New model of fluid distribution in the Cascadia Subduction Zone helps understanding seismic activity

A new three-dimensional model of the fluid stored deep within the Earth’s crust along the Cascadia Subduction Zone provides new insights into how the accumulation and release of such fluid can affect seismic activity in the region.

The liquid collects near but does not penetrate the thickened section of crust known as Siletzia lying beneath much of western Oregon and Washington. The pressure associated with this fluid could be a factor in the seismic phenomenon known as episodic tremor and slip, or ETS, said Gary Egbert, an electromagnetic geophysicist at Oregon State’s College of Earth, Ocean, and Atmospheric Sciences and lead author of the new paper. detail the findings.

Episodic tremors and slips are fault behaviors that include localized non-volcanic vibrations and slow slip events that may occur over hours or days. It occurs throughout the Cascadia Subduction Zone, from northern California to British Columbia, but is less frequent and intense under the Siletzia central core, which runs mainly down the Oregon Coast range and ends near Roseburg.

The findings, just published in the journal Nature Geosciences, have applications for improving understanding of seismic activity along the Cascadia Subduction Zone, Egbert said.

“Water is a key player in seismic activity and volcanism in Cascadia,” he said. “This is a new take on this fluid. This is information that can be used in conjunction with other data, and more detailed model studies, to better understand the major earthquakes in Cascadia.”

Egbert’s paper draws on decades of work to collect magnetotelluric data, both offshore and onshore, across the Cascadia Subduction Zone. Magnetotelluric is a geophysical technique that uses surface measurements of magnetic and electric fields to reveal subsurface variations in electrical resistivity.

“Most solid rock doesn’t conduct electricity well, but dissolved solids make water conductive, so magnetotelluric data can be very useful for detecting the presence of water below the surface,” said Egbert.

Water naturally descends from the oceans into the earth’s crust, where it accumulates and combines chemically with minerals. Where oceanic crust is pushed under continents along subduction zones, it heats up and releases water. The released fluids can weaken crustal rock, leading to crustal deformation, both from slow release stresses, such as episodic tremors and slips, and from very large and destructive earthquakes.

Improved knowledge of the distribution of subsurface fluids is particularly relevant for assessing seismic hazard in areas such as the Cascadia, where there is significant potential for catastrophic megathrust earthquakes, Egbert said.

Using software developed by Egbert and colleagues, the magnetotelluric data allowed the researchers to create a detailed three-dimensional view of where fluid is deposited in the Cascadia forearc, the area between the oceanic trench and the associated volcanic arc. Three-dimensional imaging allows researchers to see the accumulated fluids more directly and draw conclusions about their motion.

The new images show fluid trapped in elongated sheets parallel to the shoreline and also reveal areas where underthrust sediment builds up against impenetrable volcanic rock that holds little water. Beneath the Siletzia nucleus, fluid storage and transport is more focused in narrow subducting channels that exhibit less tremor and episodic slip.

“The results show how roots in Siletzia are critical to how fluids are transported and where they are within subduction zones,” said co-author Paul Bedrosian of the US Geological Survey. “The implications of this for understanding seismic variability in Cascadia are just beginning to be recognized.”

More research is needed to understand the relationship between these fluids and earthquakes along the Cascadia, but that work is not out of reach, Egbert said.

“This is a complex issue, but ultimately there are potential applications for interpreting this information in conjunction with seismic data and geodynamic modeling to determine the relationship between fluid storage and seismic motion and events,” he said.

Additional co-authors on the paper are Bo Yang, a professor at Zhejiang University in China who worked on the project as a postdoctoral researcher at Oregon State; Anna Kelbert of the US Geological Survey; Kerry Key of the Lamont-Doherty Earth Observatory; Dean of Livelybrooks and Blake Parris of the University of Oregon; and Adam Schultz of the State of Oregon.

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