• A magnitude 8.8 earthquake struck off Russia’s Far Eastern Kamchatka Peninsula on July 30, damaging buildings and generating a tsunami of up to 4 metres that prompted warnings and evacuations stretching across the Pacific Ocean.

• Several people were injured in the remote Russian region, while much of Japan’s eastern seaboard was ordered to evacuate.

• The US Geological Survey said the earthquake was shallow at a depth of 19.3 km, and was centred 119 km east-southeast of Petropavlovsk-Kamchatsky. 

• Tsunami alarms sounded in coastal towns across Japan’s Pacific coast with authorities urging people to seek higher ground.

What is a tsunami?

• A tsunami is a series of waves generated by a large and sudden displacement of the ocean. 

• Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. 

• The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Unlike wind-driven waves, which only travel through the topmost layer of the ocean, tsunamis move through the entire water column, from the ocean floor to the ocean surface.

• The word “tsunami” comprises the Japanese words ‘tsu’ (meaning harbour) and ‘nami’ (meaning wave). 

• Tsunami waves often look like walls of water and can attack the shoreline and be dangerous for hours, with waves coming every 5 to 60 minutes.

• The first wave may not be the largest, and often it is the second, third, fourth or even later waves that are the biggest. After one wave inundates, or floods inland, it recedes seaward often as far as a person can see, so the seafloor is exposed. The next wave then rushes ashore within minutes and carries with it many floating debris that were destroyed by previous waves.

• Tsunamis have claimed hundreds of thousands of lives and caused hundreds of billions of dollars in damage around the world.

• In the past 100 years, 58 of them have claimed more than 260,000 lives, or an average of 4,600 per disaster, surpassing any other natural hazard. The highest number of deaths in that period was in the Indian Ocean tsunami of December 2004.

What causes a tsunami?

• About 80 per cent of all known tsunamis are triggered by earthquakes. These seismic events move Earth’s surface, displacing the water above and generating waves that rapidly travel in all directions across the ocean or body of water. 

• Not all earthquakes create tsunamis. An earthquake must be big enough and close enough to the ocean floor to cause the vertical movement of the ocean floor that typically sets a tsunami in motion. As the ocean floor rises or drops, so does the water above it. As the water moves up and down, seeking to regain its balance, the tsunami radiates in all directions. 

• Tsunamis can also be caused by landslides, volcanic activity, certain types of weather, near-earth objects (asteroids, comets) colliding with or exploding above the ocean.

World Tsunami Awareness Day

The UN observes November 5 as World Tsunami Awareness Day, calling on countries, international bodies and civil society to raise tsunami awareness and share innovative approaches to risk reduction.

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