Hunga Tonga-Hunga Ha’apai volcano located in the Kingdom of Tonga produced a powerful eruption on January 15 just after 5 p.m. Tonga Time (TOT). The Tonga eruption was recorded on pressure sensors over a wide number of places around the world, including the Island of Hawaii about 4,900 kilometers (3,000 miles) away.
Hunga Tonga-Hunga Ha’apai is located about 64 kilometers (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 2014-2015 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 on Jan. 15 from about 5:10 to 5:30 p.m. TOT, about 6:10 to 6:30 p.m. Hawaii Standard Time on Jan. 14, and shot ash and hot gas tens of kilometers (several miles) into the atmosphere. Several centimeters (about 1 to 3 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 10,000 kilometers or 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 4 hours for the sound to travel from Tonga to Hawaii.
The USGS Hawaiian Volcano Observatory 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 Mauna Kea.
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 (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are issued daily.
Over the past week, lava has continued to intermittently erupt from the western vent within Halema‘uma‘u crater. All lava is confined within Halema‘uma‘u 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, 2022, during an eruptive pause; current SO2 emissions are likely significantly increased and comparable to emission rates measured on Feb. 1, 2022, 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. For more information on the current eruption of Kilauea, see https://www.usgs.gov/volcanoes/Kilauea/recent-eruption.
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 10 kilometers (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 Global Positioning System 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. For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.
Two earthquakes were reported felt in the Hawaiian Islands during the past week: a M3.0 earthquake 2 km (1 mi) S of Pahala at 32 km (19 mi) depth on Feb. 6 at 12:06 p.m. HST and a M2.8 earthquake 11 km (6 mi) E of Pahala at 32 km (20 mi) depth on Feb. 3 at 12:02 p.m. HST.
HVO continues to closely monitor Kilauea’s ongoing eruption and Mauna Loa for any signs of increased activity.
Please visit HVO’s website for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.
