On Jan. 15, 2022, coastal areas across California were placed under a tsunami warning. Gado via Getty Images
On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai volcano in Tonga erupted, sending a tsunami racing across the Pacific Ocean in all directions.
As word of the eruption spread, government agencies on surrounding islands and in places as far away as New Zealand, Japan and even the U.S. West Coast issued tsunami warnings. Only about 12 hours after the initial eruption, tsunami waves a few feet tall hit California shorelines – more than 5,000 miles away from the eruption.
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I’m a physical oceanographer who studies waves and turbulent mixing in the ocean. Tsunamis are one of my favorite topics to teach my students because the physics of how they move through oceans is so simple and elegant.
Waves that are a few feet tall hitting a beach in California might not sound like the destructive waves the term calls to mind, nor what you see in footage of tragic tsunamis from the past. But tsunamis are not normal waves, no matter the size. So how are tsunamis different from other ocean waves? What generates them? How do they travel so fast? And why are they so destructive?
When the Hunga Tonga-Hunga Ha'apai volcano erupted, it launched ash into the atmosphere, created a powerful shock wave and displaced a huge amount of water, generating a tsunami that raced across the ocean. Japan Meteorological Agency via WikimediaCommons, CC BY
Deep displacement
Most waves are generated by wind as it blows over the ocean’s surface, transferring energy to and displacing the water. This process creates the waves you see at the beach every day.
Tsunamis are created by an entirely different mechanism. When an underwater earthquake, volcanic eruption or landslide displaces a large amount of water, that energy has to go somewhere – so it generates a series of waves. Unlike wind-driven waves where the energy is confined to the upper layer of the ocean, the energy in a series of tsunami waves extends throughout the entire depth of the ocean. Additionally, a lot more water is displaced than in a wind-driven wave.
Imagine the difference in the waves that are created if you were to blow on the surface of a swimming pool compared to the waves that are created when someone jumps in with a big cannonball dive. The cannonball dive displaces a lot more water than blowing on the surface, so it creates a much bigger set of waves.
Earthquakes can easily move huge amounts of water and cause dangerous tsunamis. Same with large undersea landslides. In the case of the Tonga tsunami, the massive explosion of the volcano displaced the water. Some scientists are speculating that the eruption also caused an undersea landslide that contributed to the large amount of displaced water. Future research will help confirm whether this is true or not.
Propagation of the March 4, 2021 Kermadec Islands tsunami was computed with the NOAA forecast method using MOST model via inversion of the source with NOAA's TFS (Tsunami Forecast System). From the NOAA Center for Tsunami Research, located at NOAA PMEL in Seattle, WA. See https://nctr.pmel.noaa.gov/kermadec20210304/ #TsunamiPropagation
NOAA NCTR experimental research product. Not an official forecast.
Tsunami waves travel fast
No matter the cause of a tsunami, after the water is displaced, waves propagate outward in all directions – similarly to when a stone is thrown into a serene pond.
Because the energy in tsunami waves reaches all the way to the bottom of the ocean, the depth of the sea floor is the primary factor that determines how fast they move. Calculating the speed of a tsunami is actually quite simple. You just multiply the depth of the ocean – 13,000 feet (4,000 meters) on average – by gravity and take the square root. Doing this, you get an average speed of about 440 miles per hour (700 kilometers per hour). This is much faster than the speed of typical waves, which can range from about 10 to 30 mph (15 to 50 kph).
This equation is what oceanographers use to estimate when a tsunami will reach faraway shores. The tsunami on Jan. 15 hit Santa Cruz, California, 12 hours and 12 minutes after the initial eruption in Tonga. Santa Cruz is 5,280 miles (8,528 kilometers) from Tonga, which means that the tsunami traveled at 433 mph (697 kph) – nearly identical to the speed estimate calculated using the ocean’s average depth.
Many tsunamis, including the 2011 Tsunami in Japan, move inland and can flood areas far from the coast. U.S. Air Force photo/Staff Sgt. Samuel Morse via WikimediaCommons
Destruction on land
Tsunamis are rare compared to ubiquitous wind-driven waves, but they are often much more destructive. The 2004 Indian Ocean tsunami killed 225,000 people. More than 20,000 lost their lives in the 2011 Japan tsunami.
