Tsunami Hazard Assessment Along Chinese Coast Using Scaling Relations Developed for Tsunami Prediction

Potential tsunamis in the western Pacific Ocean pose great threats to the Chinese coastal areas. Among all possible tsunami source regions, the Manila subduction zone draws the most attention and there have been many research works on the tsunami hazards in the South China Sea. In this study, we evaluate the tsunami hazard along the Chinese coast by investigating more potential sources, including the subduction zones of Manila, Ryukyu, Nankai, Izu–Bonin and Mariana. Two tsunami scenarios are considered for each subduction zone, a worst scenario of earthquake magnitude 9.0 and a scenario of largest earthquake magnitude known in history in this zone. Earthquake source parameters are calculated using scaling relations that have been shown to be suitable for tsunami generation. Our results show that for the Chinese coast, tsunami hazards from the Manila and Ryukyu subduction zones are severe in the worst scenarios, and tsunami hazards from the Nankai, Izu–Bonin and Mariana subduction zones are mild. Using the largest earthquake magnitude in history, tsunami hazards from all the investigated subduction zones are almost negligible. Through a sensitivity test on earthquake magnitude, we find that earthquakes of magnitude of 8.5 or larger in the Manila and Ryukyu subduction zones cause severe tsunami hazard along the Chinese coast with wave amplitude over 2 m.

Analysis of the tsunami amplification effect by resonance in Yeongil Bay

Predicting tsunami hazards based on the tsunami source, propagation, runup patterns is critical to protect humans and property. Potential tsunami zone, as well as the historical tsunamis in 1983 and 1993, can be a threat to the east coast of South Korea. The Korea Meteorological Administration established a tsunami forecast warning system to reduce damage from tsunamis, but it does not consider tsunami amplification in the bay due to resonance. In this study, the Numerical model, Cornell Multi-grid Coupled Tsunami model, was used to investigate natural frequency in the bay due to coastal geometry. The study area is Yeongill bay in Pohang, southeast of South Korea, because this area is a natural bay and includes three harbors where resonance significantly occurs. This study generated a Gaussian-shaped tsunami, propagated it into the Yeongill bay, and compared numerical modeling results with data from tide gauge located in Yeongill bay during several storms through spectral analysis. It was found that both energies of tsunamis and storms were amplified at the same frequencies, and maximum tsunami wave height was amplified about 3.12 times. The results in this study can contribute to quantifying the amplification of tsunami heights in the bay.

Reexamination of Tsunami Source Models For the 20th Century Earthquakes Off Hokkaido and Tohoku Along the Eastern Margin of the Sea of Japan

Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, and cause both damage from the earthquake itself and from the ensuing tsunami to the coastal areas. Recently, offshore active fault surveys were conducted in the Sea of Japan by the Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (JSPJ), and their fault models (length, width, strike, dip, and slip angles) have been obtained. We examined the causative faults of M7 or larger earthquakes in the Sea of Japan during the 20th century using seismic and tsunami data. The 1940 off Shakotan Peninsula earthquake (MJMA 7.5) appears to have been caused by the offshore active faults MS01, MS02, ST01, and ST02 as modelled by the JSPJ. The 1993 off the southwest coast of Hokkaido earthquake (MJMA 7.8) likely occurred on the offshore active faults OK03a, OK03b, and OK05, while the 1983 Central Sea of Japan earthquake (MJMA 7.7) probably related to MMS01, MMS04, and MGM01. For these earthquakes, the observed tsunami waveforms were basically reproduced by tsunami numerical simulation from the offshore active faults with the slip amounts obtained by the scaling relation with three stages between seismic moment and source area for inland earthquakes. However, the observed tsunami runup heights along the coast were not reproduced at certain locations, possibly because of the coarse bathymetry data used for the simulation. The 1983 west off Aomori (MJMA 7.1) and the 1964 off Oga Peninsula (MJMA 6.9) earthquakes showed multiple faults near the source area that could be used to reproduce the observed tsunami waveforms; therefore, we could not identify the causative faults. Further analysis using near-field seismic waveforms is required for their identification of their causative faults and their parameters. The scaling relation for inland earthquakes can be used to obtain the slip amounts for offshore active faults in the Sea of Japan and to estimate the coastal tsunami heights and inundation area which can be useful for disaster prevention and mitigation of future earthquakes and tsunamis in the Sea of Japan.

