Snohomish County officials raised questions on Tuesday whether a quake may have contributed to the hillside breaking free and crashing across the valley.
The slide has killed at least 25 people. In all, 90 people are officially considered missing. The debris field covers one square mile and is 50 feet deep in places.
"That earthquake was too small, too weak to account for the slide," Craig Weaver with the USGS said Thursday.
The slide earlier this month measured 1.1 magnitude.
Seismic activity shows that there were two large slides on Saturday with the first hitting at 10:37 a.m. A second, larger mass crashed down around 10:41 a.m.
The readings, taken at the University of Washington, show the land moving for more than an hour.
Geologists can tell the ancient slide was huge, but "I can't tell you how quickly it came down, whether it took weeks or seconds," said Ralph Haugerud, a USGS geologist and the report's author.
The ancient slide occurred less than a mile west of the hillside that collapsed. It created a debris field more than a mile wide just north of where the Stilly runs today.
A map with the report shows a river valley lined by ancient landslides, all of which occurred since the last ice age. The slides might have also covered up earlier ones, the report notes.
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