Tsunamis 101

Say tsunami and up pops a mental image of a single, giant wave rising out of the ocean to swallow cities whole. In reality, tsunamis (meaning 'harbour wave' in Japanese) are a series of waves that start small and grow as they approach land. They are the result of oceans attempting to smooth out their surface after a disturbance.

Tsunamis are triggered by any phenomenon that causes a large part of the water’s surface to rise or drop relative to normal sea level. These events are usually the result of earthquakes occurring along undersea fault lines, the cracks in the earth’s crust between tectonic plates. When these plates collide or grind against each other, they can elevate, lower, or tilt major sections of the ocean floor, suddenly offsetting the level of water at the surface. The displaced water then rushes to level out, causing a tsunami. The waves travel outward in all directions from the place where the earthquake occurred, just like the ripples created when a stone is thrown into a lake.

Tsunamis can also be caused by undersea volcanic eruptions, landslides, or explosions on the surface, such as the 1917 Halifax harbour explosion that sent 10-metre-high waves crashing into the city. It has also been suggested that asteroids or other extraterrestrial bodies could cause tsunamis if they plummet into large bodies of water, but there have been no examples of this in recent history.

A popular misconception is that tsunamis are monstrous waves that scour the ocean destroying everything in their path. The displacements caused by earthquakes and other cataclysms move huge masses of water, but they do not dramatically shift the surface level. Tsunami waves travelling in the open ocean can travel hundreds of kilometres per hour, but they are usually less than one metre high and their crests can be up to a hundred kilometres apart. They can be virtually invisible from the air and, for ships, be indistinguishable from the normal movement of the ocean. It is when these waves make landfall that they achieve their destructive potential. There are stories of fishermen who had no idea that a tsunami had struck their villages because they were too far out in the ocean to see or feel any waves at all.

As the ocean becomes shallower near the coast, tsunami waves slow down, compressing and directing their energy and volume upward, some rising to amplitudes of over 50 metres and annihilating whatever they encounter. Depending on the depth and slope of the coastline, it is also possible for the tsunami waves to wash over the shore like a flood or rapid current, as they did in the widely televised video footage taken in Thailand during the recent disaster in Southeast Asia.

Generally, coasts and islands with steep fringes or surrounded by barrier reefs are safer than those with gradually rising fringes or those that are exposed to open ocean. This is because reefs can absorb much of the oncoming waves’ impact and deep coastlines do not allow tsunamis to slow down and grow into deadly towers of water.