The Mediterranean Hellenic Arc subduction zone (HASZ) has generated several Mw>=8 earthquakes and tsunamis. Seismic-probabilistic tsunami hazard assessment typically utilizes uniform or stochastic earthquake models, which may not represent dynamic rupture and tsunami generation complexity. We present an ensemble of ten 3D dynamic rupture earthquake scenarios for the HASZ, utilizing a realistic slab geometry. Our simplest models use uniform along-arc pre-stresses or a single circular initial stress asperity. We then introduce progressively more complex models varying initial shear stress along-arc, multiple asperities based on scale-dependent critical slip weakening distance, and a most complex model blending all aforementioned heterogeneities. Thereby, regional initial conditions are constrained without relying on detailed geodetic locking models. Varying hypocenter locations in the simplest, homogeneous model leads to different rupture speeds and moment magnitudes. We observe dynamic fault slip penetrating the shallow slip-strengthening region and affecting seafloor uplift. Off-fault plastic deformation can double vertical seafloor uplift. A single-asperity model generates a Mw~8 scenario resembling the 1303 Crete earthquake. Using along-strike varying initial stresses results in Mw~8.0-8.5 dynamic rupture scenarios with diverse slip rates and uplift patterns. The model with the most heterogeneous initial conditions yields a Mw~7.5 scenario. Dynamic rupture complexity in prestress and fracture energy tends to lower earthquake magnitude but enhances tsunamigenic displacements. Our results offer insights into the dynamics of potential large Hellenic Arc megathrust earthquakes and may inform future physics-based joint seismic and tsunami hazard assessments.

Tsunamis are rare, destructive events, whose generation, propagation and coastal impact processes involve several complex physical phenomena. Most tsunami applications, like probabilistic tsunami hazard assessment, make extensive use of large sets of numerical simulations, facing a systematic trade-off between the computational costs and the modelling accuracy. For seismogenic tsunami, the source is often modelled as an instantaneous sea-floor displacement due to the fault static slip distribution, while the propagation in open-sea is computed through a shallow water approximation. Here, through 1D earthquake-tsunami coupled simulations of large M>8 earthquakes in Tohoku-like subduction zone, we tested for which conditions the instantaneous source (IS) and/or the shallow water (SW) approximations can be used to simulate with enough accuracy the whole tsunami evolution. We used as a reference a time-dependent (TD), multi-layer, non-hydrostatic (NH) model whose source features, duration, and size, are based on seismic rupture dynamic simulations with realistic stress drop and rigidity, within a Tohoku-like environment. We showed that slow ruptures, generating slip in shallow part of subduction slabs (e.g. tsunami earthquakes), and very large events, with an along-dip extension comparable with the trench-coast distance (e.g. mega-thrust) require a TD-NH modelling, in particular when the bathymetry close to the coast features sharp depth gradients. Conversely, deeper, higher stress-drop events can be accurately modelled through an IS-SW approximation. We finally showed to what extent inundation depend on bathymetric geometrical features: (i) steeper bathymetries generate larger inundations and (ii) a resonant mechanism emerges with run-up amplifications associated with larger source size on flatter bathymetries.

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.