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The Lituya Bay Landslide-Generated Mega-Tsunami – Numerical Simulation and Sensitivity Analysis

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The Lituya Bay Landslide-Generated Mega-Tsunami – Numerical Simulation and Sensitivity Analysis Nat. Hazards Earth Syst. Sci., 19, 369–388, 2019 https://doi.org/10.5194/nhess-19-369-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. The Lituya Bay landslide-generated mega-tsunami – numerical simulation and sensitivity analysis José Manuel González-Vida1, Jorge Macías2, Manuel Jesús Castro2, Carlos Sánchez-Linares2, Marc de la Asunción2, Sergio Ortega-Acosta3, and Diego Arcas4 1Dpto. de Matemática Aplicada, ETSII, Universidad de Málaga, 29080, Málaga, Spain 2Dpto. de Análisis Matemático, Estadístice e Invetigación Operativa y Matemática Aplicada, Facultad de Ciencias, Universidad de Málaga, 29080, Málaga, Spain 3Unit of Numerical Methods, SCAI, Universidad de Málaga, 29080, Málaga, Spain 4NOAA/Pacific Marine Environmental Laboratory (PMEL), Seattle, WA, USA Correspondence: Jorge Macías ([email protected]) Received: 26 July 2018 – Discussion started: 15 August 2018 Revised: 21 December 2018 – Accepted: 3 January 2019 – Published: 15 February 2019 Abstract. The 1958 Lituya Bay landslide-generated mega- 1 Introduction tsunami is simulated using the Landslide-HySEA model, a recently developed finite-volume Savage–Hutter shallow wa- Tsunamis are most often generated by bottom displacements ter coupled numerical model. Two factors are crucial if the due to earthquakes. However, landslides, either submarine or main objective of the numerical simulation is to reproduce subaerial, can also trigger devastating tsunami waves. In ad- the maximal run-up with an accurate simulation of the in- dition, they are, on some occasions, extremely destructive as undated area and a precise recreation of the known trimline they form near the coast or in the same coastline if they are of the 1958 mega-tsunami of Lituya Bay: first, the accu- aerial. Sometimes landslides may generate so-called mega- rate reconstruction of the initial slide and then the choice of tsunamis, which are characterized by localized extreme run- a suitable coupled landslide–fluid model able to reproduce up heights (Lituya Bay, 1958, Miller, 1960; Fritz et al., 2009; how the energy released by the landslide is transmitted to 1934 Tafjord event, Norway, Jørstad, 1968; Harbitz et al., the water and then propagated. Given the numerical model, 1993, Taan Fjord, 17 October 2015, Bloom et al., 2016; the choice of parameters appears to be a point of major im- Higman et al., 2018; or the very recent event of Sulawesi, portance, which leads us to perform a sensitivity analysis. 29 September 2018, among many others). Based on public domain topo-bathymetric data, and on in- For seismic tsunami simulations, in general, the most crit- formation extracted from the work of Miller (1960), an ap- ical phases are generation and arrival at a coast, includ- proximation of Gilbert Inlet topo-bathymetry was set up and ing inundation. Propagation over deep basins can be mod- used for the numerical simulation of the mega-event. Once eled using the nonlinear shallow water (NLSW) equations optimal model parameters were set, comparisons with ob- or more typically using a non-diffusive linear approxima- servational data were performed in order to validate the nu- tion. With landslide-generated tsunamis, however, matters merical results. In the present work, we demonstrate that a become more complicated. The generation phase itself be- shallow water type of model is able to accurately reproduce comes critical and complex effects between the landslide and such an extreme event as the Lituya Bay mega-tsunami. The the water body must be taken into account. Most notably, resulting numerical simulation is one of the first successful the case of subaerial-landslide-generated tsunamis is where attempts (if not the first) at numerically reproducing, in de- modeling and numerical implementation becomes most criti- tail, the main features of this event in a realistic 3-D basin cal, owing to these events producing more complex flow con- geometry, where no smoothing or other stabilizing factors in figurations, larger vertical velocities and accelerations, cavi- the bathymetric data are applied. tation phenomena, dissipation, dispersion, and complex cou- pled interaction between landslide and water flow. It is evi- Published by Copernicus Publications on behalf of the European Geosciences Union. 