Sylfest Glimsdal

On the characteristics of landslide tsunamis

This review presents modelling techniques and processes that govern landslide tsunami generation, with emphasis on tsunamis induced by fully submerged landslides. The analysis focuses on a set of representative examples in simplified geometries demonstrating the main kinematic landslide parameters influencing initial tsunami amplitudes and wavelengths. Scaling relations from laboratory experiments for subaerial landslide tsunamis are also briefly reviewed. It is found that the landslide acceleration determines the initial tsunami elevation for translational landslides, while the landslide velocity is more important for impulsive events such as rapid slumps and subaerial landslides. Retrogressive effects stretch the tsunami, and in certain cases produce enlarged amplitudes due to positive interference. In an example involving a deformable landslide, it is found that the landslide deformation has only a weak influence on tsunamigenesis. However, more research is needed to determine how landslide flow processes that involve strong deformation and long run-out determine tsunami generation.

Coupling of Dispersive Tsunami Propagation and Shallow Water Coastal Response

The key issue of this article is the concept of combining a model dedicated to dispersive large scale propagation of tsunamis with ComMIT, developed and made freely available by NOAA, that is a state of the art tool for tsunami impact studies. First, the main motivation for this approach, namely the need for efficient computation of runup of tsunamis from submarine/subaerial slides and certain types of earthquake, is discussed. Then the models involved are presented. We describe in some detail the dispersive model component which is a Boussinesq type model that is recently developed for tsunami propagation purposes. Finally, the performance and flexibility of the joint model approach is illustrated by two case studies including inundation computations at selected cites. The potentially disastrous, but small probability, flank-collapse event at the La Palma Island is used as an example of slide generated tsunamis where dispersion plays an important role. The second example is a tsunami from a potential inverse thrust fault at the Lesser Antilles. In this case dispersion during propagation is important for some regions, but not for others.

The 1978 Quick Clay Landslide at Rissa, Mid Norway: Subaqueous Morphology and Tsunami Simulations

The 1978 landslide at Rissa is the largest to have struck Norway during the last century and is world-famous because it was filmed. Swath bathymetry data and seismic reflection profiles reveal detailed information about the subaqueous morphology of the mass-transport deposits (MTD). Results show that the landslide affected nearly 20% of the lake floor and that it exhibits a complex morphology including distinct lobes, transverse ridges, longitudinal ridges, flow structures and rafted blocks. The rafted blocks found at the outer-rim of the MTD travelled a distance of over 1,000 m in the early stage of the landslide on an almost flat basin floor. Simulation of sediment dynamics and tsunami modelling show that the rafted blocks most likely triggered the flood wave with a recorded maximum surface elevation of 6.8 m.

A global probabilistic tsunami hazard assessment from earthquake sources

Abstract Large tsunamis occur infrequently but have the capacity to cause enormous numbers of casualties, damage to the built environment and critical infrastructure, and economic losses. A sound understanding of tsunami hazard is required to underpin management of these risks, and while tsunami hazard assessments are typically conducted at regional or local scales, globally consistent assessments are required to support international disaster risk reduction efforts, and can serve as a reference for local and regional studies. This study presents a global-scale probabilistic tsunami hazard assessment (PTHA), extending previous global-scale assessments based largely on scenario analysis. Only earthquake sources are considered, as they represent about 80% of the recorded damaging tsunami events. Globally extensive estimates of tsunami run-up height are derived at various exceedance rates, and the associated uncertainties are quantified. Epistemic uncertainties in the exceedance rates of large earthquakes often lead to large uncertainties in tsunami run-up. Deviations between modelled tsunami run-up and event observations are quantified, and found to be larger than suggested in previous studies. Accounting for these deviations in PTHA is important, as it leads to a pronounced increase in predicted tsunami run-up for a given exceedance rate.

Submarine Landslides and Their Consequences: What Do We Know, What Can We Do?

The threats posed by submarine landslides to human civilization are the disappearance of valuable land near the shoreline, the destruction of seafloor installations like cables, pipelines or oil wells, and – most importantly – the devastation of coastal areas by landslide-generated tsunamis. Assessing and mitigating these hazards almost invariably implies the estimation of risk in situations where the probabilities associated with different scenarios are difficult to quantify. However, substantial progress has been made in the understanding of the geological processes and physical mechanisms operating at different stages of a submarine landslide event. This paper briefly reviews the state-of-the-art and points out why knowledge and methods from several disciplines of the physical sciences need to be combined to find solutions to the geotechnical engineering challenges from submarine landslides. A number of references to relevant case studies are also provided.

