Linked and fully-coupled 3D earthquake dynamic rupture and tsunami modeling for the Húsavík-Flatey Fault Zone in North Iceland

Abstract. Tsunamigenic earthquakes pose considerable risks, both economically and socially, yet earthquake and tsunami hazard assessments are typically conducted separately. Earthquakes associated with unexpected tsunamis, such as the 2018 M_w 7.5 strike-slip Sulawesi earthquake, emphasize the need to study the tsunami potential of active submarine faults in different tectonic settings. Here, we investigate physics-based scenarios combining 3D earthquake dynamic rupture with tsunami generation and propagation for the ∼100 km long Húsavík-Flatey Fault Zone in North Iceland using time-dependent one-way linked and 3D fully-coupled earthquake-tsunami modeling. Our analysis shows that the HFFZ has the potential to generate sizeable tsunamis. The six dynamic rupture models sourcing our tsunami scenarios vary regarding hypocenter location, spatio-temporal evolution, fault slip, and fault structure complexity but coincide with historical earthquake magnitudes. We find that the earthquake dynamic rupture scenarios on a less segmented fault system, particularly with a hypocenter location in the eastern part of the fault system, have a larger potential for local tsunami generation. Here, dynamically evolving large shallow fault slip (∼8 m), near-surface rake rotation (±20°), and significant coseismic vertical displacements of the local bathymetry (±1 m) facilitate strike-slip faulting tsunami generation. We model tsunami crest-to-trough differences (total wave heights) of up to ∼0.9 m near the town Ólafsfjörður. In contrast, none of our scenarios endanger the town of Akureyri, which is shielded by multiple reflections within the narrow Eyjafjörður Bay and by Hrísey Island.

We compare the modeled one-way linked tsunami waveforms with simulation results using a 3D fully-coupled approach. We find good agreement in the tsunami arrival times and location of maximum tsunami heights. While seismic waves result in transient motions of the sea surface and affect the ocean response, they do not appear to contribute to tsunami generation. However, complex source effects arise in the fully-coupled simulations, such as tsunami dispersion effects and complex superposition of seismic and acoustic waves within the shallow continental shelf of North Iceland. We find that the vertical velocities of near-source acoustic waves are unexpectedly high - larger than those corresponding to the actual tsunami - which may serve as a rapid indicator of surface dynamic rupture. Our results have important implications for understanding the tsunamigenic potential of strike-slip fault systems worldwide and the co-seismic acoustic wave excitation during tsunami generation and may help to inform future tsunami early warning systems.