On January 15, 2022, a powerful volcanic eruption occurred in the Tonga archipelago in the Pacific Ocean when the submarine Hunga volcano exploded around 04:15 (UTC). This explosion marked the climactic end of an eruptive phase that started on 19 December 2021, after several years of quiescence. Never before have atmospheric waves from such an explosion been recorded so extensively on digital sensor networks on the Earth’s surface as well as from space. Based on the amplitude measurements of the pressure wave that circumnavigated the globe several times, it has been estimated that the explosion was comparable in size to that of the 1883 Krakatau eruption.
In this blog we will further discuss some of the atmospheric and seismic observations following the climactic event. A more detailed account on this event is published in the July 2022 issue of Science (Matoza et al., 2022). The study was written by a global research team of 76 authors (from 17 countries!) with expertise in seismology, acoustics, tsunamis, ionospheric perturbations, and volcanology.
Pressure wave observed from space
In Utrecht, The Netherlands (16,500 km distance), the first Lamb wave arrival was detected around 19:00 UTC as a sinusoidal signal with an amplitude of about 1.5 hPa in the 0.1-1.0 mHz band on (micro)barometers that are part of the network of the Royal Netherlands Meteorological Institute (KNMI). Elsewhere in Europe, similar observations were made around this time.
Following the Lamb wave arrival, the pressure timeseries reveal additional small-amplitude arrivals with higher frequency content (Figures 3 and 4). This characteristic dispersion is well-known from earlier studies of large explosions, and can be interpreted as a linear superposition of various modal waveforms (Pierce and Posey, 1971). The wavetrain culminates in the detection of broadband infrasound on the microbarometer, with a frequency content of up to 10 Hz (Figure 4; top row). Such signals are unique for signals originating from the other side of the world. In Alaska (~10,000 km distance), even audible signals were detected and reported.
In addition to the pressure waves, the volcanic eruption also generated vibrations that traveled through the earth and were measured on seismometers worldwide. Some of these waves travel through the Earth’s interior, but most of the energy propagates along the Earth’s surface as Rayleigh waves, at a speed of about 3 km/s. The first Rayleigh wave arrival at NARS seismometer NE05 at Utrecht University was detected around 6:00 UTC (Figure 4; bottom row).
Another interesting feature is the coupling of the Lamb wave to the subsurface around the time the Lamb wave passes over the seismometer. It is well known that long-period (i.e., periods longer than 10 seconds) atmospheric pressure waves can contribute to horizontal and vertical displacements that can be measured on seismometers (Sorrells, 1971). Indeed, the characteristic chirp that is visible in the barometer’s spectrogram is also observed in the seismic vertical displacement spectrogram (Figure 4; bottom row).
About six hours later, a smaller antipodal Lamb wave arrival was detected as well (Figure 4; left column). The waveform shows a clear 90 degree phase shift and, similar to the first arrival, also is associated with the detection of high-frequency infrasound and a detectable vertical seismic displacement. Subsequent Lamb wave arrivals (up to a total of six) were detected with even smaller amplitudes.