Search officials say they learned of an earthquake just days before the Oso mudslide.
“You have to dig pretty deeply, but we found that there was a 1.1 earthquake approximately 100 yards right behind the slide on March 10 right near the surface,” said John Pennington, Director of Snohomish County Emergency Management.
University of Washington Seismologist Bill Steele said their data places that earthquake about two kilometers away from the fault scarp and at a depth of about 3.9 kilometers.
Steel said it’s a very small earthquake and they don’t have much coverage or good instruments located in the area. There are errors that could put the earthquake near the mudslide.
“If that is the case, it could be a result of a deformation process within that slope that could be some of the same deforming that led to the landslide, eventually,” said Steele.
As far as the actually shaking goes, Steele said “it was tiny, not felt by anyone, and that would not have impacted the landslide.”
Steele said it’s complicated, but it’s not the cause of the landslide. However, it could be “indicative of part of the process that led to the landslide.”
“We really can’t directly correlate it at this time,” said Steele.
He said they’ve been pouring over data for evidence of cracking, but they haven’t found a “smoking gun.” However, Steel said the glacial material that makes up the slopes in this area are very susceptible to landsliding – the river has been interacting with those slopes and we’ve had a lot of rain in March.
“All those factors together are definitely the dominant causes of the landslide,” said Steele.