Upper Plate Structure and Tsunamigenic Faults near the Kodiak Islands

The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modelling and geophysical data analysis. Through seismic and bathymetric data, we characterize a regionally extensive sea floor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity-rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short travel paths and little tsunami warning time for nearby communities.

Probabilistic near-field tsunami source and tsunami run-up distribution inferred from tsunami run-up records in northern Chile

Understanding a tsunami source and its impact is vital to assess a tsunami hazard. Thanks to the efforts of the tsunami survey teams, high-quality tsunami run-up data exists for contemporary events. Still, it has not been widely used to infer a tsunami source and its impact mainly due to the computational burden of the tsunami forward model. In this study, we propose a TRRF-INV (Tsunami Run-up Response Function-based INVersion) model that can provide probabilistic estimates of a near-field tsunami source and tsunami run-up distribution from a small number of run-up records. We tested the TRRF-INV model with synthetic tsunami scenarios in northern Chile and applied it to the 2014 Iquique, Chile, tsunami event as a case study. The results demonstrated that the TRRF-INV model can provide a reasonable tsunami source estimate to first order and estimate tsunami run-up distribution well. Moreover, the case study results agree well with the United States Geological Survey report and the global Centroid Moment Tensor solution. We also analyzed the performance of the TRRF-INV model depending on the number and the uncertainty of run-up records. We believe that the TRRF-INV model has the potential for supporting accurate hazard assessment by (1) providing new insights from tsunami run-up records into the tsunami source and its impact, (2) using the TRRF-INV model as a tool to support existing tsunami inversion models, and (3) estimating a tsunami source and its impact for ancient events where no data other than estimated run-up from sediment deposit data exists.

Multilayer modelling of waves generated by explosive submarine volcanism

Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with submarine volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale submarine explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a previously unreported relationship of extended initial troughs for negative disturbances at low dispersivity and high nonlinearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude range representing ejecta volumes between 0.04–0.4 km3. Waves reach all shores within 15 minutes with maximum incident crest amplitudes around 4 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating submarine explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism.

3D Linked Subduction, Dynamic Rupture, Tsunami, and Inundation Modeling: Dynamic Effects of Supershear and Tsunami Earthquakes, Hypocenter Location, and Shallow Fault Slip

Physics-based dynamic rupture models capture the variability of earthquake slip in space and time and can account for the structural complexity inherent to subduction zones. Here we link tsunami generation, propagation, and coastal inundation with 3D earthquake dynamic rupture (DR) models initialized using a 2D seismo-thermo-mechanical geodynamic (SC) model simulating both subduction dynamics and seismic cycles. We analyze a total of 15 subduction-initialized 3D dynamic rupture-tsunami scenarios in which the tsunami source arises from the time-dependent co-seismic seafloor displacements with flat bathymetry and inundation on a linearly sloping beach. We first vary the location of the hypocenter to generate 12 distinct unilateral and bilateral propagating earthquake scenarios. Large-scale fault topography leads to localized up- or downdip propagating supershear rupture depending on hypocentral depth. Albeit dynamic earthquakes differ (rupture speed, peak slip-rate, fault slip, bimaterial effects), the effects of hypocentral depth (25–40 km) on tsunami dynamics are negligible. Lateral hypocenter variations lead to small effects such as delayed wave arrival of up to 100 s and differences in tsunami amplitude of up to 0.4 m at the coast. We next analyse inundation on a coastline with complex topo-bathymetry which increases tsunami wave amplitudes up to ≈1.5 m compared to a linearly sloping beach. Motivated by structural heterogeneity in subduction zones, we analyse a scenario with increased Poisson's ratio of ν = 0.3 which results in close to double the amount of shallow fault slip, ≈1.5 m higher vertical seafloor displacement, and a difference of up to ≈1.5 m in coastal tsunami amplitudes. Lastly, we model a dynamic rupture “tsunami earthquake” with low rupture velocity and low peak slip rates but twice as high tsunami potential energy. We triple fracture energy which again doubles the amount of shallow fault slip, but also causes a 2 m higher vertical seafloor uplift and the highest coastal tsunami amplitude (≈7.5 m) and inundation area compared to all other scenarios. Our mechanically consistent analysis for a generic megathrust setting can provide building blocks toward using physics-based dynamic rupture modeling in Probabilistic Tsunami Hazard Analysis.