370 J. M. González-Vida et al.: The Lituya Bay landslide-generated mega-tsunami dent that shallow water models cannot take into account and Despite this, and independent of the eventual confinement reproduce all of these phenomena, particularly vertical ve- of the flow, many authors continue to claim the absolute need locities or cavitation. We do, however, demonstrate here that for dispersive, or even full Navier–Stokes, models (Abadie such models can, despite limitations, be useful for hazard as- et al., 2009), when dealing with the simulation of landslide- sessment in which the main features (from a hazard assess- generated tsunamis. Still other authors, not so strict in their ment point of view) of these complex events, such as run- assessment, claim that dispersive models represent a better up and main leading wave, are reproduced. The overall aim alternative than NLSW models (Løvholt et al., 2015). The is not to accurately reproduce the evolution of the displaced lack of dispersive model simulations in the literature of the solid material or the dispersive nature of the trailing waves as Lituya Bay event plays against the argument of ruling out the perturbation propagates, but, alternatively, to accurately nondispersive coupled models, such as the one proposed in reproduce the impact of tsunami waves to coasts in terms of the present work. Very recently, in February 2017 during run-up and flood area. Comparison of the numerical results the “2017 US National Tsunami Hazard Mitigation Program with the observed trimline presented here is shown to sup- (NTHMP) Tsunamigenic Landslide Model Benchmarking port our statement that a fully coupled, vertically integrated Workshop”, held on the Texas A&M Galveston, Texas, cam- shallow water Savage–Hutter model can, effectively and ac- pus, participants agreed to recommend to the NTHMP Map- curately, reproduce the run-up and coastal inundation result- ping and Modeling Subcommittee (MMS) the use of disper- ing from aerial landslides generated in fjords and enclosed sive models for numerical simulation of landslide-generated basins. This study further supports this assessment by com- tsunamis. For the case of enclosed basins, bays, and fjords, it paring model results with observed data for a paradigmatic was agreed that NLSW models remain a suitable tool. example of extreme run-up produced by an aerial landslide in Among all examples of subaerial-landslide-generated an enclosed bay, a simulation that has not been successfully tsunamis, the Lituya Bay 1958 event occupies a paradigmatic undertaken previously with more comprehensive numerical place in the records, standing alone as the largest tsunami models. ever recorded and representing a scientific challenge of ac- Full 3-D numerical modeling of landslide-generated curate numerical simulation. Based on generalized Froude tsunamis (Horrillo et al., 2013) is uncommonly used for similarity, Fritz et al.(2009) built a 2-D physical model real-world scenarios due to the highly demanding computa- of the Gilbert Inlet scaled at 1 : 675. A number of works tional resources required. Whereas common thought persists have focused their efforts in trying to numerically reproduce that NLSW equations are sufficient for simulation of ocean- Fritz et al.(2009) experiments (Mader and Gittings, 2002; wide tsunami propagation averaged models, the importance Quecedo et al., 2004; Schwaiger and Higman, 2007; Weiss of frequency dispersion for modeling landslide-generated et al., 2009; Basu et al., 2010; Sánchez-Linares, 2011). De- tsunamis lends preference to Boussinesq models. In no case, tailed numerical simulations of the real event in the whole however, can any of these standard models describe the vio- of Lituya Bay with a precise reconstruction of the bottom lent impact of subaerial landslides with flow separation and bathymetry and surrounding topography are limited. As an complex subsequent flow patterns and slide material evolu- example, Mader(1999) fails in reproducing the 524 m run- tion. We further claim that, in enclosed basins, the two main up and concludes that the amount of water displaced by a mechanisms that need to be well captured and accurately re- simple landslide at the head of the bay is insufficient to cause produced by a numerical model are first the transmission of the observed tsunami wave. As far as we know, the present energy from the slide material into the water body and, sec- work represents the first successful attempt to realistically ond, the coastal inundation by means of accurate wet–dry simulate and reproduce the 1958 Lituya Bay mega-tsunami treatment. Confinement makes the role of dispersion minor in a realistic three-dimensional geometry with no smoothing relative to other effects. in the geometry or initial conditions. In the case of tsunamigenic aerial landslides in fjords, bays, or any long and narrow water body, confinement and reflection (a process that also makes propagation and in- 2 Background teraction more complex) are relatively more important con- siderations than dispersion, which becomes less important. At 06:16 UTC on 10 July 1958, a magnitude Mw 8:3 earth- This is particularly true for the leading wave (Løvholt et al., quake occurred along the Fairweather Fault (Alaska,
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