Why did such a powerful earthquake produce such a weak tsunami?

Early Wednesday, a powerful earthquake, one of the largest ever recorded, struck off the coast of the Kamchatka Peninsula in Russia. Tsunami centers along the Pacific coastline quickly sprung into action, issuing warnings for the potential of colossal ocean waves that are often generated by big earthquakes.

But later in the morning, the centers began to downgrade or retire those warnings in some places, including parts of California, Hawaii, China and Japan, as the big waves failed to appear. What happened?

According to Diego Melgar, a geophysicist at the University of Oregon, part of the reason the tsunamis were weaker than anticipated may have to do with the size of the earthquake. “There’s big,” he said. “And then there’s really, really, really big.”

The most recent event had an estimated magnitude of 8.7 or 8.8 on the scale scientists use to measure the strength of earthquakes, Melgar said. By contrast, catastrophic tsunamis in the past, including a wave that struck Indonesia in 2004 and another that hit Japan in 2011, were about a magnitude 9.

That may sound comparable to Wednesday’s quake, Melgar said, but it is significantly bigger. That’s because the earthquake scale is logarithmic: A magnitude 9 event possesses about 10 times as much energy as a magnitude 8.7 event, and about three times as much energy as a magnitude 8.8 event.

The earthquake Wednesday occurred along a subduction zone, where one of Earth’s tectonic plates slides under another. This can cause the seafloor to move up and down, creating a wave that propagates across the ocean.

According to Melgar, current models suggest that Wednesday’s earthquake occurred across a stretch of seafloor that was hundreds of miles long. The longer the quake, the more energetic the tsunami can be, he said.

A larger earthquake typically generates a larger wave. But the wave size depends on smaller details of the quake, like the depth of the motion at different parts along the line where the two tectonic plates meet.

“Not all earthquakes are created equal,” Melgar said. “We’re still untangling the details. It’s going to take weeks to months of research to figure out exactly what happened.”

More likely than not, this earthquake just wasn’t big enough to create a catastrophic wave. “Don’t get me wrong, it’s still huge,” he said. “But those in 2004 and 2011 were behemoths.”

Current estimates of the wave height along the U.S. coastline vary by location. Alaska was mostly spared, and parts of California experienced waves around 3 feet (91 centimeters) high, according to Vasily Titov, a scientist at the National Oceanic and Atmospheric Administration who simulates tsunamis. Hawaii saw slightly higher waves.

That has generated flooding, but so far, no reports of casualties.

But the tsunami was not weak everywhere. Along the coast of Kamchatka, it was “a huge tsunami and mega-event,” Titov said, possibly comparable to the wave that struck Japan in 2011, which reached heights of nearly 130 feet (39 meters). NOAA’s tsunami models are not well-tuned in Kamchatka, which is far less populated.

“We got lucky that this energy did not go directly to U.S. coastlines,” Titov said, referring to the strength of the wave. The coast of Chile remains on alert.

Tsunamis are different from typical ocean waves, which are shallower ripples created by the wind. Tsunamis, by contrast, are generated deeper down and are more like the waves produced if you submerge your hand in water and move it up and down.

Wednesday’s event was a bit poetic, Melgar said: An earthquake struck in the same place in 1952. That triggered a 12-foot-high wave that reached the coast of Hawaii with little warning. This time, the network of warning centers in the Pacific Ocean meant the world was more prepared.

“It’s kind of a better-safe-than-sorry situation,” Melgar said. “Warnings went out. This is a big success.”

This article originally appeared in The New York Times
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