What type of landslide can generate a tsunami underwater?

A submarine landslide can generate a tsunami underwater. When a large mass of sediment or rock suddenly collapses and rushes downslope along the seafloor, it displaces seawater and creates waves that spread outward.

These failures can be triggered by earthquakes, volcanic activity, rapid sediment buildup, storms, gas-hydrate changes, or oversteepened underwater slopes. The size of the tsunami depends on the slide’s volume, speed, acceleration, and depth. Unlike many earthquake-generated tsunamis, landslide tsunamis often produce very large waves near the source but lose energy more quickly across open water.

A famous example occurred in Papua New Guinea in 1998, when an earthquake-triggered submarine landslide produced waves up to about 15 metres high and killed roughly 2,200 people. The Storegga Slide off Norway, about 8,200 years ago, also generated a major prehistoric tsunami. A debris flow or mudslide may describe how displaced material moves, but “submarine landslide” is the broad, canonical answer to this question.