Megathrust earthquakes and tsunamis may be tied to deep, slow activity

Megathrust earthquakes and their tsunamis are some of the most destructive natural disasters in the world. A team of geoscientists led by Penn State proposes that the origin of the largest earthquakes and tsunamis can be traced to deep, gradual slow-slip behaviors beneath subduction zones.  

The team discovered that unlike the shallower megathrust earthquakes that move and generate energy in the same direction as the plates move, energy from slow-slip earthquakes may move in other directions, primarily down. “What we found was pretty unexpected,” said study co-author Kirsty A. McKenzie.

Subduction zones are created when two of the Earth’s plates meet and one moves beneath the other. This typically creates a fault line, as well as a distant line of volcanoes.

The Cascadia subduction zone is typical in that the tectonic plates meet near the Pacific coast and the Cascade Mountains, a volcanic range that includes Mount St. Helens. If it ruptured across its entire length, Cascadia could produce very large earthquakes.

According to the researchers, a megathrust earthquake of magnitude 9 occurred in Cascadia in 1700 and there has not been a large earthquake there since then. Instead, slow-slip earthquakes, which are events that happen deeper and move very short distances at a very slow rate, happen continuously.

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

The researchers found that there are areas in New Zealand, which have been identified by other geologists, that slow slip the same way Cascadia does.

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

The experts analyzed how deep, slow slips may affect the timing and behavior of megathrust earthquakes. “Slow-slip earthquakes rupture over several weeks, so they are not just one event,” said McKenzie. “It’s like a swarm of events.”

In southern Cascadia, the overall plate motion is about an inch of movement per year, according to the study. In the north by Vancouver Island, the annual rate of plate motion is about 1.5 inches.

“We don’t know how much of that 30 millimeters (1 inch) per year is accumulating to be released in the next big earthquake or if some movement is taken up by some non-observable process,” said McKenzie. “These slow-slip events put out signals we can see. We can observe the slow-slip events going east to west and not in the plate motion direction.”

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

“The reason we don’t know all that much about slow-slip earthquakes is they were only discovered about 20 years ago,” said Professor Furlong. “It took five years to figure out what they were and then we needed precise enough GPS to actually measure the motion on the Earth’s surface. Then we had to use modeling to convert the slip on the surface to the slip beneath the surface on the plate boundary itself, which is bigger.”

The experts believe that understanding the effects of slow-slip earthquakes in the region at these deeper depths will allow them to understand what could initiate the next megathrust earthquake in the area. Engineers want to know how strong an earthquake will be, but they also want to know the direction of its energy.

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

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

“I would argue that it (differences in direction of motion) is happening in Alaska, Chile, Sumatra,” said Professor Furlong. “It is only in a few that we see the evidence of it, but it may be a universal process that has been missed. Cascadia exhibits it because of the slow-slip events, but it may be fundamental to subduction zones.”

The study is published in the journal Geochemistry, Geophysics, Geosystems.

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By Chrissy Sexton, Earth.com Staff Writer