Why are volcanic tsunamis difficult to predict?

Volcanic tsunamis, like the one in Tonga on January 15, baffle scientists because of their complexity.

The behavior of an earthquake-induced tsunami is relatively familiar and predictable. However, the type triggered by volcanoes, like the tsunami that hit Tonga on January 15, is elusive.

In both phenomena, the basic principle is the same: The water is suddenly displaced, creating waves. Water has no choice but to spread. However, while earthquakes cause sudden transformations, tsunamis that originate from volcanoes are a more complex underwater process with underground explosions, released gases, exploding debris, currents, and more. lava and landslides.

Risks from volcanic tsunamis may also last longer. After an earthquake, underwater aftershocks can continue to shake for weeks. But after a volcanic eruption, tremors can continue to stir calm seas for months or even years.

The eruption on January 14 occurred due to an underground volcano off the coast of Tonga, about 8,500km southwest of San Francisco, USA. Large waves reached Northern California about 12 hours later, coinciding with high tides. This is why people in Hawaii, Alaska and the Pacific coast in the US are being asked to leave or stay away from the coast and to heed new guidance from local authorities.

The tsunami hit Tongatapu, Tonga's largest island.

"There's a lot of things going on with volcanoes. More physical processes so it's hard for us to really know how water will behave and what we're going to see," said Nathan Wood, geographer. Researcher at the US Geological Survey's (USGS) Western Center for Geosciences.

"A huge bubble of gas came up, it was catastrophic. The ocean 'burps' and that belch contains both rocks and ash," he described.

Tsunamis caused by volcanoes are relatively rare, with less than 100 cases recorded over the past two centuries. One of the most recent disasters took place in 2018, when a large plate of the volcanic island of Anak Krakatau, Indonesia, collapsed into the sea, flooding the lowlands of Java and Sumatra, killing around 400 people.

Most tsunamis are caused by earthquakes, not volcanoes. In the US, most are triggered by the formation of new islands in the Aleutian chain of islands in Alaska. However, on a planet with two-thirds of its surface covered by water and constantly seismic, many coastal populations would be at risk.

Serious tsunami hazards exist on all coastlines facing open water and large, relatively young volcanic islands, according to a 2003 study published in the journal Super-Tsunami by German scientists Franziska. Whelan and Dieter Kelletat.

Forecasting volcanic eruptions is a challenge. While experts have a complex global network of seismic stations to monitor earthquakes and estimate average recurrence times, volcanoes are much more difficult to predict, especially when they are quiet. Wood.

Things get especially difficult when the volcano is underwater. To track volcanoes on land, scientists can place sensors on the mountainside. "But you can't use those devices for underwater surveillance. The job is really challenging," Wood said.

Whether caused by volcanoes or earthquakes, the path of a tsunami is tracked using the same tools. In the US, waves are closely monitored by the National Tsunami Warning System as they spread over the ocean. Travel time depends on the depth of the seabed and scientists know this data. As a result, they can often predict when a tsunami will hit a certain coast, but only after knowing when and where the volcano erupts.

The US National Tsunami Warning System includes a network of 39 stations, known as Deep Ocean Tsunami Assessment and Reporting (DART), worth $12 million, on the seabeds around the world. The system was built in 1986 following a false tsunami warning in Hawaii and upgraded after the 2004 Indian Ocean tsunami that killed more than 200,000 people.

DART's underwater sensors are anchored on the ocean floor, sensing changes in water pressure, then sending this information by sound waves to a buoy. The buoy sends data to coastal warning centers via satellite.

Tonga was most directly impacted by the new volcanic eruption. On January 15, water quickly flooded coastal areas of the capital Nuku'alofa in Tongatapu, Tonga's largest island. In the west coast of the US and Hawaii, the water rose a few hours later. They are not as tall and strong, but still dangerous, Wood said, especially in the harbor.