UC Berkeley seismologists unearth insight into relationship between tectonic plates

Heyun Jeong/Staff

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Researchers at the Berkeley Seismological Laboratory and the campus department of earth and planetary sciences gained new insight into the movement of tectonic plates as a result of the multi-year study published Friday in the journal Science.

William Hawley, a UC Berkeley graduate student in earth and planetary sciences and lead researcher on the study, said scientists used a broad network of seismometers to monitor the movement of seismic waves across the plates in the Cascadia subduction zone more precisely than ever before. Their data showed an accumulation of a viscous layer beneath the Juan de Fuca Plate that allows freer movement between it and the mantle than was previously thought.

One hypothesis for the movement of these plates is that as the mantle underneath a plate moved, it dragged the plate above it along. But because of the fluid-like substance, the researchers concluded that the mantle was not the driving force for the movement of the Juan de Fuca Plate.

“Plate tectonics is something that is a fairly simple theory,” Hawley said. “We still don’t understand why plate tectonics happen. … In particular, we don’t really understand how the movement of tectonic plate is affected by the movement flow in the mantle.”

The researchers used a technique called seismic tomography to try to better understand the movement of the Juan de Fuca Plate off the coast of California. By using hundreds of seismometers on the ground, both inland and underwater, at an unprecedented scale, researchers could remotely measure seismic movement from far away earthquakes and use that data to determine the density of the material they moved through.

“What we did not expect to find is that there is a very … low density material,” said Mark Richards, a co-author of the study, referring to the fluid-like substance, which they hypothesize is made up of volatile material such as water and carbon dioxide.

According to Peggy Hellweg, operations manager at the Berkeley Seismological Laboratory, scientists cannot predict with certainty that the phenomenon exists at other plate boundaries until they conduct similar experiments elsewhere in the world.

“If we can find (the layer) in other places, then it will be easier to generalize (the theory) … now we can go out and verify it or falsify it,” said Hellweg.

Hellweg said the use of seismometers at such a large scale allowed them to view the plates’ structure with much greater clarity — a difference similar to the one between a photograph and a CAT scan.

According to Hawley, the seismometers’ close proximity to the underwater fault line allow them to detect earthquakes as they begin and warn people on land before the seismic waves reach them.

“If we had offshore seismic instrumentation on this fault … we could give 40 to 60 seconds’ warning,” Hawley said.

While the study did not fully answer the question of how tectonics plates work, Hawley believes that in the long run, the findings will help refine seismic hazard models.

Contact Michael Lee at [email protected] and follow him on Twitter at @_HyunkyuL.

Correction(s):
A previous version of this article misspelled Mark Richards’ name.