How Things Work: Tsunamis

Akanksha Vaidya Feb 25, 2008

Standing at the edge of the beach watching the waves crash into the sand is a wonderful feeling, but not if those waves are more than 100 feet high and gush violently. In this case, onlookers can only see a shimmering wall before the waves crash down destroying everything in their path. That is a tsunami.

In Japanese, “tsu” translates into “harbor” and “nami” means “wave,” giving birth to the term “tsunami,” which translates into “harbor waves.” A tsunamis is a series of huge waves that are usually 100 to 115 feet in height near the coastline.

Normal oceanic waves are formed due to the gravitational pull exerted on the earth by the moon.

Tsunamis, on the other hand, are caused due to oceanic disturbances like earthquakes, volcanoes, or underwater landslides. While underwater landslides do not cause devastating tsunamis like the one that hit Southeast Asia in 2004, with earthquakes and underwater volcanoes it’s a whole different story.
On Dec. 26, 2004, an underwater earthquake measuring around 9.0 on the Richter scale originated about 100 miles away from Sumatra, an island of Indonesia.

According to the Guardian, the aftereffects of the natural disaster claimed nearly 230,000 lives and spanned 12 countries, including Indonesia, Sri Lanka, and India.

This is the most recent example of the devastating effects of a tsunami. Yet, tsunamis have a long history of wrecking havoc in different parts of the world.
It is hard to imagine that an earthquake in the middle of the ocean can cause such large-scale destruction miles away. However, looking at the mechanism through which the tsunami waves propagate, the results do not seem so surprising.

The outer crust of the earth is divided into sections called tectonic plates. The layer of the earth directly below these tectonic plates is not completely solid. Therefore, the tectonic plates tend to move slightly on the semi-solid layer. Earthquakes usually originate at the boundaries of the tectonic plates of the earth.

Tsunamis are caused due to earthquakes at regions of the ocean bed called subduction zones. In subduction zones, two tectonic plates exert tremendous forces on each other at the boundaries; one plate tends to push down on the other.

The two plates resemble pieces of paper overlapping and pushing against each other. When one piece pushes down on the other, the other piece folds.
Tectonic plates follow a similar process. As a result, the ocean bed forms atop the folded plate.

However, if the plates continue to push against each other, at some point the folded plate will not be able to withstand the tension and will spring out of the folded position. When the plate springs out, it causes the ocean bed to tilt and the water to rise, leading to a tsunami.

Similar displacement of water occurs during huge underwater explosions like volcanoes.

It may be surprising to learn that the tsunami that crashes on the shore is not a single wave, but a combination of smaller waves. The waves that are set up in the ocean are actually around three to six feet high, though the waves that strike the coastline can even be 100 or 160 feet high.

The large difference between the waves near the coast and those in the middle of the ocean is due to the mechanism by which the waves travel away from the origin. The waves travel at high speeds from the origin, then slow down as they approach the coast. Thus, each wave is usually moving slower than the waves behind it.

The waves collide with each other and combine into one, increasing each other’s amplitude. The result is that a wave which was originally of moderate size is now gigantic.

If civilization happens to be on a coast where this wave crashes, there is virtually no hope of survival. The tsunami that struck Southeast Asia in 2004 demonstrates this fact.

Although a number of tourists in the Sri Lankan wildlife park lost their lives because of the tsunami, reports from the park indicated that no animal casualties occurred. The animals ran to a higher ground well before the fatal waves struck the land.

Massive vibrations, called “Rayleigh waves” are sent out from the origin of the earthquake before a tsunami occurs. These waves move through the ground at speeds 10 times the speed of sound. Therefore, they reach the land a long time before the tsunami waves do. Rayleigh waves also have large wavelengths and are a type of infrasound wave.

Animals are likely to sense the infrasound waves and run away from them to a higher ground, which is what happened in the Sri Lankan park.
Humans also have infrasound sensors called pacinian corpuscles in their joints. These sensors are designed specifically to feel such waves, but since the human brain has to store much more information, it becomes insensitive to signals sent by the corpuscles.

Having said that, what contradicts this theory is that tribes in the Andaman Islands of India were able to sense the tsunami before it hit and thus found shelter on higher ground.

The only other way that humans can detect a tsunami is seismic technology. Seismologists use radars to detect disturbances in the ocean and seismographs to detect earthquakes.

Scientists also send satellites to monitor oceanic behavior.