The South Whidbey Island Fault is connected to a system of powerful earthquake fault lines stretching from Victoria, B.C., to Yakima that is capable of unleashing a devastating 7.5-magnitude earthquake.
A new study of underground magnetic fields eliminates any doubts that the South Whidbey Island fault is one of the most hazardous in Western Washington, said Craig Weaver of the U.S. Geological Survey at the University of Washington.
“This discovery is providing us with the architecture to send geologists out to see what these faults really mean,” Weaver said. “We have not changed our earthquake assessment of the South Whidbey Island fault, but now we are more certain of its potential danger.”
A 7.5-magnitude earthquake is capable of causing major destruction over a large area. It could also set off quakes on connected faults. Many of the other earthquake faults in the region could be connected to the South Whidbey in a system similar to the San Andreas fault in California.
Because the South Whidbey fault is shallow, running beneath Mukilteo and southeast to Woodinville, South Snohomish County could be at increased risk, Snohomish County emergency services director John Pennington said.
“The reality is when this earthquake hits, there will be some heavy losses,” he said.
Snohomish County is better prepared now for such a quake than it was several years ago, Pennington said.
“Our codes and standards are a lot better, and people are much more aware of the hazards than they are in many other places,” he said. “We’ve made a tremendous investment in structural collapse rescue training.”
Earthquake fault systems usually run parallel to oceans and mountain ranges, but this one crosses the volcanic arc of the Cascade Range, reaching possibly to the Tri-Cities in southeast Washington and possibly connecting to the Cascadia subduction zone off the state’s coast. The federal Geological Survey is tentatively calling the connection to Eastern Washington the Cle Elum fault.
The goal of geologists will be to assemble an earthquake fault map that will help them understand the hazards, Weaver said. The seismic hazard models they develop in turn will provide the basis for new building codes. The new research also will help the USGS when it begins work on upgrades to its earthquake warning system, complete with offshore seismometers, Weaver said.
A lack of written history before the settlers arrived here makes the study of earthquakes along the South Whidbey fault difficult, but there is evidence that it has produced multiple earthquakes over the last several thousand years, Weaver said.
Geologists believe that in 1700, a 9.0-magnitude earthquake in the Cascadia subduction zone caused a giant tsunami that was recorded in Japan. The evidence of the quake here includes coastal trees that died that same year.
Many more regional earthquakes have occurred since then, including several in the 1940s. Members of the baby boom generation who grew up in South Snohomish County may remember the 6.5-magnitude earthquake that rocked the Seattle area in 1965.
As was the case with the geology involved in those earthquakes — along with Sichuan province earthquake in China last year — the South Whidbey is a shallow fault that runs just below urban areas.
In Yakima County, earthquakes are deeper, smaller and not often felt, said county emergency services director Jim Hall.
“We had a 2.5 quake the other day and nobody reported it,” Hall said.
The counties that sit above the South Whidbey Island fault need to continue to work together to be prepared for a big earthquake, Island County Commissioner Helen Price Johnson said.
“On the islands, we also need to be self-sufficient in a disaster because we might not get help right away,” she said. “Really anywhere in the Northwest, an earthquake is a matter of when, not if.”
Gale Fiege writes for the Herald of Everett.
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