Post-2015 caldera morphology of the Hunga Tonga-Hunga Ha’apai caldera, Tonga, through drone photogrammetry and summit area bathymetry

Abstract

<p>In December 2014, eruptions began from a submarine vent between the islands of Hunga Tonga and Hunga Ha’apai, 65 km north of Tongatapu, Tonga. The “Hungas” represent small NW and NE remnants of the flanks of a larger edifice, with a ~5 km-diameter collapse caldera south of them. The 2014/15 Surtseyan explosive eruptions lasted for 5 weeks, building a 140 m-high tuff ring.</p><p>Deposits on Hunga Ha’apai and tephra fall on Tongatapu record two very large magnitude eruptions producing local pyroclastic density currents and tephra falls of >10 cm-thick >65 km away. These likely derive from the central edifice/caldera. The 2022 eruption produced slightly less tephra fall, but an extremely large explosive event, with regional tsunami indicating substantive topographic change.</p><p>Here we report the bathymetric details of the caldera as of November 2015. A multibeam sounder (WASSP) was used to mapping the shallow (<250 m) seafloor concentrating on the edges of the Hunga caldera. These results were combined with an aerial survey of the 2015 tuff cone, using a combination of drone photogrammetry and real-time kinematic GPS surveys. The bathymetry reveals that previous historical eruptions, including 1988 and 2009, and likely many other recent unknown produced a series of well-preserved cones around the rim of the caldera. Aside from the raised ground in the northern caldera produced by the 2009 and 2014/15 eruptions, the southern portion is also elevated to within a few m below sea level, with reefs present. During the 2015 visit, uplifted fresh coral showed that inflation was ongoing and that the caldera was likely in the process of resurgence.</p><p>Much of Hunga Tonga and the 2014/2015 cone was destroyed in the 2022 eruptions, with Hunga Ha’apai intact, but dropping vertically by ~10-15 m. The violence of the 2022 eruption was likely augmented by either caldera collapse or flank collapse from the upper edifice, rapidly unroofing the andesitic magma system and enabling efficient water ingress.</p><p>This data provides an essential base layer for assessing changes on the ocean floor, especially to determine any caldera or upper-flank changes. Understanding these changes is crucial for future forecasting future volcanic hazards at Hunga and other nearby large submarine volcanoes.</p>

The characteristics of the 2022 Tonga volcanic tsunami in the Pacific Ocean

“…Island and the 2014/2015 cone were destroyed by the 2022 eruption, and the vertical deformation of Hunga Ha'apai Island is ~10-15 m (Stern et al, 2022). With no more quantitative constraint of the seafloor deformation, we tentatively assume 𝐻 as 10-15 m, then the possible dimension of seafloor crustal deformation responsible for the small-scale tsunami could be in the scale of 0.8-1.8 km (Figure 9a).…”

“…Where 𝐿 is the typical dimension (length or width) of the tsunami source, H is average water depth in the source area, 𝑔 is the gravity acceleration, and T is primary tsunami period. By comparing with the post-2015 morphology of the HTHH caldera which was obtained through drone photogrammetry and multibeam sounder surveys, Stern et al (2022) estimate that much of the newly-formed Hunga Tonga…”