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Landslides are the second most important cause of tsunamis after earthquakes, and their potential for generating large tsunamis depend on the slide process. Among the world's largest submarine landslides is the Storegga Slide that generated a large tsunami over an ocean-wide scale, while no traces of a tsunami generated from the similar and nearby Traenadjupet Slide have been found. Previous models for such landslide tsunamis have not been able to capture the complexity of the landslide processes and are at odds with geotechnical and geomorphological data that reveal retrogressive landslide development. The tsunami generation from these massive events are here modeled with new methods that incorporate complex retrogressive slide motion. We show that the tsunamigenic strength is closely related to the retrogressive development and explain, for the first time, why similar giant landslides can produce very different tsunamis, sometimes smaller than anticipated. Because these slide mechanisms are common for submarine landslides, modeling procedures for dealing with their associated tsunamis should be revised.

Plain Language Summary We here study how some of the largest landslides that are observed on the planet may generate destructive tsunamis. The cases of interest are two landslides, named Storegga and Traenadjupet, which occurred about 8000 and 4000 ago below the sea offshore Norway. Their surface areas are similar to the area of Scotland. Surprisingly, only one of these two enormous landslides, Storegga, was known to generate a destructive tsunami. Here we use computer simulations to explain why the other landslide, Traenadjupet, did not produce a a large wave. The computer simulations revealed that differences in the dynamics of the two landslides led to very different tsunamis. This has broader implications on our understanding of how massive submarine landslide generate tsunamis and shows that we need better models to explain why apparently similar landslides generate tsunamis of very different size.