Tsunamis

You might be too young to remember, but a devastating tsunami took place in 2004. It is known as the 2004 Indian Ocean earthquake and tsunami, and as the name suggests, the tsunami was triggered by a massive earthquake. Some of the tsunami waves were 30m high, and over 200,000 people were killed.

But what exactly are tsunamis?

What is the main cause of tsunamis?

Earthquakes at convergent plate margins cause most tsunamis. Because of the movement from the earthquake, the seabed is displaced vertically. This, in turn, displaces the water column above. The speed of a tsunami depends on the depth of the ocean. As it approaches shallow water, the tsunami slows down and increases in height, making it very dangerous.

What are other causes of tsunamis?

It’s not just earthquakes that cause tsunamis – landslides and volcanic activity are also culprits! Let’s see why.

Tsunamis caused by landslides

Large landslides on marine margins can create tsunamis when their mass enters the ocean water or when water displaces behind and in front of an underwater landslide. Earthquakes often initiate landslides.

Tsunamis caused by volcanic activity

Here are the different ways volcanic activity can cause tsunamis:

• Underwater volcanic explosions and shock waves can cause tsunamis when the energy travels through the ocean.
• Pyroclastic flows (mixtures of hot volcanic rock, ash, and gas), lahars, and other parts of volcanoes can cause tsunamis when their mass enters and moves the water.
• If the roof of an empty magma chamber collapses, water rushes in and displaces the water column of the sea. This then gives rise to a tsunami wave.

How often do tsunamis occur?

Tsunamis causing damage near the source occur, on average, twice per year, whilst tsunamis that are greater and cause damage to distant shores occur approximately twice per decade. Smaller tsunamis occur almost every day with little effect.

What are the characteristics of tsunamis?

The characteristics of tsunami waves differ from wind-generated waves in terms of their period and wavelength.

• Wind-generated waves tend to have periods of five to 20 seconds and wavelengths of 100 to 200m.
• Tsunami waves have periods between ten minutes to two hours and wavelengths between 150 to 1000km. Due to their large wavelength, tsunamis lose very little energy as they propagate.

When a tsunami approaches the shore, its speed decreases (due to shallower water) and its height increases. At the shore, tsunamis can appear as rising and falling tides, breaking waves, or bores.

What influences the impact of tsunamis?

The impact of tsunamis is influenced by several factors, including the amplitude of the tsunami wave, how far the tsunami wave has travelled, the water column displacement, the duration and number of tsunami waves, the geological structure of the coast, when the tsunami occurs, the amount of development of the coastal area, the distance between the coast and the populated areas, and the quality of tsunami monitoring, forecasting, and warning technology.

What are the possible damages caused by tsunamis?

The possible damages caused by tsunamis include death and injuries, destruction of and damage to infrastructure, destruction of and damage to boats and marine installations, fires that cause further damage, environmental pollution, and an outbreak of diseases. Small islands hit by tsunamis are often totally destroyed.

How do we reduce the impact of tsunamis?

The impact of tsunamis can be reduced through effective forecasting and warning. For instance:

• The National Tsunami Warning Center in Palmer, Alaska, monitors earthquake activity to provide forecasts on possible tsunamis at the coastlines. Additionally, sea levels are monitored using shallow and deep water instruments to determine whether a tsunami is happening.
• Tsunami warning systems have been placed in both the Pacific and Indian Oceans. These systems use seismic sensors to detect any underwater earthquakes. They also use a system known as DART (Deep-ocean Assessment and Reporting of Tsunami), which uses seabed sensors and surface buoys to monitor changes in sea level and pressure.

Tsunami case study

We won’t go into too much detail here as we already have a case study explanation on the Tohoku Earthquake and Tsunami. But let’s briefly look at some interesting facts on the 2004 Indian Ocean earthquake and tsunami mentioned at the beginning of this explanation:

• The earthquake was an undersea megathrust earthquake measuring 9.1 moment magnitude.
• The earthquake caused a series of huge tsunami waves that caused massive destruction and loss of life. It is one of the deadliest natural disasters to occur in recent years.
• Countries affected included Indonesia, Sri Lanka, India, and Thailand.
• Donations for humanitarian relief amounted to over £10 billion.

The 2004 Indian Ocean tsunami, Thailand, David Rydevik,