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 an ocean-wide catastrophic tsunami, while no traces of a tsunami generated from the similar and nearby Trænadjupet 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.
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