A volcanic eruption in Tonga on 15 January 2022 surprised scientists by triggering two types of tsunamis: “classic” tsunamis caused by the displacement of large volumes of water, and meteotsunamis caused by fast-moving pressure disturbances in the atmosphere.
In depth: Surprising tsunamis caused by explosive eruption in Tonga
On 15 January at about 5 p.m. local time in the South Pacific Kingdom of Tonga, an undersea volcano called Hunga Tonga-Hunga Haʻapai began to erupt violently, producing massive explosions and sending gas and ash 36 miles into the atmosphere. Tsunamis triggered by the eruption swept onto Tongan islands, reaching as high as 65 feet above sea level, destroying hundreds of structures, displacing more than 1,500 people, and causing four deaths. Tsunami waves also struck shores around the Pacific, leading to two deaths and an oil spill in Peru as well as damage in Fiji, Hawaii, Chile, California, Japan, Russia, and New Zealand.
“This was an unusual event that could have taken many more lives,” said USGS research geophysicist Eric Geist. “The relatively low death toll is thanks to progress countries around the world have made in anticipating and preparing for tsunamis.”
Activity at the volcano had waxed and waned since late December 2021. When it intensified on 14 January, the Tonga Meteorological Services issued tsunami warnings and cautioned residents to stay away from low-lying areas, beaches, and harbors. These warnings, along with Tonga’s long-standing practice of running tsunami drills, likely saved countless lives.
“Global tsunami-preparedness has come a long way,” said Geist, who models the physics of tsunami generation and works on hazard assessments to help communities plan for tsunamis. “But each big tsunami surprises us with something new.”
Early arrivals in Japan
The tsunamis generated by the 15 January eruption provided plenty of surprises. One of them occurred in Japan. Based on the time of the eruption, its distance from Japan, and the average speed of tsunami waves in the Pacific Ocean—between 450 and 500 miles per hour, comparable to the speed of a commercial airliner—the first tsunami waves should have reached Japan’s Osagawara islands around 10:30 p.m. local time. But waves about 1 foot high were observed there around 8:20 p.m., more than 2 hours early. Similar early waves were recorded at many stations along the country’s Pacific coast.
What scientists were expecting in Japan was a classic tsunami—the type triggered by displacement of a massive volume of seawater at the tsunami’s source, in this case, the Hunga Tonga-Hunga Haʻapai volcano. But the first waves to arrive were not from a classic tsunami. These waves of water had been pushed across the Pacific at about 700 mph by waves of fluctuating air pressure in the atmosphere, set off by the eruption’s explosions.
Atmospheric waves that created water waves
The fastest of these atmospheric waves were a type of pressure wave called a Lamb wave. Strong Lamb waves in the atmosphere are uncommon and are usually associated with events that release tremendous amounts of energy. Examples include the 1883 eruption of Krakatau in Indonesia, the 1908 impact of a meteorite in Siberia, and the testing of large nuclear devices. Lamb waves travel at the speed of sound, which varies with the temperature of the air they pass through. The first Lamb waves to radiate away from the eruption traveled about 710 mph, similar to the speed of Lamb waves produced during the 1883 Krakatau eruption. Like those 1883 waves, the Lamb waves triggered by the recent eruption in Tonga surrounded the globe and eventually circled it three-and-a-half times.
Close behind the Lamb waves were atmospheric gravity waves. The term “gravity wave” may be unfamiliar to many readers, but it applies to a familiar phenomenon—the waves you see in the ocean are gravity waves. Gravity waves in the ocean move along the boundary between water and air, while gravity waves in the atmosphere move along a boundary between air masses of differing density. In contrast to Lamb waves, gravity waves are common in the atmosphere; many people have experienced them as clear-air turbulence during air travel. Atmospheric gravity waves move at a wide range of speeds. The first ones to ripple away from the Tonga eruption traveled about 540 to 600 mph.
