The Hunga-Tonga-Hunga Ha'apai volcano's eruption on January 15, 2022, stands as one of the most powerful volcanic events recorded, releasing energy estimated at 4-18 megatons of TNT. This immense energy release produced a variety of observable phenomena, including a tsunami, ionospheric disturbances, volcanic lightning, and a globally propagating atmospheric wave. The eruption's impact extended far beyond the immediate vicinity of the volcano. The atmospheric pressure wave, interpreted as a Lamb wave, traveled vast distances, detectable by various sensors worldwide. This study focuses specifically on the seismic signals generated by the eruption and their relationship to the atmospheric wave. Understanding the seismic record of this event is critical for advancing our knowledge of volcano-atmosphere interactions, improving eruption forecasting capabilities, and refining our understanding of Earth's response to such powerful energetic events. The research question is to investigate the seismic signals generated and their relation to the atmospheric waves, particularly the evolution of the eruption, the global propagation of atmospheric waves, and the excitation of Earth's normal modes. The study aims to provide a detailed analysis of these seismic signals and offers a unique opportunity to explore the complex interactions between the atmosphere and the solid Earth in the context of a remarkably energetic volcanic event. The importance of this study lies in its potential to enhance our understanding of the complex dynamics of large volcanic eruptions and their global geophysical consequences, leading to improved hazard assessment and mitigation strategies.

Previous research on large volcanic eruptions has documented the generation of atmospheric waves and their detection through various means, including infrasound and barometric pressure measurements. Studies like those on the Mount Pinatubo (1991) and El Chichón (1982) eruptions have shown the excitation of Earth's normal modes, resulting in long-period seismic signals. However, the Hunga-Tonga eruption's scale and the duration of its associated seismic and atmospheric effects exceed those previously observed. While the generation of Lamb waves by large volcanic events is known, the extended duration of seismic detection of this wave after multiple circumnavigations of the globe is unusual and requires detailed analysis. Prior research has established the connection between volcanic eruptions and very-long-period (VLP) seismic events, often linked to magma movements and gas releases within volcanic conduits. This study builds upon this existing knowledge by examining the unique characteristics of the seismic signals produced by the Hunga-Tonga eruption, pushing the boundaries of our understanding of atmosphere-solid earth coupling following a significant volcanic event.

The study utilized data from various global seismic networks, including the Global Seismograph Network (GSN), IRIS/IDA, Geoscope, and Geofon. Data from regional networks covering Southwestern Europe (FR, ES, and CA) were also included to assess any regional variations. The analysis focused on the LHZ (longitudinal component, vertical direction) channels of broadband seismic stations, with a sampling rate of 1 sample per second. Standard procedures within the ObsPy package were used to remove instrumental responses from the seismic data. Spectral analyses, including the calculation of spectra and spectrograms, were performed using SAC and ObsPy routines. Low-pass filtering was used in different frequency bands to isolate specific signals, allowing the identification of signals related to the initial eruption phases, the propagation of the atmospheric Lamb wave, and the excitation of Earth's normal modes. The analysis involved examination of the temporal evolution of the seismic signals, identification and characterization of the atmospheric waves, and interpretation of the low-frequency signals as Earth normal mode excitation. Careful examination of waveform characteristics, including particle motion and spectral content, were used to distinguish between signals generated by different mechanisms (e.g., the direct seismic waves from the eruption, the atmospheric wave, and the Earth's normal modes).

The seismic data revealed a complex temporal evolution of the Hunga-Tonga eruption. The eruption started earlier than previously reported in global seismic catalogues, with low-frequency seismic signals detected around 04:02 UTC, preceding the main energy release at approximately 04:15 UTC. The main eruption consisted of two distinct, high-energy pulses separated by approximately 200 seconds. Two additional major explosive events occurred around 05:30 UTC and 08:25 UTC, corroborated by satellite imagery. The analysis of the seismic data confirmed the existence of three significant seismic events corresponding to the three umbrella clouds detected in satellite imagery. The eruption generated a Lamb wave that propagated globally, with its seismic signature detected over several days, up to five circumnavigations of the Earth, an unprecedented duration for such a phenomenon. Many stations also recorded a second atmospheric wave, characterized by its dispersive nature, possibly a gravity or pressure wave. Following the main eruption, prolonged low-frequency signals were detected, lasting 10–12 hours, consistent with the excitation of Earth's normal modes. The most prominent spectral peak observed in these signals was around 3.7 mHz, interpreted as the Earth's “hum”. These signals have strong similarities to those observed after the 1982 El Chichón and 1991 Mount Pinatubo eruptions. The stacked spectrum across many global stations clearly showed the dominant 3.7 mHz mode along with smaller peaks at 4.5, 5.3 and 6.0 mHz, all corresponding to eigenfrequencies of the PREM (Preliminary Reference Earth Model). The 08:30 volcanic explosion appears to have enhanced the Earth's normal mode excitation.

The findings highlight the immense energy released by the Hunga-Tonga eruption, evidenced by the prolonged detection of the Lamb wave and the excitation of Earth's normal modes. The early onset of seismic signals, preceding the main event as recorded by global catalogs, suggests a more complex eruptive sequence than initially understood. The repetitive nature of the VLP pulses suggests episodic volcanic activity over several hours. The prolonged presence of low-frequency seismic signals underscores the strong coupling between the atmosphere and the solid Earth. The dominance of the 3.7 mHz mode suggests that this frequency is particularly effective in transferring atmospheric energy to the Earth. This study demonstrates the utility of global seismic networks in monitoring large volcanic events and provides critical insights into the interactions between volcanic eruptions, atmospheric dynamics, and the Earth's internal structure. Future modeling efforts should focus on simulating these processes to better understand the complex interactions and to improve hazard assessment and prediction for similar events.

This study provides new insights into the Hunga-Tonga eruption using global seismic data, confirming the existence of three major explosive events. The eruption generated a globally propagating Lamb wave with exceptional longevity, detectable for over 3.5 days. The excitation of Earth's normal modes, particularly the prominent 3.7 mHz mode, indicates a strong atmosphere-solid Earth coupling. Future research should focus on advanced modeling of the eruption's dynamics and atmosphere-solid Earth interaction, considering the complex temporal evolution and the interplay of different atmospheric wave modes. Further investigations could explore the use of these long-period seismic signals as potential indicators of exceptionally large volcanic events.

The study primarily relies on seismic data, with limited direct information on the atmospheric conditions. While satellite imagery provides valuable contextual information, it does not fully capture the temporal resolution of the eruptive processes. Moreover, the interpretation of the low-frequency signals as Earth's normal mode excitation relies on comparisons with previous studies and the PREM model. Future studies should integrate a wider range of geophysical data (e.g., infrasound, atmospheric pressure, GPS, and ionospheric measurements) to provide a more comprehensive understanding of the event.