[S-VC31] Active Volcanism

[SVC31-P18] The sequence of Hunga Tonga-Hunga Ha’apai eruption in January 2022 revealed by the two closest seismometer records and satellite images

*Takuro Horiuchi^1,2, Mie Ichihara², Takayuki Kaneko², Kiwamu Nishida², Masaaki Morita², Tomoki Kimura¹ (1.Tokyo University of Science, 2.Earthquake Research Institute, University of Tokyo)

Keywords:Volcanic eruption, Hunga Tonga-Hunga Ha'apai, Volcanic earthquake, Volcano monitoring, Eruption sequence

Large shallow-water eruptions pose a high risk of volcanic hazards, but they are rare. Therefore, a detailed understanding of an eruption that occurred is important to prepare for future eruptions. This study focuses on the huge eruption of Hunga Tonga-Hunga Ha’apai (HTHH), a shallow-water volcano in the Kingdom of Tonga, on January 15, 2022, at 04:00 (all times are in UTC). Ichihara et al., (2022, JpGU) analyzed the seismo-acoustic data at a station in Fiji. As a result, they reported that an eruption began at 15:00 on January 13 and continued for about 22 hours (Phase 1), followed by intermittent eruptions until about 20:00 on January 14 (Phase 2), and the main eruption started at about 04:00 on January 15 (Phase 3). However, their single-station analyses could not identify all the small or sporadic eruption signals from the volcano. In this study, we add another seismometer data at a similar distance from HTHH and more detailed observations of the satellite images. We used Himawari-8 satellite images (600-km square section centered at HTHH, downloaded from NICT-Japan and preprocessed at ERI) and the vertical component (velocity) of the seismometer data from Wallis-Futuna (FUTU, 752km) in addition to Fiji (MSVF, 758km from HTHH). Seismometer data were downloaded from IRIS Web Services, corrected for the instrument responses, low-pass filtered below 0.15Hz, and re-sampled at 1 Hz. First, to synchronize the arrival times of the M5.8 earthquake near HTHH on January 15, 04:15:45 (USGS Earthquake Catalog), we shifted the FUTU data backward by 20 s. We calculated the amplitude mean-square roots (RMS) in a 60-s window, sliding every 4 s. We used the frequency range of 0.04-0.08 Hz because it is less affected by infrasound shaking. Comparing the RMSs revealed that all the confirmed seismic signals from HTHH started in FUTU’s RMS about 0 to 20 s (real-time difference 20 to 40 s) earlier than the MSVFs. Therefore, we listed the coherent RMS rises with the FUTU-to-MSVF time difference (dt) between 0 to 20 s as the confirmed seismic signals form HTHH. We marked those with dt between -20 to 0 s or 20 to 40 s as candidates and excluded those with dt beyond the range from the list. We also searched for individual eruptive events from the enhanced satellite images and made an event catalog. The previous study identified no sporadic eruptions after 20:00 on January 14 due to high wind noise. Cronin et al. (2023, IAVCEI) noted a small eruption at 02:57 on January 15 and considered it the beginning of the main event. Here, we found many small eruptions from 20:00 on January 14 to the main eruption, including that at 02:57, by the satellite images but no associated seismic signals. We concluded that the 02:57 event was one of the intermittent eruptions in Phase 2 and not appropriate as the main eruption onset. We found the seismic RMSs significantly increased with dt of 9s at 03:47, which we regard as the precursor of the main eruption. Assuming a seismic wave propagation time from HTHH of about 3 minutes, the onset time would be around 03:44. Borrero et al. (2022) conducted a domestic tsunami survey and interviews immediately after the eruption. Their report says the eruption onset at 03:47 but includes no specific evidence. The residents might have witnessed any disturbance associated with the seismic precursor. We also identified several RMS transients during the main eruption, for some of which we confirmed new plume emissions by the satellite images. In the future, we will work on more quantitative determination of the seismic signal sources and satellite image analyses to extract atmospheric perturbations and to constrain the mechanisms of the onset and sequences of the HTHH eruption.