Offshore hazard cascades from the largest volcanic eruption this century

Volcanic eruptions in marine settings pose a diverse range of hazards, both directly and indirectly caused by the eruption. In January 2022 the partially-submerged Tongan volcano Hunga Tonga - Hunga-Ha'apai experienced one of the most powerful volcanic events seen in decades, generating a tsunami that caused damage both locally and on shorelines thousands of km away, breaking the only seafloor telecommunications cables that connect Tonga to the rest of the world and causing $90.4M of damage, equivalent to 18.5% of Tonga's Gross Domestic Product. The damage to the cable both severely hampered efforts to contact and assist Tonga in the immediate aftermath of the disaster and the time of writing (16th Feb 2022) had not yet been repaired; effectively meaning that 105,000 Tongan citizens have had to rely on low bandwidth, high latency satellite communication for over a month. Both the local tsunami and the cable break were unusual. First, while local tsunami waves ran up to 15m in some parts of Tonga, this near field tsunami was smaller and caused less damage than similar events elsewhere e.g. smaller 2018 Anak Krakatau volcanic-tsunami. Second, while the explosive event at the volcano occurred at 04:14 UTC the seabed cable faults did not occur until 05:40 UTC, 90 minutes later. This proposal will examine whether these events were caused by secondary submarine landslides or other process and will characterise their locations, dynamics and magnitudes.

In order to address these important questions we will collect new high-resolution multibeam bathymetric data in April 2022 over the region to compare with existing high-resolution data from before the eruption. This comparison will allow us to identify seafloor changes caused by Hunga Tonga -Hunga Ha'apai, map their locations and extents and calculate the volumes of material added or mobilised during this event. This study will also provide a new baseline from which future larger studies of this potentially paradigm-shifting eruption can be based and the products generated will help to constrain the boundary conditions for future tsunami modelling.

In order for data to be accurate and useful they must be collected as soon as is feasible after the eruption. The seafloor is extremely dynamic (as shown by repeat surveys at smaller offshore volcanoes), large volumes of material can be deposited over short timescales and existing shallow sediments can be remobilised by waves and storms. This opportunity is unique both because of the scale of the event and because of the high-quality data available to study it (pre-existing bathymetry, cooperation from cable operators, well constrained eruption timings and processes) and also takes advantage of extending a scheduled research cruise nearby, significantly reducing the associated costs and CO2 outputs. Cable companies can share data from the faults and repair, but their vessels are not equipped with multibeam sonars required to perform detailed seafloor surveys; hence the causes of faults, the nature of the eruptive event are unclear and cannot be addressed by satellite data either.

Hunga Tonga-Hunga Ha'apai is far from unique; there are numerous similar volcanoes both in the Tofua Arc and worldwide. However, very few of these are monitored and most are poorly surveyed; hence the risk they pose is unclear. This timely project will provide the first detailed time-lapse surveys for such a large offshore eruption, and thus enables major step changes in understanding the dynamics of extremely large eruptions, and how they generate secondary hazards, via tsunami or breaking critical seabed telecommunication cables that carry >99% of all digital traffic globally. Time is of the essence; performing a rapid response survey (by extending an already scheduled cruise that will travel close to the area) will provide robust answers to fundamental questions about submarine volcanic eruptions and their linked hazards.