The destructive powers of earthquakes are widely known.

Earthquakes occur when major tectonic activities take place along plate boundaries.

The damages earthquakes cause on land come in such forms as toppled buildings and broken roads.

But the devastating effects of earthquakes do not just take place on land.

70%of the earth is water, and many tectonic faults are found below ocean beds. So, what happens when major tectonic activity takes place at these tectonic faults under the sea?

We get Tsunamis, or giant waves.

What causes Tsunamis?

When there is major vertical tectonic movements under the seafloor, the overlying seawater gets displaced from its equilibrium position.

Water, as we know, will always maintain a level that is parallel to the ground level. When part of the seafloor is suddenly raised higher, the big mass of seawater, under the pull of gravity, will attempt to regain an equilibrium position. When such a big mass of water moves suddenly, it will cause giant waves, known as tsunamis, to form.

Subduction earthquakes, which occur when one tectonic plate moves vertically below another, are a major cause of tsunamis.

How do tsunamis affect us?

Due to the sudden major tectonic movements, tsunamis tend to be of great heights and speed. If tsunamis occur in the middle of oceans, they may have little or no effects on us. But tsunamis have high destructive powers if they approach or come into contact with land.

Tsunamis have great height and speed. Tsunamis easily reach a vertical height, also called a runup height, of up to 30 metres onshore above sea level.

As a tsunami approaches shore, it will to slow down and decrease in height. Like other waves, a tsunami loses energy as it approaches land. But, despite such energy loss, tsunamis still contain tremendous amounts of energy as they reach land.

The large amount of energy tsunamis have means they have great erosion potential. They will strip beaches of sand that may have taken years to accumulate and destroy vegetation on the land.

Tsunamis can also flood hundreds of meters inland past the typical high-water level. These fast-moving floods easily destroy buildings and other inland structures as they move along, and can result in the deaths of many people.

Recent Tsunami

One recent major tsunami occurred on July 17, 1998 in Papua New Guinea along the Sissano Lagoon. It was caused by a two-metre vertical drop in the Pacific tectonic plate along a 40-kilometre crack.

This tsunami measured about 10 metres in height. It occurred so suddenly that people along the shore had no time to run.

The tsunami flooded villages along a 30 kilometres coastline. Thousands of homes were destroyed, making many people homeless. Thousands of people lost their lives. Many of those who died were old people and young children, because they were too weak to escape the tsunami.

The West Coast & Alaska Tsunami Warning Centre (http://wcatwc.gov) tracks tectonic activities and issue tsunami warnings.