Sensitive instruments around the world recorded the passage of the atmospheric Lamb waves and gravity waves as sudden fluctuations in air pressure. As the atmospheric waves moved over the oceans, the pressure changes associated with them induced the formation of water waves that Geist and his colleagues call meteotsunamis—tsunamis caused by meteorological conditions or, more generally, by events in the atmosphere. Like classic tsunamis, meteotsunamis travel through deep water as a series of low waves with very long wavelengths, up to hundreds of miles long. As they move into ever-shallower water, the waves shorten and become higher, resulting in potentially dangerous water levels by the time they flow onto a coast.
Birth of a meteotsunami
So how do atmospheric waves create meteotsunamis? “A sharp increase in air pressure will push the sea surface down and vice versa,” said Geist. “One millibar of pressure change will generate about 1 centimeter [about 3/8 inch] of up or down change in the ocean’s surface,” creating water waves that oscillate from 1 centimeter above to 1 centimeter below sea level. Geist uses the word “coupled” to describe water waves created and pushed along by atmospheric waves.
“One or two centimeters isn’t that big,” Geist said, “but if the atmospheric disturbance—it could be a line of squalls or, in this case, atmospheric waves triggered by the eruption—if it’s moving close to the speed that tsunami waves would naturally travel, resonance effects will amplify the coupled water waves and make them much bigger.”
Scientists call this phenomenon “Proudman resonance.” It makes coupled water waves grow rapidly when the speed of the atmospheric wave matches the natural speed of a tsunami wave. Even if the match is not perfect, some amplification will occur. How did it work during the January eruption?
Speed and water depth are keys to resonance
The speed that tsunami waves naturally move through the ocean depends on depth: the deeper the water, the faster the tsunami waves. For 700 mph atmospheric Lamb waves to build up large tsunami waves through resonance, you need depths in which tsunami waves would travel near 700 mph. That requires water about 6 miles deep. For the somewhat slower atmospheric gravity waves to build up significant tsunami waves would require water depths of around 4 miles.
The average depth of the Pacific Ocean is only 2½ miles, so the water waves moving along with both types of atmospheric waves stayed almost imperceptibly low as they traveled across the Pacific toward Japan. But as the coupled ocean and atmospheric waves got closer to Japan, they began traveling over much deeper water—the Mariana Trench off Japan is almost 7 miles deep. Scientists hypothesize that as the waves swept over the deep ocean near trenches, resonance between the atmospheric waves and the ocean was sufficient to amplify the water waves and increase their height.
When these types of amplified ocean waves encounter shallow water at the edge of the continental shelf, typically less than 500 feet deep, they will slow down while the atmospheric waves race ahead. According to Geist, “This separation, or ‘decoupling,’ will further amplify the ocean waves that the atmospheric waves have left behind, and complex mechanisms associated with the decoupling will generate even more waves.” That’s likely what happened off Japan, creating the array of meteotsunami waves that surprised scientists with their unexpected arrival at tide gauges.
Geist and his colleagues think something similar happened much closer to Tonga, when atmospheric waves moving eastward from the eruption passed over the Tonga Trench. The water was deep enough in the trench (more than 6 miles deep) to create resonance with the speeding atmospheric waves and amplify the height of the coupled water waves. Decoupling of the water waves from the atmospheric waves took place where the water shallowed east of the trench. The water waves that were amplified and decoupled at the Tonga Trench continued toward South America, now at the same speed as a classic tsunami (about 450 mph). Meanwhile, the atmospheric waves kept moving eastward at nearly 700 mph, pushing along coupled water waves and producing new sets of amplified and decoupled water waves as they passed over the Peru-Chile Trench off South America.
