The largest earthquakes and tsunamis may be the result of deep slow-sliding motion

Map of the Cascadia Subduction Zone.

Massive earthquakes and subsequent tsunamis originating in subduction zones such as Cascadia – Vancouver Island, Canada, to northern California – are among the most dangerous natural disasters in the world. A team of geologists now believes that the key to understanding some of these devastating events may lie in deep, progressive slow-slip behaviors beneath subduction zones. This information may aid in planning future earthquakes in the area.

“What we found was very unexpected,” said Kirsty A. McKenzie, PhD candidate in Earth Sciences, Pennsylvania.

Unlike large, shallow earthquakes that move and release energy in the same direction that the plates are moving, the energy of slow-sliding earthquakes may move in other directions, mainly downwards.

Subduction zones happen when two Earth plates meet and one of them moves below the other. This usually creates a fault line and some distance, a line of volcanoes. The Cascadia is typical in that plate tectonics meet near the Pacific coast and the Cascade Mountains, a volcanic chain that contains Mount St. Helens, Mount Hood and Mount Rainier in the east.

Topography and bathymetry in southern Cascadia, which includes southern Oregon and northern California (sea floor depths between 200 and 3,000 meters are shown in a spectrum scale from red (shallow) to purple (deeper). Credit: Jenna Hill, USGS

According to the researchers, a magnitude 9 earthquake occurred in Cascadia in 1700 and there has been no major earthquake there since. Instead, slow-slip earthquakes, events that happen deeper and travel very short distances at a very slow rate, are constantly occurring.

Kevin B. Furlong, professor of earth sciences at Pennsylvania: “Usually, when an earthquake occurs, we find that the movement is in the opposite direction to how the plates move, causing the accumulation of slip deficit.” “For these slow-sliding earthquakes, the direction of motion is downward in the direction of gravity rather than the direction of movement of the plates.”

The researchers found that areas in New Zealand, which other geologists have identified, are slowing in sliding as do the Cascadia.

“But there are subduction zones that do not contain slow-slip events, so we don’t have direct measurements of how the deeper part of the subduction plate moves,” Furlong said. “In Sumatra, the shallow seismic zone is moving, as expected, in the direction of plate motion, but although there are no slow slip events, the deep plate motion is still mainly controlled by gravity.

Slow-sliding earthquakes occur at a deeper depth than earthquakes that cause major damage and Earth-shaking events, and researchers have analyzed how this deep slip could affect the timing and behavior of large, destructive earthquakes.

“Slow-slip earthquakes erupt over several weeks, so it’s not just one event,” MacKenzie said. “It’s like a swarm of events.”

According to the researchers, in southern Cascadia, the total movement of the plates is about one inch of movement per year, and in the north near Vancouver Island, it is about 1.5 inches.

“We don’t know how much 30 millimeters (1 inch) a year builds up to be released in the next major earthquake or if some movement is taken by an unobservable process,” MacKenzie said. “These slow-sliding events give signals that we can see. We can observe the slow-slide events as they move from east to west rather than in the direction of the movement of the plates.”

Slow slip events occur in Cascadia every 1-2 years, but geologists wonder if one of them will trigger the next earthquake.

Researchers are measuring surface motion using permanent, high-resolution GPS stations on the surface. The result is a stair pattern of loading and sliding during slow slide events. Juveniles are visible on the surface even though geologists know they are about 22 miles below the surface. They report their findings in geochemistry, geophysics, and geological systems.

“The reason we don’t know much about slow-slip earthquakes is that they were only discovered about 20 years ago,” Furlong said. “It took five years to figure out what it was, then we needed a GPS accurate enough to measure movement on the surface of the Earth. Then we had to use modeling to convert the slip on the surface to the slide below the surface on the boundaries of the board itself, which is bigger.”

Researchers believe that understanding the effects of slow-sliding earthquakes in the region at these deep depths will allow them to understand what might trigger the next massive earthquake in the region. Engineers want to know how strong the shaking will be in an earthquake, but they also want to know the direction the forces will be in. If the difference in the direction of slow slip events indicates a possible change in behavior in a large event, then this information would be helpful in planning.

“Most importantly, we don’t know what caused the great earthquake in this case,” MacKenzie said. “Every time we add new data about the physics of the problem, it becomes an important component. In the past, everyone thought that events were one-way, but they could be 40 or 50 degrees different.”

While the slow events in the Cascadia highlight the potential massive earthquakes in the region and the tsunamis that could trigger them, Furlong believes that other subduction zones may also have similar patterns.

“I would argue that (differences in direction of movement) occur in Alaska, Chile and Sumatra,” Furlong said. “We see evidence of this in only a few, but it may be a global process that we have missed. Cascadia displays it due to slow slip events, but it may be fundamental to subduction zones.”

Reference: “Bidirectional loading of the subduction interface: Evidence from the kinematics of slow slip events” by KA McKenzie, KP Furlong and MW Herman, 4 September 2020, Geochemistry, geophysics, and geological systems. DOI: 10.1029 / 2020GC008918

Matthew W. Herman, Associate Professor of Geology, California State University, Bakersfield also worked on this project.

The National Science Foundation supported this work.

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