Hunga Tonga-Hunga Ha’apai is located about 40 miles north of the Tongan capital city of Nuku’alofa. As with many volcanoes in Tonga, the part of the volcano that is visible above water is small compared to the submarine extent of the edifice and eruptions can alternately grow new land or destroy any islands that are formed.
The volcano previously erupted in 2014-15 in an extended sequence that created one larger island by connecting two separate islets. After this episode, the volcano went into a quiet period but it re-awakened in mid-December 2021.
The largest eruption occurred from about 5:10 to 5:30 p.m. Jan. 15 Tongan time, about 6:10 to 6:30 p.m. Hawaii time on Jan. 14, and shot ash and hot gas several miles into the atmosphere. Several inches of ash was deposited on the main island of Tongatapu and Nuku’alofa.
The skyward rushing ash column was amongst highest observed in modern times and this column generated strong air pressure waves which moved up from the volcano and spread outward from Tonga. Incredibly, the air pressure waves were observed over the entire Earth and people in Alaska (nearly 6,000 miles away) heard sounds from the eruption. How was this possible?
Volcanic eruptions produce significant pressure waves from the ejection of rocks and ash. These pressure waves in the air can occur at many different frequencies, from very high frequencies that we hear as sounds (audible) and at lower frequencies that we cannot hear at all (called infrasound). Both types of waves were generated by the recent eruption in Tonga.
The pressure waves might dissipate quickly if the Earth’s atmosphere were uniform. Instead, the atmosphere is layered, and these layers help sound to move efficiently. For pressure waves, the layers relate mostly to the temperature and wind speed of the air and the efficient movement of these waves occurs in the layers called the troposphere, stratosphere and thermosphere.
The speed that pressure waves travel through the Earth’s atmospheric layers is about 1,100 kilometers or 700 miles per hour. At those speeds it would take a little more than four hours for the sound to travel from Tonga to Hawaii.
The USGS Hawaiian Volcano Observatory (HVO) operates a set of microphone-like pressure sensors that are deployed to “listen” to our own volcanoes. If there is a large global eruption, and the atmospheric conditions allow, then that eruption can also be captured on the HVO pressure sensor network.
If we look at the pressure sensor data for the time about four hours after the recent Tonga eruption, we see a large pressure pulse moving over Hawaii (see the two sensor recordings in the figure) at 10:40 p.m. HST on Jan. 14. While Hawaii’s pressure sensor network is designed to detect eruptive activity for Mauna Loa, Kilauea, and along the Hawaiian rift zones, it also captured the sound waves from the distant Tonga eruption.
Close inspection of the waves observed in the figure illustrates the idea of audible and infrasound waves discussed earlier.
The pressure waves are wider at the start of the event and become narrower with time. Since the plot shows wave size with time, this means the earlier waves (left side of the plot) show slow changes of pressure with time. With more time passing, the wave pressure oscillates more quickly. This means that the first waves are low frequency, and the later waves are higher frequency.
The human ear can generally hear pressure changes higher than about 20 cycles per second (or 20 hertz). Most of the waves shown in the plots are very low frequency and would not be detected by the human ear.
In Hawaii, there were no reports of the Tonga eruption being heard, but they were seen traversing the atmosphere on top of Maunakea.
The observation of audible sound waves in Alaska and inaudible waves elsewhere is a topic of interest from scientists, who are looking at the ways that pressure waves propagate in the atmosphere. Regardless, the observation of sound waves moving over the entire Earth is remarkable.
Volcano Activity Updates
Kilauea volcano is erupting. its USGS Volcano Alert level is at WATCH. Kilauea updates are issued daily
Over the past week, lava has continued to intermittently erupt from the western vent within Halemaʻumaʻu crater. All lava is confined within Halemaʻumaʻu crater in Hawaii Volcanoes National Park. Sulfur dioxide emission rates remain elevated and were last measured at approximately 330 tonnes per day (t/d) on Feb. 8 during an eruptive pause; current SO2 emissions are likely significantly increased and comparable to emission rates measured on Feb. 1, when lava was erupting, of about 4,500 t/d. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Summit tiltmeters show several deflation and inflation patterns over the past week.
Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are issued weekly.
This past week, about 195 small-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa—the majority of these occurred at shallow depths less than 6 miles below sea level. Rates of seismicity are decreasing following a small increase in activity over the last week in the region immediately northwest of Mauna Loa summit GPS measurements show low rates of ground deformation over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone remain stable. Webcams show no changes to the landscape.
Two earthquakes were reported felt in the Hawaiian Islands during the past week: a magnitude-3.0 earthquake 1 mile south of Pahala on Feb. 6 at 12:06 p.m. and a magnitude-2.8 earthquake 6 miles east of Pahala on Feb. 3 at 12:02 p.m.
Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past Volcano Watch articles, updates, photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov. Volcano Watch is a weekly article and activity update written by USGS Hawaiian Volcano Observatory scientists and affiliates.