“Meteotsunamis definitely complicated the picture,” said Geist. Researchers are still sorting out those complications. In a paper published recently in Nature, Professor Patrick Lynett of the University of Southern California and his coauthors, including five from the USGS Pacific Coastal and Marine Science Center, used instrumental data, field observations, and modeling to unravel the mechanisms that produced tsunamis—particularly meteotsunamis—during the Tonga eruption. In their concluding paragraphs they paint a vivid picture:
As the air-pressure pulse passed over the deep-water subduction zone trenches of the Pacific Ocean, . . . Proudman resonance [induced] a phenomenon where every major trench in the Pacific Basin effectively generated a small tsunami . . . The result was tsunami energy generated from the entire extent of the Pacific Rim over the course of 12 hours, leaving the Pacific Ocean filled with tsunamis traveling in all directions, and causing the unique persistence of this event observed along coastlines and harbors throughout the Pacific.
All these meteotsunamis were unexpected because, as Geist noted, “Volcanic eruptions rarely produce them.” He recalled the 2015 eruption of Calbuco, a stratovolcano in Chile. Satellite imagery captured atmospheric gravity waves radiating out from the eruption, “but we did not see any associated meteotsunamis.” For a volcano to produce meteotsunamis seems to require a hugely explosive eruption, like the recent one in Tonga. “Before that,” said Geist, “you have to go back to the 1883 eruption of Krakatau in Indonesia.” That event produced meteotsunamis on tide gauges as far away as Honolulu, San Francisco, Panama, the island of South Georgia in the southern Atlantic Ocean, and the English Channel.
In contrast to volcanic meteotsunamis, weather-generated meteotsunamis occur fairly frequently. In some parts of the world they are so common that they have local names, such as “rissaga” in Spain’s Balearic Islands, “šćiga” in Croatia, and “abiki” in Japan. In the U.S., meteotsunamis have occurred on the East Coast, Gulf Coast, Great Lakes, and Pacific Northwest. A combination of variables such as geography, weather patterns, and the size, shape, and depth of the water body determine their likelihood.
Many meteotsunamis are too small to notice, but some have been large enough to cause considerable damage and even deaths. In 1954, for example, a meteotsunami in Lake Michigan swept several people off piers near Chicago, Illinois, leading to seven deaths. In 2008, a meteotsunami in Boothbay Harbor, Maine, emptied and flooded the harbor at least three times over 15 minutes, damaging boats and structures on shore. And in 1992, a 10-foot meteotsunami wave crashed onto the shore shortly before midnight at Daytona Beach, Florida, injuring 75 people, damaging 100 vehicles, and causing property damage.
Tide gauges captured January’s volcanic meteotsunamis
The meteotsunamis caused by the January 15 eruption were mostly small, detectable mainly in tide gauge records. In a graphic provided by NOAA NOS Senior Scientist Greg Dusek, for example, you can see a wave about 20 centimeters (8 inches) high in the upper graph. This water wave arrived at the tide gauge in Port San Luis, California, just after the arrival of the first atmospheric wave (marked by a sudden spike in barometric pressure, lower graph). A few hours later you see the arrival of larger (up to about 2.5 meters [8 feet]) waves at the time the classic (“predicted”) tsunami was expected. Dusek noted that early waves were observed at many Pacific tide gauges.
Meteotsunamis generated by the eruption’s atmospheric waves were observed all around the globe. At a tide gauge in Mayagüez, Puerto Rico, a water level change of about 10 centimeters (4 inches) occurred at noon local time, just minutes after a spike in barometric pressure that marked the arrival of the first atmospheric wave. The timing can be seen in the graphs below.
In the Mediterranean Sea, water levels rose nearly a foot on the coast of Spain, and as much as a foot and a half in the Balearic Islands (an area prone to weather-generated meteotsunamis). Small meteotsunami waves, ranging from a few inches to about a foot high, were also evident on tide gauge records in the Indian Ocean.
The meteotsunamis caused by the eruption were a surprise, but the classic tsunamis—the ones that must have been produced by a large displacement of seawater at the eruption site—were nearly as puzzling.
“We are more accustomed to thinking about earthquake-generated tsunamis, which account for about 70 to 80 percent of the tsunamis that have been recorded,” said Geist. Earthquakes trigger tsunamis by causing a sudden vertical shift in the seafloor that displaces all the water above it. The energy of this disturbance radiates away from the source as tsunami waves. Seismic stations around the globe provide copious data about large earthquakes, enabling scientists to deduce the likely changes to the seafloor and model the resulting tsunami.
