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A slow, deep slide may guide the largest earthquakes and tsunamis

 


University Park, Pennsylvania – Massive earthquakes and subsequent tsunamis that originate 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.

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. “For these slow-sliding earthquakes, the direction of motion is directly downward in the direction of gravity rather than in the direction of plate motion.”

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 do not have direct measurements of how the deeper part of the eruption 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 they are 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 mm (1 inch) per year that accumulates to be released in the next major earthquake or if some movement is taken by a process that cannot be observed,” said McKinsey. “These slow-sliding events give signals that we can see. We can observe the slow-sliding events moving from east to west rather than in the direction of plate movement.”

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 because 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 actually measure the movement on the surface of the Earth. Then we had to use modeling to convert the slip on the surface to the slide down the surface onto the board the limit 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 triggered 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 has been overlooked. Cascadia predicts it due to slow slip events, but it may be fundamental to subduction zones.”

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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