A tsunami, or tidal wave, is created when a substantial volume of water in the oceans is displaced, resulting in a wave front which eventually reaches human coastal settlements. A tsunami can be caused by an earthquake, volcanic eruption, landslide or meteorite impact, though earthquakes are by far the most common cause. While a normal tsunami taken alone is a quite impressive, unique events (massive landslides or asteroid impacts) can trigger tsunamis so large and different from their smaller counterparts that they have been called supertsunamis, or mega tsunamis, for the purpose of classifying them separately.
While a normal tsunami appears not so much like a giant wave but more like a tide that just keeps rising, a supertsunami would be an unmistakable wall of water, killing its victims more by crushing than drowning. Conventional tsunamis are about a few meters high (about 10 feet) when they hit the shore, whereas a supertsunami would be dozens or even hundreds of meters high. Achieving the level of water displacement necessary to trigger a supertsunami would require a landslide of cubic kilometers of rock, large but plausible given the size of mountains on island chains both above and underwater.
One location being watched for signs of weakness and a possible catastrophic landslide is the Cumbre Vieja volcano, on the southern half of La Palma, one of the Canary Islands. A block of unstable rock more than 15 kilometers (9.3 miles) long could collapse if a large eruption or series of eruptions were to occur. The result would be a supertsunami sending waves around the globe, smashing into the east coast of the United States with a height of about 50 meters (164 feet) and a speed of hundreds of kilometers per hour. Rather than damaging the infrastructure of coastal cities in a way that is repairable, a supertsunami could toss skyscrapers like toys, reach miles inland, and permanently reshape the coastline. Luckily, eruptions on Cumbre Vieja happen only about every 200 years, with the most recent one occurring in 1949. Also, many experts believe that one eruption would not provide sufficient destabilizing force to dislodge the block.
More worrisome than a natural landslide is the possibility of deliberate meddling used to dislodge a massive chunk of land and create an artificial supertsunami. This could probably be achieved on La Palma with several large nuclear bombs. Terrorists of the future may see this as an opportunity to create the largest possible quantity of damage with limited resources.
