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What we know about Washington earthquake risk might be wrong

Distant earthquakes can cause underwater landslides according to a recently published University of Washington study.
Distant earthquakes can cause underwater landslides according to a recently published University of Washington study.

Can an earthquake under the Indian Ocean cause an underwater landslide off the coast of Washington?

In 2012, a magnitude-8.6 Indian Ocean earthquake caused underwater landslides more than 8,000 miles away according to a study published earlier this month in the “Journal of Geophysical Research.” University of Washington oceanographer Paul Johnson was the lead author for the study.

“The basic assumption … is that these marine landslides are generated by the local earthquakes,” Johnson said in a statement released by the UW. “But what our paper said is, ‘No, you can generate them from earthquakes anywhere on the globe.’”

The findings came after an analyzes of data from ocean bottom seismometers off the Washington-Oregon coast tied to the 2012 Indian Ocean earthquake to a series of underwater landslides on the Cascadia Subduction Zone that occurred intermittently for nearly four months after the quake.

The findings could complicate sediment records used to estimate earthquake risk, according to the UW statement. These underwater landslides could also cause tsunamis and damaging underwater communications cables.

The discovery came after Johnson learned by chance about the underwater seismometers at a scientific meeting. The study states that the steeper-than average submarine slopes off the Northwest coast make them particularly prone to slides. Sediment accumulates atop the slopes and it can be knocked loose when seismic waves arrive from quakes around the world.

“So these things are all primed, ready to collapse, if there is an earthquake somewhere,” Johnson said.

Scientists study sediment layers to calculate the time between past earthquakes to help them predict when quakes might occur. However, the new data shows that distant earthquakes often cause smaller landslides that could impact these predictions. Johnson said more core sampling over a wider area would be needed for more accurate estimates of earthquake risk.

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