Mini Special Issue on Tsunami Numerical Modeling Benchmarks – Challenges of Tsunami Modeling Hackathon –

Numerical simulation and modeling became an essential technology in tsunami research and disaster management. Various numerical models were proposed and utilized for the development of tsunami risk assessment, inundation maps, and evacuation plans. The model verification and validation standards would be crucial to ensure sufficient reliability of tsunami risk assessment, inundation maps, as well as a consistency among various efforts. Common approach to ensure sufficient accuracy and reliability of numerical modeling is developing benchmark problems of hydraulic experiments and to use them for numerical model’s verification and validation. To satisfy this requirement, “Tsunami Modeling Hackathon” was held in September 2020 to organize new benchmark problems in numerical modeling of tsunamis and to improve their reliability and accuracy. Hackathon is an intensive-gathering event of computer programmers and others involved in software development to create outcomes by the end of the event. This event was organized by Prof. Tomoyuki Takahashi of Kansai University and his colleagues, who led the tsunami research subcommittee in Japan Society of Civil Engineers (JSCE). Tsunami modeling hackathon, in which about 23 teams and 162 researchers joined, included experiment and modeling teams in seven benchmark problems: urban tsunami inundation, landslide tsunami, tsunami loading on seawalls and coastal structures, sediment transport, drift of floating objects. The modeling groups performed the blind tests to cross-validate and interpret the results of their simulations in seven benchmark problems given by the experiment groups and discussed the improvement. This special issue reports the outcomes of the tsunami modeling hackathon, and includes six papers (five in this issue, one in the regular issue). We hope this issue will provide useful insights for tsunami modelers and contribute to establishing a standardized way to ensure that various tsunami numerical models would be validated through the benchmark problems.

Linking dynamic earthquake rupture to tsunami modeling for the Húsavík-Flatey transform fault system in North Iceland

<p>Earthquake generated tsunamis are generally associated with large submarine events on dip-slip faults, in particular on subduction zone megathrusts (Bilek and Lay, 2018). Submerged ruptures across strike-slip fault systems mostly produce minor vertical offset and hence no significant disturbance of the water column. For the 2018 Mw 7.5 Sulawesi earthquake in Indonesia, linked dynamic earthquake rupture and tsunami modeling implies that coseismic, mixed strike-slip and normal faulting induced seafloor displacements were a critical component generating an unexpected and devastating local tsunami in Palu Bay (Ulrich et al., 2019), with important implications for tsunami hazard assessment of submarine strike-slip fault systems in transtensional tectonic settings worldwide. </p><p>We reassess the tsunami potential of the ~100 km Húsavík Flatey Fault (HFF) in North Iceland using physics-based, linked earthquake-tsunami modelling. The HFF consists of multiple fault segments that localise both strike-slip and normal movements, agreeing with a transtensional deformation pattern (Garcia and Dhont, 2005). The HFF hosted several historical earthquakes with M>6. It crosses from off-shore to on-shore in immediate proximity to the town of Húsavík. We analyse simple and complex fault geometries and varying hypocenter locations accounting for newly inferred fault geometries (Einarsson et al., 2019), 3-D subsurface structure (Abril et al., 2020), bathymetry and topography of the area, primary stress orientations and the stress shape ratio constrained by the inversion of earthquake focal mechanisms (Ziegler et al., 2016).</p><p>Dynamic rupture models are simulated with SeisSol (https://github.com/SeisSol/SeisSol), a scientific open-source software for 3D dynamic earthquake rupture simulation (www.seissol.org, Pelties et al., 2014). SeisSol, a flagship code of the ChEESE project (https://cheese-coe.eu), enables us to explore simple and complex fault and subsurface geometries by using unstructured tetrahedral meshes. The dynamically adaptive, parallel software sam(oa)²-flash (https://gitlab.lrz.de/samoa/samoa) is used for tsunami propagation and inundation simulations and solves the hydrostatic shallow water equations (Meister, 2016). We consider the contribution of the horizontal ground deformation of realistic bathymetry to the vertical displacement following Tanioka and Satake, 1996. The tsunami simulations use time-dependent seafloor displacements to initialise bathymetry perturbations. </p><p>We show that up to 2 m of vertical coseismic offset can be generated during dynamic earthquake rupture scenarios across the HFF, which resemble historic magnitudes and are controlled by spontaneous fault interaction in terms of dynamic and static stress transfer and rupture jumping across the complex fault network. Our models reveal rake deviations from pure right-lateral strike-slip motion, indicating the presence of dip-slip components, in combination with large shallow fault slip (~8 m for a hypocenter in the East), which can cause a sizable tsunami affecting North Iceland. Sea surface height (ssh), which is defined as the deviation from the mean sea level, and inundation synthetics give an estimate about the impact of the tsunami along the coastline. We further investigate a physically plausible worst-case scenario of a tsunamigenic HFF event, accounting for tsunami sourcing mechanisms similar to the one causing the Sulawesi Tsunami in 2018.</p>