USGS finds clue to 1872 quake that blocked Columbia River
Is a land formation near Entiat the key to finding the epicenter of Washington’s largest-known earthquake? A research geologist for the U.S. Geological Survey hopes to find out.
The Wenatchee World
ENTIAT, Chelan County — After years of searching, scientists are hoping that a newly discovered land feature near Entiat is a fault that will hold the clues they need to pinpoint the epicenter of Washington’s largest known earthquake.
The Dec. 15, 1872, quake left behind reports of intense ground shaking from Oregon to British Columbia, a landslide near Entiat that blocked the Columbia River for several hours, and a geyser near Chelan Falls that shot water 30 feet into the air for days.
Geologists have used such reports and other data to try to determine the epicenter of that quake, and studies in the past dozen years put it somewhere near the southern end of Lake Chelan.
With an estimated magnitude of between 6.8 and 7.2, this massive quake should have left some permanent mark on the land, said Brian Sherrod, research geologist for the U.S. Geological Survey.
Sherrod believes he may have found that mark in Spencer Canyon, three miles south of Entiat, where he hopes to bring a crew this summer to find out whether the cliff-like feature is what’s known as a fault scarp, and if it is, whether it can be linked to the 1872 quake.
He’s applied for a permit with the Entiat Ranger District to bring a crew in to hand-dig the trenches for further investigation. “We’re hoping we can find material — volcanic ash or charcoal — and figure out the maximum age of the scarp,” he said.
A fault scarp is “a place where the fault tip breaks the ground surface and displaces it. One side goes up relative to the other side, and you get a cliff-like feature,” he said.
Sherrod said he already knows from trees growing on the scarp that it’s been there for at least 130 years. He’ll use materials found in the trench to help determine just how old it is.
Sherrod discovered the 8-foot tall scarp from technology known as lidar, or light detection and ranging, which uses lasers to scan the earth from a plane or helicopter and creates a detailed ground map that ignores plants and man-made structures.
The USGS used lidar to map this region several years ago, and didn’t see anything. “But I have a habit of going back to old lidar data and pulling it up to see if I see anything new,” he said.
Three years ago, he did, and it was located on the Entiat Ranger District, at the very edge of the map up Spencer Canyon. “It’s a very subtle feature and on very steep topography,” Sherrod said, making it difficult to pick out on the map.
Last year, he coordinated another lidar survey of the area.
Pete Wier, environmental coordinator for the Entiat Ranger District, said the proposal involves no heavy equipment, has few impacts, and the area will be rehabilitated when the investigation is complete.
He expects to put it out for public review within a month.
Sherrod said larger questions could eventually be answered about the big quake if the scarp was, in fact, created by it. It’s unlikely to ever answer questions about when or where the next big earthquake will be, he said, unless scientists eventually find evidence of repeated earthquakes.
But Sherrod said he’ll be satisfied if he can tie the small cliff he found to the 1872 earthquake to further pinpoint its epicenter.
It’s important not only to help unravel one of the state’s biggest geologic mysteries, but also for regional development.
“If we can narrow it down, it will reduce the uncertainty,” Sherrod said.