On January 15, 2022, at 4:14 UTC, the Hunga Tonga-Hunga Ha'apai (HTHH, 175.39°W, 20.55°S) volcano erupted and triggered a global tsunami, located in the Tonga subduction zone volcanic chain region of the South Pacific. Focusing on observation methods for volcanic eruptions and tsunami waves, the latest monitoring results of volcanic eruptions and tsunamis by seafloor sensors, satellite images of changes in atmospheric waves, and ionospheric disturbances are presented. Based on the geological structure and volcanic eruption history data of the region, the characteristics of the tsunami caused by the HTHH volcanic eruption in Tonga in 2022 and the latest observation methods were reviewed, and suggestions were put forward to conduct detailed marine geophysical surveys of volcanic tsunamis in subduction zones, and to comprehensively quantify the potential impact of submarine volcanic eruptions on global climate based on observations.
Subduction zones are frequent areas for major natural disasters such as volcanic eruptions, earthquakes, and large landslides. The occurrence of these seabed tectonic events can trigger a megatsunami that will cause a large number of casualties and huge economic losses to adjacent coastal areas. On January 14, 2022, the Hunga Tonga-Hunga Ha'apai (hereinafter referred to as HTHH) submarine volcano eruption (Figure 1) in the post-arc area of the Tonga subduction zone in the South Pacific Ocean released energy of 5.8 on the Richter scale and triggered a global tsunami, which quickly spread to the Pacific Rim within 10 hours, causing Pacific Rim countries to issue tsunami warnings. The tsunami signal then reached the Atlantic, Indian Ocean, Caribbean and Mediterranean regions. In addition, satellite observations show that the eruption plume reached as high as 58 km, reaching the middle layer of the atmosphere, causing ionospheric disturbances, and propagating from the eruption area in the form of fluctuations in the atmosphere to a global scale, and the change in the fluctuating air pressure was successfully recorded by the global barometer network (Figure 1).
Fig. 1 Distribution map of Pacific Rim submarine manometer (blue and green inverted triangle) and barometer (red circle) stations (a) and geological map of Tonga volcanic area (b)
Focusing on the latest observations of this rare volcanic eruption and the tsunami it caused, this paper comprehensively collates and analyzes the deformation characteristics of the land surface before the 2022 HTHH volcanic eruption, the tsunami caused by the eruption, atmospheric waves, ionosphere and other new observations, and puts forward prospects and suggestions for the study of marine geological disasters such as volcanic tsunamis in subduction zones.
Tectonics of the Tonga subduction zone and surface deformation characteristics before the HTHH volcanic eruption
01 - Tectonic background and historical activities
In the Southwest Pacific region, the fast-moving Pacific Plate subducts into the Australian Plate, forming the second deepest Tonga-Kermadek Trench in the world after the Mariana Trench (Figure 1). The convergence rate of this subduction zone is about 20cm/a, and it has the characteristics of very active post-arc expansion and plate retraction. Over the past 45 million years, the Tonga subduction zone has undergone multiple periods of post-arc expansion and formed the Tonga volcanic arc chain in the post-arc region (Figure 1(b)), which spans a long span from the subduction zone of Fiji to the northeast of New Zealand, and covers areas with active tectonic movements and frequent geological disasters such as volcanoes, earthquakes and tsunamis.
The latest eruption, the HTHH crater, is located about 68 km north of Nuku'alofa, the capital of Tonga (Figure 1(b)). The crater connected two uninhabited islands of Hunga-Ha'apai and Hunga-Tonga after successive eruptions in 2014-2015. The highest point of the HTHH volcano before the 2022 eruption is about 100m above sea level, the crater is below the seawater, it is about 5km in diameter, and the entire volcano is about 20km wide. Since the beginning of records in the 20th century, the HTHH volcano has erupted six times, in 1912, 1937, 1988 (lasting 3d), 2009 (lasting 1 week), and 2014-2015 (lasting 5 weeks). Among them, the volcanic eruption index of the 2009 eruption was 2, and it was traced to the eruption stage of the volcano; Garvin et al. analyzed the erosion rate of volcanic cone formed by volcanic eruptions from 2014 to 2015 through satellite data of about 0.00256/a. The above historical data shows that the eruption interval of HTHH volcano is about 20~50a, and the latest eruption interval in 2022 is only 7a, indicating that the tectonic and volcanic activity of this area has increased significantly.
02 - Surface deformation characteristics before the 2022 HTHH volcanic eruption
For the surface deformation characteristics of HTHH volcano before the eruption, Hu Yufeng et al. analyzed based on Sentinel-1 satellite data and obtained the cumulative deformation time series of satellite radar line of sight (LOS) from 2019-07-06 to 2021-12-10 before the eruption (Figure 2(a)~(h)). where red is negative, indicating that it is moving away from the satellite along the LOS; Blue is a positive value, indicating approaching the satellite along the LOS direction. The results show that in the observation time before the eruption, the volcanic body has obvious subsidence phenomenon and the deformation field is similar to the semi-ring shape of the volcanic surface, and the displacement rate of the crater to the bottom of the mountain gradually decreases. Further analysis of the time series of two reference feature points and time series in the crater deformation zone (Fig. 2(i) and (j)), the cumulative displacement of the points during the observation period subsided by more than 6cm, the cumulative displacement of the points subsided by about 5cm, and the sinking rate also showed phased changes, which were basically stable before June 2020 and August 2020, and then the points sank at a rate of about 3.1cm/a and 2.2cm/a, respectively, quantitatively revealing the spatiotemporal evolution characteristics of slight deformation of the surface before the eruption.