What makes tsunamis so much more destructive than normal waves?
As waves approach shore, they get pushed upward by the rising seafloor. Régis Lachaume via Wikimedia Commons, CC BY-SA
In the open ocean, tsunami waves can be small and may even be undetectable by a boat at the surface. But as the tsunami approaches land, the ocean gets progressively shallower and all the wave energy that extended thousands of feet to the bottom of the deep ocean gets compressed. The displaced water needs to go somewhere. The only place to go is up, so the waves get taller and taller as they approach shore.
When tsunamis get to shore, they often do not crest and break like a typical ocean wave. Instead, they are more like a large wall of water that can inundate land near the coast. It is as if sea level were to suddenly rise by a few feet or more. This can cause flooding and very strong currents that can easily sweep people, cars and buildings away.
Luckily, tsunamis are rare and not nearly as much of a surprise as they once were. There is now an extensive array of bottom pressure sensors, called DART buoys, that can sense a tsunami wave and allow government agencies to send warnings prior to the arrival of the tsunami.
If you live near a coast – especially on the Pacific Ocean where the vast majority of tsunamis occur – be sure to know your tsunami escape route for getting to higher ground, and listen to tsunami warnings if you receive one.
The eruption of the Hunga Tonga-Hunga Ha’apai volcano severed the main communication cable that connects the people of Tonga to the rest of the world. While the science of tsunamis can be fascinating, these are serious natural disasters. Only a few deaths have been reported so far from Tonga, but many people are missing and the true extent of the damage from the tsunami is still unknown.
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Sally Warner has received funding from the National Science Foundation and the Office of Naval Research.
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Why the Tonga volcano eruption was so big and what's next
The Tonga volcano eruption, explained
People around the world looked on in awe at the spectacular satellite images of an undersea volcano erupting in a giant mushroom cloud in the Pacific. Many wondered why the blast was so big, how the resulting tsunami traveled so far, and what will happen next.
New Zealand scientists Shane Cronin, a volcanology professor at the University of Auckland, and Emily Lane, a tsunami expert at the National Institute of Water and Atmospheric Research, help explain.
Photo: In this photo provided by the New Zealand Defense Force, volcanic ash covers roof tops and vegetation in an area of Tonga, Monday, Jan. 17, 2022. Thick ash on an airport runway was delaying aid deliveries to the Pacific island nation of Tonga, where significant damage was being reported days after a huge undersea volcanic eruption and tsunami.
Explosive but brief
The eruption on Saturday was incredibly explosive but also relatively brief. The plume rose into the air more than 30 kilometers (19 miles) but the eruption lasted only about 10 minutes, unlike some big eruptions that can continue for hours. Cronin said the power of the eruption of the Hunga Tonga Hunga Ha’apai volcano ranks among the world's biggest over the past 30 years, and the height of the plume of ash, steam and gas was comparable with the huge 1991 eruption of Mount Pinatubo in the Philippines, which killed several hundred people.
Photo: This satellite image provided by Maxar Technologies shows an overview of Hunga Tonga Hunga Ha'apai volcano in Tonga on Jan. 6, 2022, before a huge undersea volcanic eruption.
Why so big?
The magma inside the volcano was under enormous pressure and had gasses trapped within it. A fracture in the rock likely induced a sudden drop in pressure, allowing the gas to expand and blast the magma apart. Cronin said the crater was sitting about 200 meters (650 feet) below the sea surface, a kind of Goldilocks depth for a big explosion in which seawater pours into the volcano and turns instantly into steam, adding to the rapid expansion and energy of the explosion. Any deeper and the extra pressure of the water would have helped contain the eruption.
Photo: In this satellite photo taken by Planet Labs PBC, an island created by the underwater Hunga Tonga Hunga Ha'apai volcano is seen smoking Jan. 7, 2022.
Farflung tsunami
Many scientists were surprised that a single eruption could produce a Pacific-wide tsunami of about 1 meter (3 feet) that smashed boats in New Zealand and caused an oil spill and two drownings in Peru. Lane said that oceanwide tsunamis are usually triggered by earthquakes that extend across huge areas rather than from a single volcano, essentially a tiny dot in the ocean. She said other factors may have been at play, such as an underwater flank of the volcano collapsing and displacing water. She said one interesting theory is that the shock wave, or sonic boom, from the volcano that traveled twice around the world may have pumped more power into the tsunami waves.
