How powerful earthquakes can trigger deadly, destructive tsunamis

After an earthquake shakes a region, it’s not uncommon for scientists to issue tsunami watches or warnings in an area that could face an additional threat from yet another devastating natural phenomenon.

Subduction-zone megathrust earthquakes, which are the most powerful on Earth, can generate large and deadly tsunamis, according to the Incorporated Research Institutions for Seismology.

While not all earthquakes are strong enough to trigger a massive wave of ocean water rushing onto land, it has occurred on several occasions.

In December 2004, an extreme megathrust, 9.1 magnitude earthquake struck near Sumatra, Indonesia, triggering a tsunami that killed an estimated 230,000 people.

The magnitude 9 Tōhoku earthquake shook Honshu, Japan, in March 2011, causing a devastating tsunami that claimed at least 15,800 lives and led to the Fukushima nuclear disaster.

How earthquakes trigger tsunamis

“If you suddenly uplift or subside a large piece of the ocean floor, just by a meter or two or more, you displace the water faster than it can respond to gravity and go back to sea level,” said Dr. Charles Ammon, professor of geosciences at The Pennsylvania State University.

The process of restoring sea level begins a set of oscillations, exciting the tsunami waves that propagate outward from the area that was uplifted, Ammon said.

“You need an earthquake that’s going to deform the sea floor in a vertical direction, either up or down, over a fairly large region,” he added.

Some fault ruptures are mostly horizontal in nature, as is the case with strike-slip earthquakes, while others are vertical, including earthquakes triggered by normal and thrust faulting, according to AccuWeather Meteorologist James Andrews.

“It is the latter kind that, when triggered underwater, are most likely to displace the ocean to trigger tsunamis,” Andrews said.

Importance of depth, location and magnitude

Andrews added that focal depth, which is how deep the actual earthquake rupture is, can be important, as earthquakes very deep in the earth tend to cause less focused displacement of the seafloor.

Shallow earthquakes create the most dangerous tsunamis, said Cindi Preller, geologist and duty scientist for the Pacific Tsunami Warning Center.

“We stop worrying when an earthquake nears 100 kilometers deep,” Preller said. “An event sourcing that deep does not displace much rock on the surface, and the energy is absorbed inside the earth.”

The Pacific region is known as a hot spot for tsunamis due to the high number of large earthquakes associated with subduction zones that occur along the Pacific basin.

This region is known as the Ring of Fire, where 90 percent of the world’s earthquakes are located, according to the Pacific Tsunami Museum.

Subduction zones near areas including Japan, Chile and Indonesia have produced massive earthquakes which triggered destructive, deadly tsunamis.

Earthquakes that strike near coastlines are both the most hazardous and effective at generating a tsunami, according to Preller, because a temblor that occurs far inland wouldn’t have a significant impact on the water and one that rattles too far offshore diminishes seafloor displacement.

Tsunamis are most often triggered by 7.0 magnitude earthquakes or greater, Preller said.

“That’s when we worry,” she said. “[Magnitudes 8 and greater] are the ones that can transfer enough energy into the ocean to really cause some damage.”

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