Fig. 2 Record of surface deformation before the HTHH volcanic eruption
Deformation characteristics of the 2022 HTHH volcanic eruption
01 - Causing a global tsunami
The tsunami triggered by the eruption of the HTHH volcano in Tonga spread rapidly across the globe within a few to tens of hours. A global network of coastal tidal stations has seen significant tsunami waves widely in the Pacific, Atlantic, Indian Ocean, and Caribbean and Mediterranean regions. Kubota et al. analyzed the tsunami signal recorded by the seafloor manometer and barometer of the Pacific Rim (Figure 3) and found that the actual arrival time of the tsunami (the black line in Figure 3(a)) was more than 2 hours earlier than the theoretical arrival time of the tsunami caused by conventional submarine volcanic eruptions (the green line in Figure 3(a)), and the propagation time was more rapid. The latest report from Tonga suggests that the tsunami could reach at least 15m, and as high as 2.7m when it reaches Japan. It is worth noting that the characteristics of tsunami waves along the Pacific coast are markedly different from those in other parts of the world. These coastal stations recorded that pilot tsunami waves with smaller amplitudes were monitored before the largest tsunami waves (up to 3m) were reached; Outside the Pacific Ocean, these initial small-amplitude tsunamis were mainly monitored.
Fig. 3 Signal recording (a) and barometric pressure recording (b) of the Pacific Rim seafloor after the HTHH volcanic eruption
The tsunamis triggered by the HTHH volcanic eruption in Tonga are significantly different from those caused by common subduction zone earthquakes. In general, tsunamis triggered by earthquakes disperse seawater from the bottom of the sea along fault lines that are tens or hundreds of kilometers long, while tsunamis caused by volcanic eruptions spread outward more like a point source. The current tsunami prediction model used for research is basically based on earthquake triggering simulation, which is not very suitable for volcanic tsunami situations. In addition, the pressure changes on the seafloor caused by this eruption last longer and the mechanisms of tsunami generation are more complex. The tsunami that first reaches the coast has a small amplitude and is coupled with the propagation velocity (about 300 m/s) of fast-moving atmospheric pressure waves produced by volcanic eruptions, and is thought to be the result of interaction with seawater during the propagation of atmospheric waves.
02 - Excitation of atmospheric waves
In theory, large explosions such as volcanic eruptions and nuclear tests would produce atmospheric waves of varying lengths and frequencies. On short wavelength scales in the horizontal direction, Lamb waves, sound waves, and gravitational waves can be elicted. In addition, volcanoes can form a continuous source of waves after the initial eruption through updrafts and convection of heated plumes. The plume over the HTHH volcano continuously released latent heat for more than 12 hours and was the single most important gravitational source of this atmospheric wave, excitation of circular wave surfaces across the entire Pacific Ocean, and a global response of this magnitude has never been observed before from a single source (Figure 4). Observations by the geostationary environment satellite GOES showed that volcanic eruptions began to be seen around 04:15 UTC, forming plumes that rose to 30 km high within 30 minutes, covering a width of 200 km. After the plume rises 20~30min, obvious atmospheric waves can be seen in the near-infrared geostationary image with 10min resolution. Through the reverse projection analysis of ground pressure data, Wright et al. determined that the trigger source occurred at UTC04:28±0:02, and the front wave surface propagated at the near-ground phase velocity of (318.2±6)m/s in the surface layer, and identified it as a Lamb wave according to the analysis of the signal's high phase velocity, large amplitude and non-dispersion characteristics. Theoretically, the energy density of the Lamb wave decays exponentially with altitude, and observations show that it propagates in the stratosphere at phase velocities of (308±5) to (319±4)m/s. Lamb waves spread globally, passing through the Algerian opposite point 18.1 h (±7.5 min) after the eruption, and traveling around the Earth at least 3 times over the next few days. Based on data from the Atmospheric Infrared Sounder (AIRS), Cross Track Infrared Sounder (CrIS), Infrared Atmospheric Sounding Interferometer (IA⁃SI), and Polar Orbital Thermal Infrared (IR) data, initial gravitational waves are also widely identified worldwide after the Lamb wave. The gravitational wave passed through the opposite point of HTHH volcano for the first time on January 16 UTC00:30-02:30 (within 20~22h after eruption), and propagated in the stratosphere with a phase velocity of (238±3) to (269±3)m/s. Comprehensive observational analysis shows that the propagation distance difference between the initial Lamb wave and the gravitational wave increases over time.
