Deep, slow-sliding actions can target major earthquakes and tsunamis

Deep, slow-sliding actions can target major earthquakes and tsunamis

Map of the Cascadia subduction zone. Credit: Public domain

Megathrust earthquakes and subsequent tsunamis that originate in subduction areas such as Cascadia – Vancouver Island, Canada, to northern California – are some of the worst natural disasters in the world. Now, a team of geoscientists believes that the key to understanding some of these destructive events may lie in the deep, gradual, slow-sliding behaviors below the subduction zones. This information could help plan for future earthquakes in the area.

“What I found was quite unexpected,” said Kirsty A. McKenzie, a doctoral student in science at Penn State.

Unlike larger, shallower earthquakes that move and produce energy in the same direction as the plates move, the energy of slow-sliding earthquakes can move in other directions, mainly downward.

Subduction zones occur when two of the Earth’s plates meet and one moves beneath the other. This usually creates an error line and at a certain distance, a line of volcanoes. The cascade is typical in that the tectonic plates meet near the Pacific coast and the Cascade Mountains, a volcanic area containing Mount St. Helena, Mount Hood and Mount Rainier, form to the east.

According to researchers, in 1700 there was an earthquake with a magnitude of 9 in Cascadia and there has been no major earthquake since then. Rather, slow-sliding earthquakes, events that happen deeper and travel very short distances at a very slow pace, happen continuously.

“Usually, when an earthquake occurs, we find that the motion is in the opposite direction to how the plates moved, accumulating that slip deficit,” said Kevin P. Furlong, professor of geosciences, Penn State. “For these slow-slip earthquakes, the direction of motion is directly downward in the direction of gravity instead of the directions of motion of the plate.”

Researchers have found that areas in New Zealand, identified by other geologists, are sliding slowly like Cascadia.

“But there are subduction zones that don’t have these slow-sliding events, so we don’t have direct measurements of how the deeper part of the subduction plate moves,” Furlong said. “In Sumatra, the shallower seismic zone, as expected, is moving in the direction of plate movement, but even if there are no slow sliding events, the movement of the deeper plate seems to be controlled primarily by gravity.”

Slow-sliding earthquakes occur at a deeper depth than earthquakes that cause major damage and earthquakes, and researchers have looked at how this deep landslide can affect the timing and behavior of larger and more harmful public address earthquakes.

“Slow-slip earthquakes break in a matter of weeks, so they’re not just an event,” McKenzie said. “It’s like a swarm of events.”

According to the researchers, in the south of Cascadia, the general movement of the plate is about one centimeter of movement per year, and in the north of Vancouver Island, it is about 1.5 inches.

“We don’t know how much of the 30 millimeters (1 inch) a year accumulates to be released in the next big earthquake or if a movement is taken over by an unobservable process,” McKenzie said. “These slow-sliding events send out signals that we can see. We can observe slow-sliding events that go from east to west and not in the direction of plate movement.

Slow-slip events in Cascadia take place every two to two years, but geologists are wondering if one of them will trigger the next public address earthquake.

Researchers measure surface movement using permanent high-resolution surface GPS stations. The result is a pattern of loading and sliding steps during slow slip events. The events are visible on the surface, even though geologists know they are about 22 miles below the surface. They report their results in Geochemistry, Geophysics, Geosystems.

“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 us five years to figure out what these were, and then we needed a GPS that was accurate enough to effectively measure the movement of the Earth’s surface. Then we had to use modeling to convert the slip on the surface to the slip under the surface on the board. the border itself, which is larger. “

The researchers believe that understanding the effects of slow-sliding earthquakes in the region at these deeper depths will allow them to understand what could trigger the next megatrust earthquake in the area. Engineers want to know how strong the tremor will be in an earthquake, but they also want to know the direction of the forces. If the difference in direction of slow slip events indicates a potential change in behavior in a large event, that information would be helpful in planning.

“More fundamentally, we don’t know what triggers the big earthquake in this situation,” McKenzie said. “Every time we add new data about the physics of the problem, it becomes an important component. In the past, everyone thought events were one-way, but they could be 40 or 50 degrees different.”

While the slow events in Cascadia shed light on the potential megatrust earthquakes in the area and the tsunamis they can trigger, Furlong believes that other subduction areas may have similar patterns.

“I would argue that (differences in direction of movement) are happening in Alaska, Chile, Sumatra,” Furlong said. “Only in a few do we see evidence, but it may be a universal process that has been missed. Cascadia presents it because of slow-slip events, but it can be fundamental to subduction areas.”

Slow motion interplate slip detected in the Nankai Channel near Japan

More information:
KA McKenzie et al., Two-way loading of the subduction interface: evidence from the kinematics of slow slip events, Geochemistry, Geophysics, Geosystems (2020). DOI: 10.1029 / 2020GC008918

Provided by Pennsylvania State University

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