On March 11, 2011, a magnitude 9 earthquake struck off the coast of Japan. According to LiveScience, the quake was centered 45 miles east of Tohoku and 15 miles below the surface. A series of tsunami waves, reaching heights up to 128 ft, barreled inland and flooded approximately 217 square miles. As of Oct. 9, 2015, the confirmed death toll is 15,893, reported CNN.
Now, a team of scientists is creating the first map of mantle flow beneath a tectonic plate, specifically the Juan de Fuca plate, situated offshore of Oregon, Washington and British Columbia.
“Our goal is to understand large-scale plate tectonic processes and start to link them all the way down to the smallest, to specific earthquakes in the Pacific Northwest,” said Prof. Richard Allen, chair of earth and planetary science at the Univ. of California, Berkeley. Allen is the senior author of a paper on the subject appearing in Nature Geoscience.
Since 2012, the research team has made 24 two-week voyages to position and retrieve seismometers. A key finding is the mantle beneath a microplate in the area is moving differently than the plate, causing segmentation of the subduction zone.
“The experiment was unprecedented in that there were 70 seismometers deployed at a time, sitting there for 10 months, which is much bigger than any other ocean-bottom experiment ever done before,” said co-author and graduate student Robert Martin-Short.
Though small compared to other plates, roughly around the size of California and 50 to 70 km thick, the Juan de Fuca plate is capable of generating earthquakes similar to the Tohoku earthquake.
“The offshore environment is much simpler, the plates are thinner and more uniform than continental plates and we can see through them to get a better sense of what is going on beneath,” said Allen.
While the asthenospheric mantle, 100 miles beneath the Juan de Fuca plate, travels eastward with the plate, the mantle underneath a nearby plate located off the northern California coast behaves differently. Known as the Gorda Plate, the section is uncoupled from the mantle beneath it, allowing independent movement.
“The Juan de Fuca plate is clearly influencing the flow of the mantle beneath it, but the Gorda Plate is apparently too small to affect the underlying mantle,” said Allen.
Instead, the researchers believe the mantle is influenced by the gargantuan Pacific plate, which is moving eastward and plowing under the western edge of the Americas.
The research is part of the Cascadia Initiative, which aims to develope underwater and on-shore seismic instruments that measure interactions between plates and the mantle. Additionally, the initiative is interested in monitoring quake and volcanic activity where the Juan de Fuca plate subducts under the North American plate.
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