“We have less experience with volcano-generated tsunamis, and less data about them,” said Geist. “They are interesting because there are a number of different mechanisms that can cause them.” Scientists are still trying to figure out exactly what displaced enough seawater in the 15 January eruption to trigger the large tsunamis that inundated Tongan islands. Was it the underwater part of the eruptive blast? A collapse of the caldera? An underwater landslide on the flank of the volcano?
There was likely more than one cause, as evidenced by an eyewitness account from Tongatapu—the largest island in the Kingdom of Tonga and about 40 miles south of Hunga Tonga-Hunga Haʻapai. The managers of a resort on the island’s northwest tip were barely able to get their guests safely off the property before they themselves had to flee from waves flowing into the resort. The guests had time to drive away, but the managers had to leave their vehicles and escape by foot out the back. The resort managers reported that the waves arrived many minutes before they heard the eruption’s first major explosion, and before much larger tsunami waves washed right over the peninsula. Did an undersea landslide trigger the early waves? Did some larger displacement associated with the main explosive episode of the eruption create the much bigger waves? “One of the things that’s really going to help,” said Geist, “is when somebody gets a seafloor survey out there.”
Seafloor surveys point to pyroclastic flows
As of this writing (early August), two seafloor surveys have been completed and a third is underway. The Tonga Eruption Seabed Mapping Project (TESMaP) began in early April when the 230-foot research vessel Tangaroa set sail for Tonga. Owned and operated by New Zealand’s National Institute of Water and Atmospheric Research (NIWA), the Tangaroa spent a month mapping 8,500 square miles of seafloor around Hunga Tonga-Hunga Haʻapai. The researchers expected to find the volcano obliterated by the explosive eruption, but their mapping showed it to be largely intact. In an area of about 3,000 square miles surrounding the volcano, however, they found dramatic changes. NIWA marine geologist Kevin Mackay, who led the expedition, reported in a media release issued on 23 May that they had observed “fine sandy mud and deep ash ripples as far as 50 kilometers [30 miles] away from the volcano, with gouged valleys and huge piles of sediment.” Tonga’s domestic internet cable, severed during the eruption, was covered by 100 feet of sediment and ash. The scientists recorded up to one and two-thirds cubic miles of displaced material—the equivalent of 3 million Olympic-sized swimming pools—and think there is likely more to be seen.
A second seafloor survey was conducted from 17-19 May by University of Auckland professor Shane Cronin and a team from Tonga Geological Services (TGS). Unlike the Tangaroa survey, which mapped up to but not over the caldera, Cronin and the TGS team motored over the top of the volcano to map the caldera. Comparing their results with a survey led by Cronin in 2015, they discovered that the caldera floor had deepened from about 500 feet below sea level in November 2015 to 2,800 feet below sea level in May 2022. The scientists also detected minor eruptions in the caldera, which they did not notice until they saw them in their sonar records. They estimated that about one and a half cubic miles of material was missing from the caldera since the 2015 survey. In addition to their seafloor survey, Cronin and the TGS team conducted ash surveys, compiled eyewitness accounts, and measured tsunami runups—the heights above sea level reached by the waves on land.
A third seafloor survey, phase 2 of the TESMaP project, is now underway. SEA-KIT International’s uncrewed surface vessel (USV) Maxlimer, a 40-foot robotic boat, is mapping the caldera while operated from SEA-KIT’s headquarters in the United Kingdom. Among the findings of the survey’s first week are signs of continuing volcanic activity in the caldera. Ongoing work by Maxlimer will include mapping areas where communication cables broke.