Figure 4 Atmospheric wave propagation triggered by HTHH volcanic eruptions (Geostationary Environment Satellite GOES observation)
03 - Causes ionospheric disturbance
Atmospheric waves caused by sudden movements of the Earth's surface propagate through the atmosphere to the upper atmosphere, eventually causing disturbances (TIDs) in the moving ionosphere. This change has been detected in the past in scenarios such as earthquakes, tsunamis, volcanic eruptions, and underground nuclear explosions. After the eruption of the HTHH volcano, the vertical total electron content (TEC) of the ionosphere over the volcano was significantly reduced, and the Global Navigation Satellite System (GNSS) receiving network monitored the movement of ionospheric disturbances over Japan. Saito observed ionospheric disturbance signals with 2 different characteristics in the disturbance of the TEC. The first disturbance signal arrived in Japan 3 hours after the eruption, about 7800 km from the crater, with an amplitude of about ±0.5 TECU. The second disturbance signal arrived in Japan 7 hours after the eruption with an amplitude of about ±1.0 TECU.
In addition to Japan, ionospheric disturbance signals from the HTHH eruption have also been monitored in other regions of the world. Themens et al. used measurements from 4,735 GNSS receivers around the world to track the movement signals of ionospheric disturbances associated with the HTHH eruption for the first time (Figure 5). By selecting ionospheric anomaly signals from different regions of the world for analysis (New Zealand, Australia, Hawaii, Japan, eastern North America, South Africa and Northern Europe, Figure 5(b)~(h)), two different large-scale moving ionospheric disturbances and several subsequent mesoscale moving ionospheric disturbances were found, both of which propagated radially outward from the eruption point. Within 3000 km of the eruption center, large-scale moving ionospheric disturbance signals with wavelengths of 1600 km were initially observed to propagate at speeds of about 950 m/s and about 555 m/s, respectively, and then slowed down significantly to about 600 and about 390 m/s. This signal is the primary signal of the global ionospheric response, consistent with atmospheric surface pressure disturbances associated with volcanic eruptions. The mesoscale moving ionospheric disturbance signal was observed within 6h after the volcanic eruption, and its velocity was 200~400m/s.
Figure 5 Observations of ionospheric effects triggered by HTHH volcanic eruptions on a global scale
conclusion
This paper summarizes the latest observations and studies on eruptions and tsunamis since the 2022 HTHH volcanic eruption in Tonga. The unusual geophysical changes triggered by the HTHH volcanic tsunami around the world reflect the complexity of the interaction of the entire eruption process with the atmosphere, lithosphere and oceans during submarine volcanic eruptions, and the impact of marine geological disasters on a global scale. The lessons learned from this event and suggestions for future research are as follows.
1) The characteristics of the tsunami caused by this volcanic eruption are complex, especially the geological structure changes near the volcanic body and its contribution to the tsunami are not yet clear, and detailed marine geophysical surveys are urgently needed to solve this mystery. Existing models are insufficient to provide accurate tsunami simulations, so a new non-seismic tsunami model needs to be developed and incorporated into the global tsunami early warning system to provide timely emergency warning, evacuation and rescue measures for coastal cities and residents with similar geological characteristics of subduction zones.
2) The volcanic eruption released a large amount of energy and gases of different components into the atmosphere, which stimulated gravitational waves and atmospheric waves on a global scale, causing ionospheric disturbances, etc., and it is urgent to comprehensively quantify the potential impact of submarine volcanic eruptions on the global climate based on these latest observations, and interdisciplinary cooperation to explore the relationship between marine geological disasters and atmospheric response, and the interaction between different layers of the earth.
3) Use new observation methods to strengthen the observation of submarine volcanic tsunamis, and establish and improve the tsunami warning and climate monitoring system along the global subduction zone.
4) Huge marine geological disasters are global in scope, and it is recommended to strengthen international cooperation and rescue to jointly create a sustainable and livable home.
The authors of this article: Zheng Tingting, Qiu Qiang, Lin Ma
About author:ZHENG Tingting, Key Laboratory of Marginal Sea and Ocean Geology, Chinese Academy of Sciences, South China Sea Institute of Oceanology, Chinese Academy of Sciences, South China Sea Eco-Environmental Engineering Innovation Institute, Southern Oceanography and Engineering Guangdong Laboratory (Guangzhou), China-Pakistan Geoscience Research Center, Chinese Academy of Sciences-Pakistan Higher Education Commission, Assistant Researcher, Research Direction is Marine Geology and Geophysics; Lin Jian (corresponding author), Key Laboratory of Marginal Sea and Ocean Geology, Chinese Academy of Sciences, South China Sea Institute of Oceanology, Chinese Academy of Sciences, South China Sea Eco-Environmental Engineering Innovation Institute, Southern Oceanography and Engineering Guangdong Laboratory (Guangzhou), China-Pakistan Geoscience Research Center, Chinese Academy of Sciences-Pakistan Higher Education Commission, Department of Marine Science and Engineering, Southern University of Science and Technology, Woods Hole Oceanographic Institution, Professor, research interests in marine geology and geophysics, geodynamics.
The original article was published in the second issue of Science and Technology Review in 2023, welcome to subscribe to view.