With no reported seismic activity, early indications are wet weather, erosion and previous history may have triggered the Highway 530 mudslide near Oso Saturday.
According to the Department of Natural Resources, the landslide re-activated from a similar one in the same area in 2006.
In that incident, weeks of heavy rain caused the hillside to slide six inches, cutting off Darrington from Arlington, Everett and I-5. Since then, geologists say erosion has caused the base of the slide to weaken.
Recent heavy rainfall in the area hasn’t helped.
The National Weather Service reports up to 200% the normal rainfall there in the last 45 days, at least 15 inches above normal.
“As the moisture gets more and more it’s pushing the particles apart. It makes them slippery and has less friction. The force of gravity is constantly trying to pull things down,” said Brent Bower, hydrologist.
On Monday, DNR geologists will use radar imagery to figure out the volume of the slide, which will help determine if people near Oso are still in danger.