Array-based source estimation of the tsunami later phases associated with the 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption

The eruption of the Hunga Tonga-Hunga Ha'apai volcano in January 2022 triggered a tsunami detected around the world. The first wave of this tsunami is now widely known to be related to the Lamb waves because (1) the pressure change started earlier than the expected tsunami arrival time and (2) the atmospheric pressure change was observed at the same time (e.g., Kubota et al., 2022).

On the other hand, the source has not been detected for the clear later phases. In this study, we applied the Vespa analysis (e.g., Davis et al., 1971) to the records of DONET and S-net, ocean-bottom pressure observation networks around Japan (e.g., Aoi et al., 2020).

The Vespa analysis assumes the horizontal slowness or back azimuth as input and estimates the other (Rost and Thomas, 2002). Since the phase speed of tsunamis can be determined by the bathymetry with the linear-long wave approximation, we adopted the assumption of the horizontal slowness and estimated the back azimuth of tsunamis.

Figure 1a shows the result of the Vespa analysis. The first signal corresponds to the Lamb wave. There are other clear phases slightly earlier than the expected tsunami arrival time, and its incident angles are different from the great circle between the array and Tonga island. In other words, the later phases were not originated from the atmospheric waves or sea-bottom displacement near the volcano.

Garrett (1970) indicated that the air wave at large bathymetry change can generate free ocean waves. Based on the later phase arrival time in Figure 1a, we estimated the location where the free wave was generated (Figure 1b). We also conducted a synthetic test and confirmed that such free waves can yield the same signal as shown in Figure 1a (Figure 1c).

In conclusion, the later phases observed between the Lamb and tsunami theoretical travel times were the free wave generated at a sharp bathymetry boundary in the south of Nauru Island.