Researchers at Oregon State University have found a new explanation as to why foreshocks often precede large earthquakes.

Large quakes appear to follow a short period of “shallow mantle creep” and “seismic swarms,” according to the study, which was published Monday in the the academic journal Nature Geosciences.

The findings shed new light on a phenomenon called “silent slip,” in which parts of the Earth’s crust are displaced along a fault line, but without any seismic activity.

In simpler terms, silent slip is when the Earth moves, but there’s no earthquake. That silent slip can lead to actual seismic activity, though, said co-author Vaclav Kuna, a graduate student in geology and geophysics at the university.

Vaclav and other researchers deployed 55 seismometers along the Blanco Transform Fault off of Oregon’s coast for a year.

“It’s a very seismically active fault that generates significant earthquakes at higher rates than the majority of faults on land, making it ideal for studying the process of earthquake generation,” Kuna said in a statement.

Transform faults occur at the edges of tectonic plates where the movement is mostly horizontal. The Cascadia Subduction Zone, the Pacific Northwest’s most widely known producer of large quakes, is a subduction fault, where one plate is sliding beneath another. Still, Kuna’s findings were remarkable.

“Slow slip directly triggers seismic slip – we can see that,” Kuna said in a statement. “The findings are very interesting and may have some broader implications for understanding how these kinds of faults and maybe other kinds of faults work.”

To see how those faults work, Kuna and other researchers had to look beneath the Earth’s crust, which can vary from 40 miles thick on land to just 2 miles thick on undersea ridges. The boundary area between the crust and the next layer beneath it, the upper mantle, is called the Moho.

Movement deep beneath the Earth's surface silently builds up stress that leads to earthquakes, according to researchers from Oregon State University.

Slips in the Moho, which don’t always result in earthquakes, can be precursors for quakes in the layers above, said co-author John Nabelek.

“We see slow, aseismic slips that occur at depth in the fault beneath the Moho and load the shallower part of fault,” he said in a statement. “We can see a relationship between mantle slip and crust slip. The slip at depth most likely triggers the big earthquakes. The big ones are preceded by foreshocks associated with creep.”

The Blanco fault is close enough to the Cascadia Subduction Zone, Nabelek said, that action on one could portend action on the other.

“A slip on Blanco could actually trigger a Cascadia Subduction slip,” Nabelek said. “It would have to be a big one, but a big Blanco quake could trigger a subduction zone slip.”

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale

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