Photo: Oil pollutes Cavero beach in Ventanilla, Callao, Peru, Tuesday, Jan. 18, 2022, after high waves attributed to the eruption of an undersea volcano in Tonga caused an oil spill. The Peruvian Civil Defense Institute said in a press release that a ship was loading oil into La Pampilla refinery on the Pacific coast on Sunday when strong waves moved the boat and caused the spill.
Tonga mainly spared
Another mystery is why the tsunami wasn't bigger and more destructive in Tonga, which sits almost on top of the volcano.
“That's the million dollar question,” Cronin said. “Looking at the images so far, the level of devastation is less than I was fearing.”
Authorities by Wednesday had confirmed three deaths in Tonga, with concerns remaining about people on some of the hard-hit smaller islands. Dozens of homes were destroyed.
Lane said that Tongans at least got some warning, both from the increased activity at the volcano the day before the eruption and from the incredibly loud bang when it erupted but before the tsunami hit, allowing many to scramble to higher ground. She said reefs, lagoons and other natural features may also have protected parts of Tonga, while amplifying the waves in certain areas.
Photo: In this photo provided by the New Zealand Defense Force, volcanic ash covers roof tops and vegetation in an area of Tonga, Monday, Jan. 17, 2022. Thick ash on an airport runway was delaying aid deliveries to the Pacific island nation of Tonga, where significant damage was being reported days after a huge undersea volcanic eruption and tsunami.
Falling
The ash that has coated Tonga is acidic but not poisonous, Cronin said. Indeed, he has been advising Pacific responders that people may still be able to drink from their rainwater supplies even if some ash has fallen in, which will make the water more acidic and salty. He said it was a question of applying the taste test and if water became scarce, it would be better drinking ash-tainted water than stagnant water that might be contaminated with bacteria. New Zealand and other nations are trying to get water and other supplies to Tonga as quickly as possible. Cronin said all of Tonga's soil comes from volcanic ash and the latest dump of ash would quickly wash into the ground and make the nation more fertile.
Photo: In this photo provided by the New Zealand Defense Force, an Orion plane flies over Tonga where volcanic ash covers roof tops and vegetation, Monday, Jan. 17, 2022. Thick ash on an airport runway was delaying aid deliveries to the Pacific island nation of Tonga, where significant damage was being reported days after a huge undersea volcanic eruption and tsunami.
No global cooling
Huge volcanic eruptions can sometimes cause temporary global cooling as sulfur dioxide is pumped into the stratosphere. But in the case of the Tonga eruption, initial satellite measurements indicated the amount of sulfur dioxide released would only have a tiny effect of perhaps 0.01 degree Celsius (0.02 Fahrenheit) global average cooling, said Alan Robock, a professor at Rutgers University.
Photo: In this photo provided by the New Zealand Defense Force, a crew member from an Orion aircraft looks out the window as it flies over an area of Tonga that has heavy ash fall from a volcanic eruption in an area of Tonga, Monday, Jan. 17, 2022. Thick ash on an airport runway was delaying aid deliveries to the Pacific island nation of Tonga, where significant damage was being reported days after a huge undersea volcanic eruption and tsunami.
What's next
Cronin envisions two main scenarios for the volcano. The first is that it has exhausted itself for now and will go quiet for the next 10 to 20 years as magma slowly returns. A second scenario is that new magma rises up quickly to replace that which exploded, in which case there might be ongoing eruptions. But he believes the cracks and rifts caused by Saturday's big explosion will allow more gas to escape, and subsequent eruptions won't be as big, at least for now. Both Cronin and Lane agree there needs to be much better monitoring of the volcano — and others in Tonga — to help better predict future events.
Photo: This combination of satellite images taken by Himawari-8, a Japanese weather satellite operated by Japan Meteorological Agency and released by National Institute of Information and Communications Technology (NICT), shows an undersea volcano eruption of the Pacific nation of Tonga Saturday, Jan. 15, 2022.
Associated Press Science Writer Seth Borenstein in Kensington, Maryland, contributed to this report.
