Tsunami

Understanding the Devastating Power of Tsunamis

Tsunamis are not just giant waves; they’re a force of nature that can reshape coastlines and claim lives in an instant. Imagine a tsunami as a colossal wave, much like a tidal surge but on a scale so vast it could swallow entire cities whole. These waves are caused by the displacement of water due to earthquakes, volcanic eruptions, landslides, or even meteorite impacts. But what exactly makes them such a formidable threat?

Causes and Mechanisms

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. This displacement can occur due to earthquakes, landslides, volcanic eruptions, or glacier calvings. The 2011 Tōhoku earthquake in Japan, for instance, was a megathrust earthquake, which generated one of the most powerful tsunamis ever recorded.

The term ‘tsunami’ comes from the Japanese word for harbour wave and has become commonly accepted in English. However, its literal meaning is not descriptive of the waves, which do not occur only in harbours. Tsunamis are sometimes referred to as tidal waves, but this usage is discouraged by geologists and oceanographers due to their different causes.

History and Impact

The 2004 Indian Ocean tsunami stands out as one of the most devastating events in modern times, killing around 230,000 people. The Sumatran region is also accustomed to tsunamis, with earthquakes regularly occurring off its coast. Tsunamis are an often underestimated hazard in the Mediterranean Sea and parts of Europe.

Historical records show that the oldest human record of a tsunami dates back to 479 BC, in the Greek colony of Potidaea, thought to be triggered by an earthquake. The Roman historian Ammianus Marcellinus described the typical sequence of a tsunami after the 365 AD tsunami devastated Alexandria.

Causes and Generation Mechanisms

The principal generation mechanism of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement can be caused by earthquakes, landslides, volcanic eruptions, glacier calvings, or other rare events.

Tsunamis have a small wave height offshore, a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea. They grow in height when they reach shallower water through a process described as wave shoaling. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

Examples of Tsunamis

The 8.6 Mw Aleutian Islands earthquake generated a tsunami that inundated Hilo on the island of Hawaii with a 14-metre high surge, killing 165-173 people. Examples of tsunamis originating at locations away from convergent boundaries include:

• Storegga about 8,000 years ago
• Grand Banks in 1929
• Papua New Guinea in 1998 (Tappin, 2001)

The cause of the Storegga sediment failure is unknown and may have been caused by overloading of sediments, an earthquake, or a release of gas hydrates.

Volcanic Eruptions and Meteorological Conditions

Volcanic eruptions can generate waves through various mechanisms. Pyroclastic flow submergence, caldera collapse, or underwater explosions have caused over 20% of all fatalities during the past 250 years. Tsunamis triggered by volcanic eruptions are lesser understood than seismic tsunamis and pose a large problem of awareness and preparedness.

Meteorological conditions can displace bodies of water enough to cause meteotsunamis, which have lower energies but can still cause localized damage and loss of life. Human-made or triggered tsunamis include the Halifax Explosion in 1917 and studies of using explosives to induce tsunamis as a tectonic weapon, with nuclear testing having poor results.

Prevention and Mitigation

Tsunami-prone countries have taken measures such as building walls, floodgates, and channels to reduce damage. However, these measures have been questioned due to tsunamis often overtopping barriers. The Fukushima Daiichi nuclear disaster was triggered by a tsunami in 2011, despite having tsunami barriers.

Some zoologists suggest that certain animals may be able to sense subsonic waves from earthquakes or tsunamis, potentially providing advance warning of disasters, but this evidence is controversial. Elephants may have heard tsunami warnings as they approached the coast and moved away from the noise. Humans who went to investigate suffered devastating consequences.

Conclusion

Tsunamis are a natural phenomenon that can be both unpredictable and devastating. Understanding their causes, mechanisms, and impacts is crucial for preparing and mitigating their effects. As we continue to study these waves of destruction, the hope is that one day we might find ways to predict them more accurately and protect those in harm’s way.