What is a landslide tsunami again?
November 03, 2020
Tsunamis caused by submarine (below water) and subaerial (above the water) landslides are a serious hazard in bays and fjords of coastal Alaska, particularly in Southeast and South-central Alaska. This region has a long history of tsunami waves generated by submarine and subaerial landslides, avalanches, and rockfalls. These have produced some of the largest tsunami waves recorded and, unlike earthquake-induced tsunamis, they can strike with no warning.
Submarine landslide-induced tsunamis
The most common trigger of submarine landslides is slope over-steepening due to high rates of sediments being deposited on already steep slopes. Other triggering factors include earthquakes, large tidal ranges that can expose unstable sediments and weaken their hold on the slope, construction activities in coastal areas, or some combination of these. The animation below shows how an underwater landslide causes the water above it to be pulled down. When the water rebounds back up towards the surface, it produces a local tsunami. Water can be displaced in front of or behind (as in the animation) the moving landslide.
Subaerial landslide-induced tsunamis
Subaerial landslide-induced tsunamis are the easiest to visualize. These are caused by a massive amount of material directly impacting the water surface from above. The water is displaced, forcing a wave outward from the impact point. This can be seen in the animation below from the National Weather Service. These landslides are most often caused by steep slopes rising directly out of the water, rapid erosion caused by glacial retreat or calving, heavy rains in coastal areas, and frequent earthquakes. They have also occurred because of volcanic eruptions. Subaerial landslides usually produce the largest waves because of the extra volume of material impacting the surface and causing a leading crest wave. The largest tsunami wave heights ever recorded were caused by subaerial landslides.
Alaska’s landslide tsunami history
Alaskan history is filled with stories of landslide tsunamis. They have caused devastating damage and produced megatsunamis. In 1958, landslides in Lituya Bay following a M7.8 earthquake on the Fairweather Fault produced a megatsunami with 1,720 feet of run-up. Run-up is the elevation above sea level that a tsunami wave reaches. You can read more about this event in 60 years ago: The 1958 earthquake and Lituya Bay megatsunami. During the M9.2 1964 Great Alaska Earthquake, more than twenty local tsunamis were generated by submarine and subaerial landslides in coastal Alaska. These local tsunamis accounted for 76% of the 122 tsunami-related fatalities. The tsunamis arrived almost immediately after the shaking began, leaving no time for warning or evacuation. In 2015, the giant wave of Icy Bay, with a run-up of 600 feet, was caused by the collapse of a mountainside near Tyndall Glacier into Taan Fjord after a period of heavy rains. You can read more in Landslide tsunamis: why they're different and how to prepare and Lessons for Alaska from the Palu Tsunami.
Currently, there are two areas that have been identified as potential landslides. One in Barry Arm, Prince William Sound and one in Tidal Inlet, Glacier Bay. There are many other potential landslides that have yet to be identified.