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.

An unusual devastating tsunami occurred on 28 September 2018 after a strike-slip faulting earthquake in Sulawesi, Indonesia. The induced tsunami struck Palu city with 4-m wave height and flow depth. We performed a two-step analysis to investigate the source of the tsunami. We first conducted the teleseismic source inversion and obtained the slip distribution of the strike-slip fault. Our tsunami simulation from the coseismic deformation of the seismically-estimated strike-slip faulting produced a tsunami comparable to the leading part of the observation at Pantoloan. We then jointly utilized the tsunami waveform and Synthetic Aperture Radar (SAR) data to reconstruct the detailed slip distribution on the fault plane. Because of the lack of SAR data in the bay, the tsunami data is necessary to constrain the offshore slip distribution, which directly induces the tsunami. The inverted source model shows a strike-slip fault which consists of three segments extending from the epicenter to the south of 1.4°S with two bends and two asperities around Palu city. The joint inversion model accurately reconstructs the observed surface displacements and the leading part of the tsunami waveform. Our result exhibits the significant contribution of the strike-slip faulting to the tsunami, but it also suggests additional tsunami sources, such as landslides, for the high inundations near Palu bay. The result also indicates that regional devastating tsunamis can result from an onshore strike-slip fault with localized large dip slip.

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.