Tonga eruption’s Lamb waves were stronger westward, reshaping ionospheric GPS signals

When the Hunga Tonga–Hunga Ha’apai submarine volcano exploded on January 15, 2022, it sent a colossal pressure pulse racing around the planet several times. That wave, known as a Lamb wave, is a special kind of atmospheric oscillation trapped near the Earth’s surface that can travel thousands of kilometers with remarkable efficiency. Now a detailed reanalysis of barometric and GPS records shows that this wave was not symmetric: the branch that raced westward across Asia and Oceania carried substantially more energy than the branch heading eastward toward the Americas, and the same west–east imbalance shows up in the ripples it left in the ionosphere.

The new study, published in Earth, Planets and Space, analyzed one-minute-cadence atmospheric pressure data from a network of barometric stations spread across the Asia-Oceania region and the Americas, together with globally distributed GPS stations used to measure total electron content, or TEC, a proxy for the amount of electrons in the ionosphere along a signal path. The researchers compared the amplitude of the pressure wave at each station for the first and second passages of the wave in each direction, labeled W1 and W2 for westward passages and E1 and E2 for eastward ones.

The result was striking. At stations west of the volcano, such as PHIM in Asia, the pressure jump at the first westward arrival was far larger than the jump recorded when the eastward branch later swept past the same longitude. The same held true at stations east of the volcano, such as SRER in North America: the westward wave, arriving after a longer great-circle journey, still outmatched the eastward wave that had traveled a shorter distance. Because the two branches pass over essentially the same routes for any given station, differences in travel distance or topography cannot easily explain the gap.

Averaged across the station networks, the mean pressure amplitude was 2.30 hectopascals for the first westward passage versus 1.62 hectopascals for the first eastward passage. Converting pressure amplitudes into wave energy flux using standard acoustic relations, with an assumed atmospheric density of 1.293 kilograms per cubic meter and a wave speed of 315 meters per second, the team found that the westward-propagating wave carried roughly 80 percent more energy flux than the eastward one. The mean integrated energy flux over 250 minutes at the first passage was about 23,000 joules per square meter; integrating over a cylindrical surface at the mean station distance yields a total transported energy of roughly 7×10^15 joules, equivalent to about 1.4 megatons of TNT, or about one percent of the total eruption energy estimate of 100 to 200 megatons previously derived from International Monitoring System data.

The wave shapes themselves differed as well. The westward wave front displayed a more complex structure, particularly at southern-hemisphere stations such as LEAR, NTIS and CLEE, while the eastward front showed a simpler classic blast-wave profile of a sharp positive pressure rise followed by a negative excursion. The authors also noted a north–south asymmetry: first-arrival amplitudes were generally larger at southern-hemisphere stations than at northern-hemisphere ones, a pattern they tentatively attribute to enhanced dissipative absorption of Lamb waves by atmospheric turbulence in the equatorial region.

The ionosphere told a matching story. Short-period fluctuations in GPS-derived TEC, with periods shorter than about 30 minutes, were enhanced worldwide around the times the pressure wave passed overhead. When the researchers superposed the TEC records by aligning them to the estimated arrival time of the Lamb wave at each station, the amplitude increase after the first westward passage at western stations was about twice that seen after the first eastward passage at eastern stations. The second westward passage again produced a clear TEC enhancement on both sides, while the second eastward passage did not. If the TEC ripples are driven by the Lamb wave, this asymmetry is consistent with the ground-pressure asymmetry.

The TEC response also lingered, continuing for three hours or more after the wave front passed, and in some cases began before the wave should have arrived at the station. The authors suggest this early onset may be a signature of magnetic conjugacy: electric fields associated with the pressure-driven disturbance can be transmitted along geomagnetic field lines to the opposite hemisphere, where conductance is high, moving plasma there. At the PTGG station in the Philippines, for example, the volcano lies 7,960 kilometers away, but its geomagnetic conjugate point is only 6,390 kilometers from the source, so the wave-induced effect could arrive there more than an hour before the direct Lamb wave reaches the station, matching the observed timing.

What could make a volcanic explosion prefer one compass direction? The authors propose, explicitly as a speculation, a role for the Earth’s rotation. A propagating blast wave carries momentum, and in the rotating frame of the Earth a horizontal flow experiences a vertical Coriolis force at low latitudes. For an eastward wave speed of about 350 meters per second at latitude 20 degrees south, this force amounts to roughly 0.048 meters per second squared, about half a percent of gravitational acceleration, directed upward on the eastern side of the volcano and downward on the western side. Because most Lamb wave energy resides in the lower troposphere, within an energy scale height of around 16 kilometers, a slight downward tilt of the blast momentum on the western side could transfer energy into the horizontal Lamb wave more efficiently there, making the westward wave stronger.

The authors are careful to note that other explanations cannot be fully excluded. A chance asymmetric explosion is considered unlikely, since geostationary satellite imagery of the eruption plume from Himawari and GOES-17 showed no clear asymmetry in the ash cloud, and the undersea setting makes a lopsided blast harder to envision. Distance and topography also fall short as explanations: the eastern stations were on average only about 20 percent farther from the volcano than the western ones, yet the western energy flux was about 80 percent larger, and the terrain between the volcano and most stations is open ocean with no high mountains.

Intriguingly, the same pattern may have a historical precedent. In the 1883 Krakatoa eruption, a contemporary report’s map of regions where the explosion sounds were heard covers a wider area to the west than to the east, and pressure traces at Bombay, west of Krakatoa, show two distinct positive peaks 4,519 kilometers apart, while such clear peaks are absent at Melbourne to the east. The authors argue that whatever the precise mechanism, some common physical process may impose an east–west asymmetry on Lamb waves from major low-latitude eruptions. They caution that their barometric coverage is regionally limited, that the transition from the initial blast wave to a mature Lamb wave is still not well understood, and that numerical simulations on a rotating Earth, plus wider high-resolution pressure networks, are needed to confirm the proposed Coriolis mechanism.

Subject of Research: East–west asymmetry in Lamb wave amplitude and GPS-TEC variations generated by the 2022 Hunga Tonga volcanic eruption

Article Title: East–west asymmetry of Lamb wave amplitude generated by the HTHH eruption and its effect on GPS-TEC variation

Article References: Iyemori, T., Aoyama, T., Yokoyama, Y., Pangsapa, V., Jarupongsakul, T., Sano, Y., Odagi, Y., Tanaka, Y., Taguchi, S., Saito, A., Hozumi, K., Iguchi, M., Thomas, C., & Matsumoto, K. (2026). East–west asymmetry of Lamb wave amplitude generated by the HTHH eruption and its effect on GPS-TEC variation. Earth, Planets and Space, 78(1), Article 230. https://doi.org/10.1186/s40623-026-02551-3

Image Credits: AI Generated

DOI: 10.1186/s40623-026-02551-3

Keywords: Hunga Tonga eruption, Lamb wave, atmospheric pressure waves, GPS-TEC, ionosphere, Coriolis force, wave energy flux, Krakatoa 1883, barometric observations, geomagnetic conjugacy, Earth rotation, volcanic eruption