Discoveries About 'Water' Beneath Oregon / Washington Coast May Help Major Quake Assessments

Published 07/18/22 at 4:55 AM PST
By Oregon Coast Beach Connection staff

(Newport, Oregon) – A new angle on researching megathrust earthquakes along the Oregon and Washington coast has been opened up as one group of Oregon State University (OSU) scientists announces some breakthroughs regarding fluids that exist beneath the Pacific Ocean. This could play a role in more accurately predicting the effects of major quakes (and thus tsunamis) that happen along this offshore region, such as the big one in 1700 and the massive one that is still coming. (Above: Humbug Mountain on the south coast - courtesy Manuela Durson - see Manuela Durson Fine Arts for more)

The fluid – primarily ocean water that has leaked further down – exists in a section near this region, but not in the deeper crust that lies beneath much of Oregon and Washington called Siletzia. How much pressure these fluids create beneath the Earth's crust could be factors in how the Cascadia Subduction Zone will react during what are known as the “Big Ones.”

The research - headed up by Gary Egbert, an electromagnetic geophysicist in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences – reveals more than a few surprises geologically, simply by way of explanation. One standout: the other “ocean” of water beneath the Earth's surface and well under the Oregon coast or Washington coast.

Knowing where the fluids are and aren't may soon yield more clues as to what parts of the zone may create the worst damage.

Egbert said the research will eventually give new insights into a seismic phenomenon known as episodic tremor and slip, or ETS. These fault behaviors have to do with sudden tremors and slow-slip events that may happen over days. It takes place up and down the Cascadia Subduction Zone, which is where two continental plates meet. A major event along that zone will someday result in a magnitude 9.0 quake or higher that will cause severe destruction on the coastlines and throughout the inland Pacific Northwest.

Those smaller actions take place along the zone, Egbert said, but they are less frequent and less intense in the vast area beneath the surface known as Siletzia, which runs mostly through the Oregon Coast Range and ends near Roseburg.

“Water is a key player in both seismic activity and volcanism in Cascadia,” he said.

That water doesn't really penetrate Siletzia, a formation that is at least 50 million years old, but the water runs through many parts of the subduction zone near Siletzia.

The discovery of the whereabouts of all this subsurface fluid was made possible by new software created by Egbert and his colleagues on the project. With it, they were able to use magnetotelluric data, both offshore and on land, throughout the Cascadia Subduction Zone. Magnetotellurics is a geophysical technique that uses surface measurements of magnetic and electric fields to reveal subsurface variations in electrical resistivity.

Grayland, Washington; photo courtesy Steve Ginn / Flickr

This in itself is another revelation to those interested in coastal geology: electricity can help you find subsurface water.

“Most solid rocks don’t conduct electricity very well, but dissolved solids cause water to be conductive, so the magnetotelluric data can be very useful for detecting where water exists in the subsurface,” Egbert said.

Subsurface formations like Siletzia are a particularly bizarre and surprising category of Oregon and Washington coast geology all their own. This formation, like a myriad of others much older on the south Oregon coast, were “accreted” to this continent tens of millions of years ago, meaning they were slowly joined to this region by tectonic plate movement. In spite of generally being below the visible surface, many parts of it are exposed around the coast, coast range and inland Oregon, mixed in with newer geologic features. (Also see Weirdest Science: Possible 'Ocean' Beneath the Oregon Coast, North America)

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