Understanding and reducing the disaster risk of landslide-induced tsunamis: a short summary of the panel discussion in the World Tsunami Awareness Day Special Event of the Fifth World Landslide Forum

A World Tsunami Awareness Day Special Event was held in hybrid mode on 5 November 2021, during the Fifth World Landslide Forum, in Kyoto, Japan. In this context, a panel discussion was organized across America, Europe, and Asia, with the goal to better understand and reduce the disaster risk of landslide-induced tsunamis, consistent with the Kyoto Landslide Commitment 2020. This article presents a short summary of this panel discussion.

Shinji Sassa (S. S.) served as the coordinator of the World Tsunami Awareness Day Special Event during WLF5 and organized the Panel Discussion. The panelists were Stephan T. Grilli (USA), David R. Tappin (UK), Kyoji Sassa (Japan), Dwikorita Karnawati (Indonesia), Viacheslav K. Gusiakov (Russia), and Finn Løvholt (Norway) (S. G., D. T., K. S., D. K., V. G., F. L., respectively). This article presents the framework, essential content, and a short summary of the Panel Discussion.
Panel discussion S. S. first introduced and presented the framework of the Panel Discussion. Each panelist then provided a few slide-talks covering what they considered to be essential for better understanding and reducing the disaster risk of landslide-induced tsunamis. This was followed by a general discussion moderated by the coordinator. At the conclusion, the coordinator presented a short summary of the Panel Discussion which was endorsed by all panelists. The framework, essential content, and a short summary of this Panel Discussion are concisely shown in Fig. 2.
Specifically, for better understanding and reducing the disaster risk of landslide-induced tsunamis, S. G. presented three fundamental and important issues, namely: 1. Triggering: when, where, how? 2. Tsunami generation and propagation: magnitude, where, how? 3. Landslide tsunami detection: magnitude/where? D. T. presented a global map of submarine landslide tsunami locations ( Fig. 1), noting that a broad global understanding of the hazard and mapping is required. K. S. presented simulations of coastal and submarine landslide-induced tsunamis and highlighted the important role of landslide motion in tsunami generation. D. K. presented controlling factors and characteristics of areas typically prone to landslides and provided a mitigation strategy based on hazard maps and evacuation plans. V. G. demonstrated the close relationship between oceanic sedimentation zones and landslide-triggered tsunamigenic potential, which could directly contribute to improving tsunami early warning and long-term risk assessment. Finally, F. L. stressed the lack of data for landslide volume probability, resulting from limited seafloor sub-bottom mapping, and highlighted the uncertainty in landslide dynamics leading to tsunami genesis.
During the general Panel Discussion, each of the key items summarized in Fig. 2 was critically discussed among the panelists (S. G., D. T., K. S., D. K., V. G., F. L.) and the coordinator (S. S.), and News/Kyoto Commitment similarities and connections were recognized between items. Key discussion points are briefly listed in the following. Triggering and source mechanisms need to be well constrained (S. G.), and one needs to better understand how both of these affect tsunami generation, in order to more accurately predict/model landslide-induced tsunamis (S. G., S. S.). There is still a lot to learn (D. T.) for better understanding and mitigating the disaster risk of landslide-induced tsunamis, regarding hazard mapping and improving warning (D. T., S. S.). Dual and multiple mechanisms must be considered to achieve improved mitigation (D. T.). However, our limited understanding and characterization of past events (D. T., F. L.) makes it difficult to well constrain landslide dynamics (F. L.). Landslide dynamics controls tsunami generation (genesis) but is inherently uncertain (F. L.); hence, a better integrated understanding of landslide Fig. 1 Global distribution of mapped submarine landslides (SLs): green, SLs on passive margins; yellow, SLs located along convergent margins; orange, SLs on strike slip margins; purple, volcanoes; red, tsunamis associated with SLs (Tappin and Grilli 2020). Submarine landslide tsunamis (in red) are mainly located along convergent margins, but also along passive and strike slip margins and on flanks of volcanoes Fig. 2 The framework, essential content, and a short summary of the panel discussion in the World Tsunami Awareness Day Special Event of the Fifth World Landslide Forum dynamics as well as multi-phased physics of landslide-water interactions is crucial to reducing landslide tsunami disaster risk (S. S., F. L.). Landslide dynamics itself features complex physics, such as liquefaction and evolutions of pore water pressures involving phase change processes (S. S., K. S.), and the in-depth understanding of such phenomena is important for improving landslide tsunami hazard assessment technologies (K. S., S. S.). A closer investigation of landslides that have caused tsunamis is thus very important (K. S.). In this respect, multiple triggering sources of tsunamis such as landslides caused by earthquakes and volcanic eruptions must be better understood (D. K.). Large volcanic flank collapses have induced large tsunamis, e.g., recently Anak Krakatau 2018 (S. G.); hence, sites of potential future events should be closely monitored and new instruments, allowing for early detection and warning, deployed (S. G., D. K.). In contrast, some landslide-induced tsunamis may impact shores very rapidly, such as in the cases of coastal and submarine landslide-induced tsunamis, e.g., recently Palu 2018 (D. K., S. S., K. S.). Therefore, in order to prepare for such fast arriving tsunamis, hazard mapping is critically important (S. S., D. K.). Accordingly, both hazard maps and 24-h warning are vital as mitigation strategies (D. K., S. S.). In light of multiple mechanisms, oceanic sedimentation conditions/zones have been overlooked as having high submarine landslide tsunami generation potential (V. G.); these can play a significant role in improving early warning and hazard maps (V. G., S. S.). Coastal tsunami zoning is also fundamental for the long-term tsunami risk assessment (V. G.).
In closing, many panelists stressed that more data is needed to better constrain geological and geotechnical conditions for hazard mapping (D. T., F. L.). Landslide-water interactions are currently poorly constrained, and their better understanding, together with that of the complex multi-phased physics this entails, is crucial towards developing early warning systems (F. L., S. S., S. G.). Overall, all the panelists and the coordinator agreed that developing a better understanding of the multiple mechanisms and multi-phased physics governing landslide tsunami hazard is important and necessary for improving landslide tsunami hazard mapping and early warning.

Summary
The Panel Discussion held across America, Europe, and Asia in the World Tsunami Awareness Day Special Event of the Fifth World Landslide Forum has produced an important outcome. All the panelists and the coordinator concur that hazard mapping and improved early warning are essential for better understanding and reducing the disaster risk of landslide-induced tsunamis. Achieving this will require developing a better integrated understanding of the multiple mechanisms and multi-phased physics of landslide tsunami hazard. Thus, it is desirable to develop an international collaborative network/platform, at both the regional and global scales, for the benefit of society and best practice. A more detailed outcome of this panel discussion and the World Tsunami Awareness Special Event will be reported in the new open access book series titled "Progress in Landslide Research and Technology".
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