Study Says Big Quake Poses Lower Risk for Eugene Area

By CHRISTIAN HILL, The Register-Guard | August 8, 2018

New research from the University of Oregon delivers what may be a sliver of good news – if it can be called that – for the Eugene-Springfield area as the Pacific Northwest prepares for what scientists say will be the worst natural disaster in the region’s recorded history.

The research by Miles Bodmer, a doctoral student at the university, offers a theory about why the Cascadia Subduction Zone, a 700-mile-long fault in the Earth’s crust, is less likely to produce a major earthquake off the Oregon Coast and more likely to generate one off Northern California and Washington state.

The research offers an explanation of why the odds favor a partial rupture – and by partial we means kilometers long – to the north and south of the Oregon Coast, which could spare the local area from the very worst shaking and damage from a massive magnitude 8 quake that such an event could produce. Nevertheless, such an event still could do heavy damage to the local area.

The Eugene-Springfield area wouldn’t be so lucky in the event the fault breaks along its entire length, which researchers said would create one of the most powerful earthquakes in recorded history at magnitude 9 or greater. Researchers predict a quake of that scale would cause widespread death and property destruction across the region, inundating coastal areas with a massive tsunami and isolating inland communities for weeks and months.

“Our study is worse news for Portland northward to Seattle and for southern Cascadia, but central Cascadia is not off the hook,” said Doug Toomey, a professor of earth sciences at the UO and a member of Bodmer’s research team.

Bodmer’s theory about why a fault rupture off the Oregon Coast is less likely relies on the titanic forces at work deep below the Earth’s surface.

Imaging of the Earth’s crust below the ocean floor by Bodmer’s team has identified anomalies along the fault off the coasts of Northern California and Washington where these powerful quakes most often occur.

The subduction zone is where the oceanic Juan de Fuca tectonic plate – one of the giant masses of solid rock that form the rigid outermost shell of our planet – is subducting, or sliding under, the North American tectonic plate that holds our continent.

Bodmer’s just-published research paper hypothesizes that these anomalies are areas of higher buoyancy where the Earth’s mantle – superheated, pliable rock – is pressing against the subducting Juan De Fuca plate and pinning it with greater force to the North American tectonic plate than in other areas along the fault. Bodmer likened the mechanism to trapping helium balloons under a bed sheet.

Subduction zone earthquakes occur when the forces become too much and the plates slip, causing a rupture either along a segment of the fault or through its entire length that releases unimaginable amounts of energy that those of us on the Earth’s surface feel as an earthquake.

“We expect that these areas (of the fault above the anomalies) can build up more stress because the plates are more coupled there,” Bodmer said. “The strength of an event depends on two things: the amount of built-up stress it can release, and the area of the rupture. It doesn’t mean that every event in the region would be stronger but that the region has the potential for larger events.”

And the research doesn’t rule out ruptures of the fault along the Oregon Coast, although they are less likely, he said.

The last Cascadia subduction zone earthquake was more than 300 years ago, on Jan. 26, 1700.

Bodmer’s team identified the anomalies by using years of data from a network of nearly 300 sensors underwater and on land that measure the speed of energy released from distant major earthquakes as it moves around the earth. The team transformed the data into an image of the fault – similar to a CAT scan of the human body – that identified the anomalies as areas below the fault where the energy moved slower than elsewhere underneath it.

Two years ago, a study co-authored by Oregon State University concluded Cascadia earthquakes occur with somewhat greater frequency than researchers had initially suspected. The study concluded that there’s a 15 to 20 percent chance of a quake off the Oregon Coast in the next 50 years.

Subduction-zone earthquakes are the Earth’s strongest, but there are others that local residents are familiar with.

The first is shallow earthquakes caused along fractures created within the North American plate by the constant friction. The Fourth of July earthquake near Walterville in 2015 is the most recent example felt by area residents.

The second is deep earthquakes that originate in the portion of the Juan De Fuca plate that already has slipped beneath the edge of the North American plate. The 2001 Nisqually earthquake near Olympia, with a magnitude 6.8, is the last recent example locally.

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