Study finds microplate is moving in divergent direction from mantle beneath it

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A microplate off the coast of Northern California is moving in a divergent direction from the mantle beneath it, researchers discovered in a study investigating the direction of seismic wave speeds through the mantle’s viscous asthenosphere.

The study was published Monday in the scientific journal “Nature Geoscience” by a team of researchers from UC Berkeley and Imperial College London after the first three years of seismological data collection near the Juan de Fuca Plate.

Robert Martin-Short, the report’s first author and campus graduate student in the earth and planetary science department, noted that the study’s scale was unprecedented in the quantity of ocean-bottom seismometers, or OBSs, and the area it covered. It is the first study to assess the entirety of a plate from ridge to trench.

Researchers analyzed data from 70 OBSs that were fairly evenly placed at 160 sites inside and outside the Juan de Fuca Plate, the Pacific Plate and the North American Plate. This is more than double the number of seismometers used in a previous study along the Pacific-North American plate boundaries led by California Insitute of Technology and CSU Northridge scientists.

The Juan de Fuca Plate is approaching complete subduction under the North American Plate along the Cascadia fault line, which stretches nearly 1,000 kilometers (621 miles) from Cape Mendocino, California, to Vancouver Island.

“Epicenters usually happen along the subduction zone,” Martin-Short said. “Cascadia is very strange — it’s one of the few subduction zones that doesn’t have many large earthquakes.”

The Cascadia Fault is divided into three segments — the first stretching from Mendocino to southern Oregon, the second in central Oregon and the third stretching from northern Oregon through Washington to Vancouver Island.

“Interestingly, studies of ancient giant earthquakes along the Cascadia subduction zone indicate that their rupture patterns follow this segmentation. The giant quakes typically break one or more of these three segments in its entirety,” Martin-Short said in an email.

Tectonic plate movement is influenced by the mantle — the thickest layer of the Earth directly below the crust. This was not the case for the Gorda Plate, which is caught between three larger plates.

Mark Richards, a professor in the department of earth and planetary science, explained the study’s findings that the Gorda Plate moved in a different angle than the mantle below.

“The Pacific Plate is essentially dominating the flow below the Gorda Plate,” Richards said.

The offshore and onshore seismometers were deployed by the Cascadia Initiative, which received funding from the National Science Foundation as part of funding from the American Recovery and Reinvestment Act.

“The community is in the process of deciding where they want to put the seismometers next,” Martin-Short said.

Alaska, home of the Pacific Plate’s northern border and the destination of a nationwide collection of seismological equipment, is speculated to be a top potential candidate for the seismometers’ next stop.

The data collected by the Cascadia Initiative is available to download for free from the NSF’s Incorporated Research Institutions for Seismology.

Contact Pamela Larson at [email protected] and follow her on Twitter at @PamReporting.

A previous version of this article incorrectly stated that Mark Richards is the dean of the campus physical sciences division. In fact, he no longer holds that position.
On second reference, a previous version of this article misspelled Richards’ last name. It also misidentified ocean-bottom seismometers as OCBs. In fact, the abbreviation is OBS.