WENATCHEE — Engineers at North Central Washington’s Columbia River dams have double-checked their data and concluded that the discovery last year of a major seismic fault west of Entiat will not oblige them to rethink the dams’ vulnerability to ground shaking.
“We definitely had our experts look at the work we’ve done to ensure that the study we were relying on was robust, even with. the recent discovery,” Bill Christman, hydro engineering manager at the Chelan County PUD, said Thursday.
A team led by U.S. Geological Survey geologist Brian Sherrod discovered the fault in November with the aid of aerial images captured using “Light Detection and Ranging (LIDAR), a technology that can “see” through trees and other vegetation to capture detailed images of the contours of the ground surface.
The fault is just west of Entiat and very near the Chelan PUD’s Rocky Reach Dam. The area sits amid a “seismic swarm,” an area of high seismicity and frequent tremors.
After inspecting the site and path of the fault at ground level, Sherrod developed a hypothesis that the fault was likely the cause of “the 1872 earthquake,” a temblor that would have measured an estimated 6.8 to 7.2 on the Richter Scale and is thought to be the region’s most severe.
Before Sherrod’s discovery, the epicenter of the big quake was thought to be north of Chelan.
The Chelan, Douglas and Grant PUDs in 2012 finished a five-year analysis of how their dams would perform in a major quake.
The analysis revealed that the ground-shaking potential from a big earthquake is about three times greater than was thought possible when their dams were built decades ago. But, as they expected, their study already takes into account the effects that a 1872-type quake would have anywhere within their mid-Columbia study region, Christman said.
The engineers have said that their dams would likely hold up well to that degree of shaking.
“Seismic loads are very important for our projects, and we need to and are taking them seriously,” Christman said. “We looked carefully, immediately after becoming aware of Brian Sherrod’s findings and hypothesis. I couldn’t imagine a better outcome.”
The study review took about a month, Christman said and was reported to Chelan PUD General Manager Steve Wright, but has not yet been shared with commissioners during a public session.
