The Royal Society features Norwegian tsunami research

A landslide tsunami in Årdalsfjorden, Norway, caused great damage in 1983.

The world’s oldest scientific journal publishes a theme issue on tsunamis this week. Finn Løvholt of the Norwegian Geotechncial Institute, NGI, is the main author of one of the papers. He and his colleagues summarize the state-of-the-art in international research on landslide tsunamis, but also point out new perspectives.

In the article, they present new analyses, which for the first time show how various types of landslides represent great variations with respect to tsunami genesis. The Indian Ocean tsunami in 2004 opened the eyes of scientists and the public. As a result, the tsunami science came high, not only on the scientific, but also on the public agenda.

The Royal Society is the independent scientific academy of the UK and the Commonwealth. In 2015, they celebrate the 350th anniversary of Philosophical Transactions, the world's first science journal. When planning the theme issue on 10 years of tsunami research, the editors contacted Finn Løvholt, who is a senior engineer at NGI and associate adjunct professor at the University of Oslo. For several years, he has had a central position in international tsunami research. He recently headed a project that led to the first global tsunami risk maps, published by the UN organization ISDR.

Great variations
”The dynamics of different landslides can vary greatly, thus creating different tsunami generation processes. Since there are few observations of landslide tsunamis compared to earthquake tsunamis, we have to resort to calculations, to a large extent, in order to unveil the mechanisms. Previously, we have reviewed the occurrence of landslide tsunamis worldwide and their related hazard. In the Royal Society article we wanted to explore their basic mechanisms further. By combining theory and experiments from international research on landslide tsunamis with own new analyses at NGI and the University of Oslo, we wanted to uncover and present the main principles of wave generation,” explains Finn Løvholt, who is the main author of the article On the characteristics of landslide tsunamis.

Papua New Guinea an eye-opener
The article covers research on tsunamis caused by subaerial as well as submarine landslides.The authors explore calculation models in simple geometries, well suited to bring forth the main principles. These calculations are used to delineate how the main mechanisms in real events, such as the Storegga landslide outside the coast of Norway some 8,000 years ago, and the Papua New Guinea slump in 1998, takes place, but also how they differ greatly.In the latter event, more than 2,000 persons were killed, thus creating a historical eye-opener. Until then, many scientists had refused the theory that submarine landslides can indeed cause large tsunamis. For the same reasons, the authors of the article find reasons to believe that historical figures for tsunamis related to submarine landslides have been underreported.

Compared to an airplane
“We look into how the velocity, acceleration, landslide dimensions and depths of generation combine to influence the shape and size of the tsunami. The generation can be compared to an airplane moving close to the speed of sound. The speed of the tsunami waves increases with depth. A landslide moving fast in shallow waters will generate waves in an efficient manner, which is the case with rock slide impacts as well as coastal slope failures in fjords. However, for the largest submarine landslides, those that occur at great depths, the generation process is primarily governed by the acceleration of the landslide,” explains Finn Løvholt, who in January 2014 was awarded a research grant from the Research Council of Norway, as part of a program for young research talents.

The grant finances the Tsunamiland project, where Løvholt looks into connections between large landslides and tsunamis. The project aims to increase the understanding of retrogressive landslides, unleashed in step-by-step processes, and how this process influences the generations of waves. These landslides are similar to those in quick clay, well known from land, but the submarine retrogressive landslides can have much larger volumes.Field studies indicate that many of the historical tsunamis were triggered by retrogressive landslides, although this is area discipline where little research has been performed.

The latest issue of The Royal Society journal Philosophical Transactions A, published on the 21^st of September, includes 14 articles. The theme is ”Tsunamis: bridging science, engineering and society”.

Co-authors of the article are Geir Pedersen, UiO; Carl B. Harbitz, NGI and UiO; Sylfest Glimsdal, NGI; and Jihwan Kim, UiO.