Volcanic tsunamis: Why they are so difficult to predict

Tonga's underwater explosion triggered a big mess of gas and debris.

The behavior of an earthquake-caused tsunami is familiar – and predictable.

A tsunami – like the one threatening the California coast Saturday –  that’s caused by a volcano? That’s a whole different story.

In both events, the underlying principle is the same: Water suddenly gets displaced, creating waves. The water has nowhere to go but … away.

But while an earthquake triggers sudden drama, a volcano-generated tsunami is a more complicated underwater production of underwater explosions, gas releases, exploding debris, lava flows and landslides.

The risk from a volcanic tsunami may also last longer. After an earthquake, underwater aftershocks may rattle for weeks. But after a volcano, rubble can continue careening into tranquil seas for months, even years. Saturday’s eruption occurred about 5:14 p.m. local time (8:14 p.m. Friday evening in California) in a Pacific Ocean volcano off the coast of Tonga, about 5,300 miles southwest of San Francisco. Waves created by the tsunami rolled into Northern California about 12 hours later, coinciding with high tide.

That’s why residents of Hawaii, Alaska and the U.S. Pacific coast were being asked Saturday to leave or avoid the coastline and be attentive to updated instructions from local emergency management officials.

“Volcanoes have a lot going on,” said Nathan Wood, supervisory research geographer with the USGS Western Geographic Science Center in Portland, Oregon. “There are more physical processes, which make it harder to really get a sense of how the water is going to behave – and what we’re going to see.

“A huge bubble of air comes up, catastrophically,” he said. “The ocean burps and that burp includes pebbles and ash.”

Volcanic tsunamis are relatively rare, with fewer than 100 recorded over the past two centuries. One of the most recent occurred in 2018, when a big chunk of Indonesia’s Anak Krakatau volcanic island collapsed into the ocean, swamping the lowlands of Java and Sumatra and killing about 400 people.

Most tsunamis are caused by earthquakes, not volcanos. In the United States most are triggered by the creation of new islands in Alaska’s Aleutian chain.

But on a planet that is two-thirds water – and seismically restless – many coastal populations are at risk.

“Severe tsunami hazards exist on all coastlines that face both the open sea and large and relatively young volcanic islands,” according to a 2003 journal article on mega-tsunamis by German scientists Franziska Whelan and Dieter Kelletat.

Forecasting volcanic eruption is a challenge. While there is a sophisticated global network of seismic stations to monitor earthquakes, and estimate average recurrence intervals, “volcanoes are a lot more unpredictable, especially when they’re quiet,” said Wood.

It’s especially tough when the volcano is underwater.

To monitor land-based volcanoes, sensors can be easily placed on the flank of a hill.

“But with underwater monitoring, that stuff isn’t at their disposal,” Wood said. “It’s very, very challenging.”

The National Tsunami Warning Center in Palmer, Alaska, mostly focuses on earthquakes. If its computers detect an underwater magnitude 7 earthquake, for instance, the system automatically goes into high alert.

But whether a tsunami is generated by a volcano or earthquake, its journey is monitored using identical tools.

Incoming waves are closely tracked by the National Tsunami Warning System as they spread across the ocean. Travel time is dependent on seafloor depth, and scientists know the seafloor depth for the entire earth. Therefore, once they know when and where a volcano erupts, they can generally predict when a tsunami will arrive at any given coastline.

The system has a $12-million network of 39 stations, called Deep-ocean Assessment and Reporting of Tsunamis (DART), on sea floors around the world. It was built in 1986 after a false tsunami alert in Hawaii and upgraded after the 2004 Indian Ocean tsunami, which killed more than 200,000 people.

DART’s deep underwater sensors, anchored to the ocean floor, sense the pressure changes of water, then send this information via soundwaves to a floating buoy. The buoy sends data, by satellite, to coastal warning centers.

Tonga suffered the most direct impact of Saturday’s eruption, with a sudden underwater explosion and shock waves, coupled with sea waves. On Saturday, water quickly flooded parts of the coastal area of the capital of Nuku’alofa on Tonga’s largest island, Tongatapu.

On the U.S. West Coast and Hawaii, rising waters came several hours later. They aren’t as high, or as powerful, said Wood. But they’re still dangerous, he added, especially in ports and harbors, due to physical damage to docks and piers.

“Waves can get trapped inside a harbor and bounce around, like a pinball machine, for several hours,” he said.