A Landslide Probabilistic Tsunami Hazard Analysis for the Åkerneset and Hegguraksla rockslides (Norway)

Landslide tsunami hazard analysis is characterized by substantial uncertainty, particularly in estimating exceedance probabilities for tsunami heights. As a result, no standardized hazard methodology exists for landslide tsunamis, and most studies rely on deterministic or scenario-based approaches, while Probabilistic Tsunami Hazard Analysis (PTHA) methods are seldom applied. The limited use of probabilistic frameworks is largely due to scarce observational data on past landslide tsunamis and the increased modelling complexity required to represent landslide sources, especially subaerial failures producing impact tsunamis. 

To address these challenges, we apply a Landslide Probabilistic Tsunami Hazard Analysis (LPTHA) framework previously developed for Norwegian fjord environments, focusing here on the potentialÅkerneset and Hegguraksla rockslides in western Norway. The LPTHA combines landslide occurrence rates derived from slope stability assessments with an event-tree formulation describing uncertainty in key landslide kinematic parameters controlling tsunami generation, such as impact velocity, impact frontal area, and runout distance. Quantification of epistemic uncertainty requires large ensembles of simulations spanning wide parameter ranges, which necessitates computationally efficient modelling approaches. 

In this study, tsunami generation is represented using a "rounded block” landslide source model, coupled with a linear dispersive wave propagation model and a non-linear shallow-water model for nearshore propagation and inundation. The resulting LPTHA provides probabilistic estimates of tsunami and run-up heights within the fjord system, explicitly accounting for uncertainty in landslide dynamics and tsunami response. The analysis highlights the sensitivity of hazard estimates to landslide source parameterization and demonstrates the applicability of LPTHA as a systematic framework for probabilistic tsunami hazard assessment in fjord settings affected by large unstable rock slopes. Finally, the methodology enables a presentation of run-up heights with corresponding probabilities as required by the Norwegian Planning and Building Act.