Landslide Tsunamis

The Other Kind of Tsunami

Landslide collapses building in southern China. Getty Images

By now everyone on Earth knows about tsunamis, like the dreadful ones from 2004 and 2011, especially for people unfamiliar with the earlier tsunamis of 1946, 1960 and 1964. Those tsunamis were of the ordinary type, seismic tsunamis caused by earthquakes that suddenly lift or drop the seafloor. But a second type of tsunami can arise from landsliding with or without an earthquake, and shorelines of every kind, even lakes on land, are susceptible.

Landslide tsunamis are harder to predict, harder for scientists to model and harder to defend against.

Landslide Tsunamis and Earthquakes

Landslides of various kinds can push water around. Mountains may crumble to the sea, as the song goes. Mudslides may plop into lakes and reservoirs. And land that lies entirely beneath the waves may fail. In all cases, the landslide material displaces water, and the water responds in very large waves that spread rapidly out in all directions.

Many landslides occur during earthquakes, so landslides can complicate seismic tsunamis. The Grand Banks earthquake in eastern Canada on 18 November 1929 was tolerable, but the ensuing tsunami killed 28 people and ruined the economy of southern Newfoundland. The landslide was quickly detected by the fact that it broke 12 submarine cables linking Europe and America with communications traffic.

The role of landslides in tsunamis has become more important as tsunami modeling has advanced.

The deadly Aitape tsunami in Papua New Guinea on 17 July 1998 was preceded by an earthquake of magnitude 7, but seismologists couldn't make the seismic data match the tsunami observations until seafloor surveys later showed that a large submarine landslide was also involved. Now awareness has been raised.

Today the best advice is to beware of a tsunami any time you experience an earthquake near any body of water. Alaska's dire Lituya Bay, a steep-walled fjord on a major fault zone, has been the site of several stupendous landslide tsunamis related to earthquakes including the largest one on record. Lake Tahoe, high in the Sierra Nevada between California and Nevada, is prone to both seismic and landslide tsunamis.

Human-Caused Tsunamis

In 1963, a massive landslide pushed some 30 million cubic meters of water over the new Vajont Dam, in the Italian Alps, which killed some 2500 people. The filling of the reservoir destabilized the adjoining mountainside until it gave way. Amazingly, the reservoir designers were attempting to let the mountainside collapse gently by manipulating the water level. Dave Petley, writer of the Landslide Blog, doesn't use the word tsunami in his description of this manmade tragedy, but that's what it was.

Prehistoric Megatsunamis

Recently with the improved maps of the world's seafloor, we have found evidence suggesting truly gigantic disturbances that must have created landslide tsunamis equal to today's worst events. Like the supposed threat of "supervolcanoes" based on the great size of ancient volcanic deposits, the idea of impending "megatsunamis" has gotten a lot of credulous attention.

Very large seafloor landslides could occur in many places, where they could have produced tsunamis. Consider the fact that rivers are constantly depositing sediment on the continental shelves on the edge of every continent. At some point there will be one sand grain too many, and a runaway landslide over the edge of the shelf could move a lot of material beneath a lot of water. If a distant earthquake isn't the trigger, a large local storm could be.

Also to be considered is the long-term climate, including ice ages. Rising water temperatures or falling sea levels that accompany different stages of an ice age could destabilize the delicate methane hydrate deposits in subarctic regions. That kind of slow destabilization is one common explanation for the enormous Storegga Slide in the North Sea off Norway, which left widespread tsunami deposits on surrounding lands about 8200 years ago.

Given that sea level has been steady ever since, we can discount the possibility that a repeat slide is imminent even though the average ocean temperature is likely to rise with global warming.

Another postulated tsunami mechanism is the collapse of volcanic islands, which are generally considered to be more fragile than continental rocks. There are large chunks of Molokai and other Hawaiian islands found far away on the Pacific Ocean floor, for instance. Similarly, the volcanic Canary and Cape Verde islands in the North Atlantic are known to have collapsed at times in the past.

Scientists who modeled these collapses got a lot of press a few years ago when they suggested that eruptions on these islands could cause them to fall apart and raise truly killer waves all around the Pacific or Atlantic shoreline. But there are compelling arguments that nothing like this is likely today. Like the thrilling threat of "supervolcanoes," megatsunamis would be foreseeable many years in advance.