Volcanogenic tsunami and wave hazard remains less understood than that of other tsunami sources. Volcanoes can generate waves in a multitude of ways, including subaqueous explosions. Recent events, including a highly explosive eruption at Hunga Tonga-Hunga Ha’apai and subsequent tsunami in January 2022, have reinforced the necessity to explore and quantify volcanic tsunami sources. We utilise a non-hydrostatic multilayer numerical method to simulate 20 scenarios of sublacustrine explosive eruptions under Lake Taupō, New Zealand, across five locations and four eruption sizes. Waves propagate around the entire lake within 15 minutes, and there is a minimum explosive size required to generate significant waves (positive amplitudes incident on foreshore of >1 m) from the impulsive displacement of water from the eruption itself. This corresponds to a mass eruption rate of 5.8x10^7 kg s^-1, or VEI 5 equivalent. Inundation is mapped across five built areas and becomes significant near shore when considering only the two largest sizes, above VEI 5, which preferentially impact areas of low-gradient run-up. In addition, novel hydrographic output is produced showing the impact of incident waves on the Waikato river inlet draining the lake, and is potentially useful for future structural impact analysis. Waves generated from these explosive source types are highly dispersive, resulting in hazard rapidly diminishing with distance from the source. With improved computational efficiency, a probabilistic study could be formulated and other, potentially more significant, volcanic source mechanisms should be investigated.

The tsunami generation potential of pyroclastic density currents (PDCs) entering the sea is poorly understood, due to limited data and observations. Thus far, tsunami generation by PDCs has been modeled in a similar manner to tsunami generation associated with landslides or debris flows, using two-layer depth-averaged approaches. Using the adaptive partial differential equation solver Basilisk and benchmarking with published laboratory experiments, this work explores some of the important parameters not yet accounted for in numerical models of PDC-generated tsunamis. We use assumptions derived from experimental literature to approximate the granular, basal flow component of a PDC as a dense Newtonian fluid flowing down an inclined plane. This modeling provides insight into how the boundary condition of the slope and the viscosity of the dense granular-fluid influence the characteristics of the waves generated. It is shown that the boundary condition of the slope has a first-order impact on the interaction dynamics between the fluidized granular flow and water, as well as the energy transfer from the flow to the generated wave. The experimental physics is captured well in the numerical model, which confirms the underlying assumption of Newtonian fluid-like behaviour in the context of wave generation. The results from this study suggest the importance of considering vertical density and velocity stratification in wave generation models. Furthermore, we demonstrate that granular-fluids more dense than water are capable of shearing the water surface and generating significant amplitude waves, despite vigorous overturning.

A tsunamigenic earthquake occurred in the Southern New Hebrides subduction zone on the 10th of February 2021. The tsunami was observed at coastal gauges in the islands around the source area, and at a new DART buoy network that was designed to enhance the tsunami forecasting capability of the Southwestern Pacific (Fig.1). We used the tsunami waveforms in an inversion to estimate the fault slip distribution. The estimated major slip region is located near the trench with maximum slip amount of 4 m (Fig.2). The computed seismic moment for the source model of 3.39 × 1020 Nm (Mw 7.65) is consistent with the Global Centroid Moment Tensor and USGS W-phase Moment Tensor solutions. The estimated slip distribution (Fig.2a) was then used as reference model to evaluate our tsunami forecasting methods. We have developed a database of threat level maps for tsunami warning regions along the coast of New Zealand from earthquake scenarios with magnitudes ranging from 6.9 to 9.3 around the Pacific Ocean. Tsunami heights in coastal regions can be obtained by interpolating pre-computed results from selected scenarios around the earthquake location. The pre-computed waveforms can also be interpolated and then compared with the observation to verify the tsunami forecast. We found that the interpolated tsunami waveforms at the DART stations match the observations better than the waveforms from the pre-computed scenarios. We used the pre-computed scenarios to obtain a collection of B values that are required to enable the calculation of tsunami magnitude from tsunami observations observations (following the methods originally developed by Abe (1979) and extended by Baba et al. (2004)). A tsunami magnitude of Mt 7.72 was obtained from the tsunami peak amplitudes recorded at DARTs NZC, E, G, I along the Hikurangi-Kermadec-Tonga subduction zone. The tsunami magnitude was then used to predict tsunami heights in the tsunami warning regions. The predicted tsunami threat levels from both interpolation and tsunami magnitude methods can match those from the reference map in most of the warning regions.

The tsunami source of the MW8.1 2021 Raoul Island earthquake in the Kermadec subduction zone was estimated by inverting the tsunami signals recorded by DART bottom pressure sensors and coastal tide-gauges. The rupture propagated unilaterally northeastward from the hypocenter with maximum slip value of about 5 m, with features compatible with the aftershock distribution and rapid back-projection analysis. Three earthquakes of Mw ~8 or larger which also produced moderate tsunamis happened in the 20th century in the same portion of the subduction zone. This is the first great tsunamigenic event captured by the new New Zealand DART network in the South West Pacific, which proved valuable to estimate a robust image of the tsunami source. We also show a first proof of concept of the capability of this network to reduce the uncertainty associated with tsunami forecasting and to increase lead time available for evacuation for future alerts.

A tsunamigenic earthquake with thrust faulting mechanism occurred off the Loyalty Islands, New Caledonia, in the Southern New Hebrides subduction zone on the 10th of February 2021. The tsunami was observed at coastal gauges in the surrounding islands and in New Zealand. The tsunami was also recorded at a new DART network that was designed to enhance the tsunami forecasting capability of the Southwestern Pacific. We used the tsunami waveforms in an inversion to estimate the fault slip distribution. The estimated major slip region is located near the trench with maximum slip amount of 4 m. The computed seismic moment for the source model of 3.39 × 1020 Nm (Mw 7.65) is slightly smaller than the Global Centroid Moment Tensor or USGS W-phase Moment Tensor solutions. We evaluate two tsunami forecasting approaches of selecting a pre-computed scenario and interpolating pre-computed scenarios for coastal regions in New Zealand. For the evaluation, we first computed the tsunami threat levels in New Zealand coastal regions from the earthquake source model to make a reference threat level map. The results show that the tsunami threat level maps from a pre-computed Mw 7.7 scenario located closest to the epicenter and from an interpolation of two scenarios matched the reference threat levels at most of the coastal regions. We also report on utilization of the coastal gauge and DART buoy data for updating forecasts in real-time during the event and discuss the differences between the rapid-response forecast and post-event retrospective forecasts.