Strong earthquakes and tsunamis may be associated with deep and slow • Earth.com activity

Massive earthquakes and tsunamis are among the most devastating natural disasters in the world. A team of geologists led by Pennsylvania suggests that the origin of the largest earthquakes and tsunamis can be traced back to deep, gradual slow slide behaviors below subduction zones.

The team discovered that unlike massive, shallow earthquakes that move and generate energy in the same direction that the plates move, energy from slow-sliding earthquakes may move in other directions, mainly downwards. “What we found was quite unexpected,” said study co-author Kirsty A. McKenzie.

Subduction zones are created when two plates of Earth meet and one plate moves below the other. This usually creates a fault line, as well as a distant line of volcanoes.

The subduction zone is a typical Cascadia where tectonic plates meet near the Pacific coast and the Cascade Mountains, a volcanic chain that includes Mount St.Helens. If torn across its entire length, Cascadia can produce very large earthquakes.

According to the researchers, a massive 9-magnitude earthquake occurred in Cascadia in 1700 and there has been no major earthquake since then. Instead, slow-sliding earthquakes, which are events that occur deeper and move over very short distances at a very slow rate, are occurring continuously.

Professor Kevin B. Furlong: “Usually, when an earthquake occurs, we find that movement is in the opposite direction to how the plates move, leading to an 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 plate motion.”

The researchers found that there are areas in New Zealand that have been identified by other geologists that are slowing in sliding as the Cascadia does.

“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 subduction plate moves,” said Professor Furlong. “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.”

Experts analyzed how deep and slow slides might affect the timing and behavior of massive earthquakes. “Slow earthquakes erupt over several weeks, so it’s not just one single event,” said MacKenzie. “It’s like a swarm of events.”

In South Cascadia, the total movement of the plates is about an inch of movement per year, according to the study. In North Vancouver Island, the annual rate of plate movement is about 1.5 inches.

“We don’t know how much 30 millimeters (1 inch) per year would accumulate to be released in the next major earthquake or if some movement is taken by an unobserved process,” McKinsey said. These slow slide 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 plate movement. “

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

Professor Furlong said: “The reason we don’t know much about slow-slip earthquakes is that they were only detected about 20 years ago.” “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 board itself, which is bigger.”

Experts believe that understanding the effects of slow-sliding earthquakes in the region at these deep depths will allow them to understand what could trigger the next massive earthquake in the region. Engineers want to know how strong an earthquake is, but they also want to know the direction of its energy.

“Most importantly, we don’t know what triggered the major 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.”

Professor Furlong thinks other subduction zones may also have similar patterns.

Professor Furlong said, “I would argue that (differences in direction of movement) occur in Alaska, Chile and Sumatra.” “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.”

The study has been published in Geochemistry, Geophysics, Geosystems.

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Written by Chrissy Sexton, Earth.com Staff Writer

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