Probabilistic Tsunami Hazard Analysis: Multiple Sources and Global Applications

Applying probabilistic methods to infrequent but devastating natural events is intrinsically challenging. For tsunami analyses, a suite of geophysical assessments should be in principle evaluated because of the different causes generating tsunamis (earthquakes, landslides, volcanic activity, meteorological events, and asteroid impacts) with varying mean recurrence rates. Probabilistic Tsunami Hazard Analyses (PTHAs) are conducted in different areas of the world at global, regional, and local scales with the aim of understanding tsunami hazard to inform tsunami risk reduction activities. PTHAs enhance knowledge of the potential tsunamigenic threat by estimating the probability of exceeding specific levels of tsunami intensity metrics (e.g., run-up or maximum inundation heights) within a certain period of time (exposure time) at given locations (target sites); these estimates can be summarized in hazard maps or hazard curves. This discussion presents a broad overview of PTHA, including (i) sources and mechanisms of tsunami generation, emphasizing the variety and complexity of the tsunami sources and their generation mechanisms, (ii) developments in modeling the propagation and impact of tsunami waves, and (iii) statistical procedures for tsunami hazard estimates that include the associated epistemic and aleatoric uncertainties. Key elements in understanding the potential tsunami hazard are discussed, in light of the rapid development of PTHA methods during the last decade and the globally distributed applications, including the importance of considering multiple sources, their relative intensities, probabilities of occurrence, and uncertainties in an integrated and consistent probabilistic framework.

Modeling Potential Tsunami Generation by the BIG'95 Landslide

The BIG’95 landslide was emplaced 11,500 years ago and is one of the largest known submarine landslides in the Mediterranean Sea. The simulated landslide dynamics matches the observed run-out and deposited thickness. Water elevation simulated by using a dispersive tsunami model exceed 10 m close to the landslide area and at the nearest shorelines. Modeling further indicates that the tsunami probably had widespread consequences in the Mediterranean. Compared to previous studies, this new simulation provides larger waves. There is, however, still a need to better constrain the landslide dynamics in order to illuminate the uncertainties related to the tsunamigenic power of this, and other, submarine landslides.

Modelling of the 1888 Landslide Tsunami, Trondheim, Norway

The modelling of the tsunami generated by the 1888 landslide close to the Trondheim harbour, Norway is revised. Results for the tsunami generation, propagation, and inundation are shown. Improvements for the modelling are made for the generation phase. A special filter for the sea-surface response from the landslide is applied. In this manner the generated waves are more realistic, and spurious trailing waves due to too sharp gradients on the surface are effectively damped. Further, the landslide in the numerical model follows the most likely track for the 1888 event, and not only a straight line as in previous studies.

The Mjølnir Tsunami

Propagation characteristics of impact-generated tsunamis are different from most tsunami originating from other sources in that both nonlinearity and dispersion remain important for a long time after generation. This is particularly true for bolides with diameters that are comparable to, or larger than, the ocean depth. Submarine earthquakes and mass gravity flows on the other hand generally produce waves with amplitudes of only a few meters. Such tsunamis are linear during generation as well as propagation, while nonlinear effects become significant only close to the shore. Tsunamis of yet other origins, such as airborne slides, huge rock falls, or exploding/collapsing volcanoes, may locally display features reminiscent to impact tsunamis, but the far-field propagation is again linear. Oceanic impacts of asteroids and comets, however, may produce huge waves in mid ocean that stay strongly nonlinear during propagation over hundreds and thousands of km.

Bayesian Risk Assessment of a Tsunamigenic Rockslide at Åknes

This chapter introduces a comparison between two methods for estimating the risk of a tsunamigenic rockslide at Aknes, Norway. The first method follows a classical approach based on best estimates of the risk factors (i.e., hazard, vulnerability, and elements at risk). The second method follows a more recent approach based on Bayesian networks, which introduces the notion of causal effects and defines full probability distributions for each risk factor. The Bayesian approach is thought to be more powerful in terms of number and quality of inferences. It allows for conducting diagnosis and prognosis risk assessment analyses and it traces the influence of new evidence as it becomes available, either from experimental observations, model predictions, informed expert beliefs, a combination of them, or even “interventions” in the model to reproduce optimal decision-making processes (e.g., by introducing the stochastic model of an early warning system). Both methods illustrate the interaction of multiple natural threats when implemented in the Storfjord area where the Aknes rockslide is located. Results generated from the proposed methods are based on available evidence; however a key component on both approaches is the evidence assimilation from the experts who provided technical information, but also their beliefs in terms of probability measures (i.e., informed expert’s beliefs). The use of informed expert’s beliefs introduced a unique approach for incorporating fine engineering judgment into risk measures in a systematic manner. Results obtained from each method showed significant qualitative differences in terms of inference capabilities, but in terms of the expected risk estimates, their orders of magnitude were relatively similar, which validated the state of risk at the Aknes rockslide.