An earthquake did not immediately precede the deadly mudslide in Oso, Washington on March 22, the U.S. Geological Survey says.

Local officials in Snohomish County, where the landslide occurred, had claimed that a small 1.1-magnitude earthquake about two weeks prior to the landslide might have contributed to the event, which has killed at least 16 people.

USGS is not contradicting that report, according to a press release sent to Mashable, but it is simply stating there were no seismic events "around the time of the landslide."

"Seismograph readings show no indication of an earthquake in association with the landslide," USGS says in the statement. The agency is tasked with monitoring earthquakes as well as studying landslides, which are also known as mudslides, among other duties. According to USGS, the seismograph readings from the University of Washington's Pacific Northwest Seismic Network, which is operated in conjunction with USGS, showed two wave signals.

The first signal was shortly before 10:30 a.m. Pacific Standard Time on March 22, and was associated with part of the landslide. The second is the larger landslide that occurred a few minutes later.

Image: USGS/University of Washington

Aside from the landslide itself, the USGS says, there were no other seismic events in that area.

"There are no earthquakes or other local seismic events on the records around the time of the landslide," the statement said. The landslides generated "elevated levels of local ground shaking" for more than an hour, USGS said.

Landslides occur in every state in the country, and cause up to $2 billion in damage each year. Landslides typically kill more than 25 people annually in the U.S., with a far higher death toll worldwide. Scientists know landslides can be triggered by heavy rains, earthquakes and other factors, but the USGS only spends about $3 million per year to study and monitor landslide-prone areas around the U.S. There was no such monitoring in place for the steep, unstable terrain above the Steelhead Drive housing development, despite the fact that this area has seen repeated landslide events in the past, most recently in 2006.

The Snohomish County landslide was likely caused by a combination of above average rainfall and soil moisture conditions that made the steep slope less stable, but scientists are examining the area to determine a more precise reason for the event.

As of Thursday morning, at least 90 people were still unaccounted for in the landslide, and the death toll is expected to rise.

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