The new surveys have already shed light on what caused classic tsunamis during the January eruption. Researchers believe that ash and rock ejected from the caldera fell into the sea and rushed down the flanks of the volcano as hot, turbid currents called pyroclastic flows. These currents moved far out over the ocean floor. Barren zones in the NIWA map suggest that they wiped out all life in their path. They are prime suspects for breaking communication cables and, along with the eruptive explosions, triggering tsunamis. At a May 24 news conference in Tonga, Shane Cronin reported that several groups of scientists are using the new survey data to develop tsunami models—a crucial step in understanding the waves that devastated Tongan islands, including what caused the pre-explosion waves observed by the resort managers.
Mitigating impacts of volcanic tsunamis
For the future, Geist said, “It would be great if we could produce hazard assessments for volcanic tsunamis.” Hazard assessments show which geographic areas are vulnerable to particular hazards, how frequently those hazards might be expected to occur, and what their effects are likely to be. Such assessments can help communities plan for and mitigate the impacts.
“We have done hazard assessments for regions likely to be threatened by earthquake-generated tsunamis, and we are making progress on assessments for regions vulnerable to meteotsunamis. Volcanoes would be tougher.”
Typically, tsunami-hazard assessment begins with assessing the likelihood of the triggering mechanism: the large earthquakes that trigger tsunamis, for example, and the types of weather events that produce meteotsunamis. There are so many underwater volcanoes around the world, and so little data about them, that assessing the likelihood of their producing tsunami-generating eruptions is a daunting task. But something useful might be within our reach: forecasting eruptions in real time by monitoring earthquake activity.
Monitoring earthquakes near the volcano
Scientists have found that many volcanic eruptions are preceded by distinctive stages of earthquake activity caused by magma moving beneath the volcano. To detect these stages, they generally rely on seismometers on or very close to the volcano. Nothing like that was available for Hunga Tonga-Hunga Haʻapai—at the time of the January eruption the nearest functioning seismic station was nearly 500 miles away. To tackle that problem, a team from the USGS National Earthquake Information Center (NEIC) quickly developed methods to discover earthquakes that NEIC’s automated system was unable to detect. Working closely with the USGS Volcano Disaster Assistance Program (VDAP), they constructed a near-real time catalog of earthquakes occurring close to the volcano in the days and weeks after the eruption.
By monitoring ongoing earthquakes, the USGS scientists hoped to detect any signs of further eruptions and to gain insight into what was happening beneath the volcano. A group including volcano seismologists Stephanie Prejean and Jeremy Pesicek of VDAP have been analyzing the earthquakes of magnitude 4.5 and greater that followed the eruption. They see an unusual pattern of quakes that might record post-eruption injection of magma into dikes or along rifts near the volcano. Alternatively, earthquake swarms like the one in the NEIC catalog have resulted from the collapse of calderas or subsurface magma chambers after an eruption. The group hopes to clarify the cause with continued analyses of earthquake and geologic data—including data from seafloor surveys.
The detection methods developed by NEIC also revealed three magnitude 4.7 earthquakes that occurred before the massive eruption, one on 14 January at about 2 p.m. local time, during an eruptive phase that had begun early that morning, and two at 5:07 and 5:13 p.m. local time on 15 January, just minutes before the violent explosion at 5:15.
Pesicek noted: “The detection of a few M4.7 earthquakes prior to the big explosion by distant seismic stations indicates that we might have been able to forecast this eruption if we had had more local monitoring stations. We are certainly missing many, many smaller earthquakes without having closer stations to record them.”
Installing and maintaining seismometers would be helpful in populated areas near potentially active volcanoes, such as Tonga. In response to the January eruption, VDAP has donated a seismic station and infrasound array to provide Tonga with basic volcano monitoring. The seismic station will detect earthquakes that might indicate moving magma; the infrasound array will detect atmospheric pressure waves produced by volcanic explosions. Installation of the instruments is planned for later in 2022. GNS Science (New Zealand), Geoscience Australia, and the World Bank all have plans to contribute additional equipment or training to allow for better seismic monitoring of the region. Farther into the future, the NEIC hopes to get more seismic stations in the Tonga region as part of the Global Seismographic Network.
Meanwhile, education and tsunami drills, like those conducted by the Tongan government, are low-cost and highly effective ways to save lives.