Tonga eruption recorded on Hawaii infrasound array
(BIVN) – In the latest Volcano Watch article, scientists detail how last month’s powerful Hunga Tonga-Hunga Ha’apai volcano eruption was detected in Hawaiʻi. From this week’s article, written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory:
The 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 in a number of locations around the world, including the island of Hawai’i about 4,900 kilometers (3,000 miles) away.
Hunga Tonga-Hunga Ha’apai is located approximately 64 kilometers (40 miles) north of Tonga’s capital, Nuku’alofa. As with many volcanoes in Tonga, the part of the volcano visible above water is small compared to the underwater extent of the edifice and eruptions can alternately push up new land or destroy islands that are forming.
The volcano previously erupted in 2014-2015 in an extended sequence that created a larger island by connecting two separate islets. After this episode, the volcano entered a quiet period but woke up in mid-December 2021.
The largest eruption occurred on January 15 from approximately 5:10 p.m. to 5:30 p.m. TOT, approximately 6:10 to 6:30 p.m. hot for tens of kilometers (several miles) in the atmosphere. Several centimeters (about 1-3 inches) of ash was deposited on the main island of Tongatapu and Nuku’alofa.
The column of ash rushing skyward was among the highest seen in modern times and this column generated strong air pressure waves that rose from the volcano and spread out from Tonga. Incredibly, the air pressure waves were seen all over the Earth and people in Alaska (nearly 10,000 kilometers or 6,000 miles away) heard the sounds of the eruption. How was it possible?
Volcanic eruptions produce large pressure waves from the ejection of rock and ash. These air pressure waves can occur at many different frequencies, from very high frequencies that we hear as sound (audible) to lower frequencies that we cannot hear at all (called infrasound) . Both types of waves were generated by the recent eruption in Tonga.
Pressure waves could dissipate quickly if the Earth’s atmosphere were uniform. Instead, the atmosphere is layered, and those layers help sound move efficiently. For pressure waves, the layers mainly relate to the temperature and wind speed of the air and the effective movement of these waves occurs in the layers called troposphere, stratosphere and thermosphere.
The speed at which pressure waves pass through the Earth’s atmospheric layers is approximately 1,100 kilometers or 700 miles per hour. At these speeds, it would take just over 4 hours for the sound to travel from Tonga to Hawaii.
The USGS Hawaiian Volcano Observatory (HVO) operates an array of microphone-like pressure sensors that are deployed to “listen” to our own volcanoes. If there is a large worldwide eruption and atmospheric conditions permit, this eruption can also be captured on the HVO pressure sensor network.
If we look at the pressure sensor data for the roughly four hour period after Tonga’s recent eruption, we see a large pressure pulse moving over Hawaii (see the two sensor records in the figure) at 10:40 p.m. HST on January 14. While Hawaii’s pressure sensor array is designed to detect eruptive activity at Mauna Loa, Kīlauea, and along the Hawaiian rift zones, it also captured sound waves from the distant Tonga eruption.
Close inspection of the waves seen in the figure illustrates the idea of audible and infrasonic waves discussed earlier.
The pressure waves are wider at the start of the event and narrow over time. Since the graph shows wave size over time, this means that previous waves (left side of graph) show slow changes in pressure over time. The more time passes, the faster the wave pressure oscillates. This means that the first waves are of low frequency and the last waves are of higher frequency.
The human ear can usually hear pressure changes above about 20 cycles per second (or 20 hertz). Most of the waves shown in the graphs are very low frequency and would not be detected by the human ear.
In Hawaii, there have been no reports of Tonga erupting, but they have been seen breaking through the atmosphere atop Mauna Kea.
Observing audible sound waves in Alaska and inaudible waves elsewhere is a topic of interest to scientists studying how pressure waves propagate through the atmosphere. Either way, the observation of sound waves traveling all over the Earth is remarkable.