern edge of the volcanic basalts, making it a boundary area where earthquake hazards are typically found.

A cluster of four earthquakes with magnitudes of 1.5 to 2.3 struck along the Little Spokane River on the city’s north side in September. Three similarly small quakes occurred earlier this month in southern Stevens County.

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

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