Why some earthquakes lead to tsunamis?


logo By Afroz Ahmad Shah

Some of the past earthquakes, which have been followed by tsunamis have been of the magnitude 9.0 (1952), of Kamchatka, the magnitude 9.1 (1957), of Andrean of Islands, Alaska, the magnitude 9.5 (1960) Chile, and the magnitude 9.2 (1964), Prince William Sound, Alaska. However; the scale of destruction and the damage caused by these quakes was far less than the destruction which followed the 26th December, 2004, Aceh-Andaman earthquake and its tsunami. This catastrophe was a warning bell and it changed the perspective of people regarding earthquake dangers in oceans and therefore opened a variety of avenues to understand a virtually dead subject of oceanic earthquake research. More than 2, 30,000 people were killed and it affected several millions more.

Tsunamis unlike the earthquakes on land, have one advantage, it gives us time to evacuate because the waves take some time to travel until they reach the coast and thus provides an opportunity to save lives. However, during 2004, the warning system was a complete failure, particularly in far-off places like, Thailand, Sri Lanka, India and East Africa. There was no tsunami warning in place because people had very short memories about disasters of the past, which caused further loss to life. This was again realised in September 2009, earthquake that produced a tsunami in southwest Pacific and killed nearly 200 people, again pointing at the miserable failure of authorities to deliver a timely warning system! This however, changed to some extent in Japan, where they have successfully adopted a strict building code, which saved several lives, when in March, 2011 an earthquake of magnitude 9 rocked this small country. The earthquake resistant structural engineering marvel of Japan is a huge success and therefore saved buildings with minor destruction. A giant tsunami, which followed the quake, caused a serious loss of life and property. Its source was close to the coast and therefore, the warning system did not help. Therefore some 20,000 people lost their lives to the deadly devastating tsunami. Japan to a large extent is a well-prepared nation for earthquakes and tsunami.  But they hadn’t been expecting an earthquake and tsunami of the magnitude that occurred on March 11, 2011, which suggests an immediate need to thoroughly understand and map all the seismogenic (capable of producing earthquakes) faults and more so, the megathrust.

All of these earthquakes, like the one on March, 2011, were megathrust events (see below), occurring where one tectonic plate dives beneath another.

What is a Tsunami?

A tsunami means a big wave or a series of big waves. They can be caused by any big disturbance in the ocean or any other body of water. For example during an earthquake under water, an enormous amount of energy is released when a fault slips. It can cause the crust to move up or down, therefore forcing a great column of water to follow it. This can cause a total imbalance in water body and can form a big tsunami.  For example, the recent Tsunami along the coast of Sumatra was caused by an earthquake offshore, where a fault line some 1000 miles long ruptured. A volcanic eruption in or close to the Ocean can also result in formation of big waves. The eruption of Krakatoa, two centuries ago, caused a tsunami throughout South East Asia and the tremors traveled throughout the world. Likewise a meteorite or land sliding can potentially cause big or small tsunamis.

Tsunamis are described as shallow-water waves and are different from the normal sea waves, which are generated by the wind. Normally, wind-generated waves have small periods (time between two successive waves), five to twenty seconds and wavelengths (100 to 200 meters). However, these numbers magnify to a large extent during a tsunami and thus it can have a period in the range of ten minutes to two hours and wavelengths greater than 500 km and because of its very large wavelength, a tsunami tends lose little energy as it moves ahead. The water near the shore can initially, be the trough of the coming wave and the water may swirl out away from the shore. But as the leading edge of the fast moving wave comes into shallow water, it is slowed down. The water behind doesn’t know this, and carries on pushing forward. The edge of the wave gets higher. As more and more fast moving wave push into the slowing wave front, it gets higher and higher; and steeper and steeper. Eventually it can become a moving vertical wall of water, whose height depends on the geometry of the shore and the characteristics of the tsunami.

The earthquakes and the associated tsunamis are caused because of the friction along the megathrust faults (see below). For example, had there been no resistance to the continuous push and therefore, a smooth journey of the Pacific plate underneath the North American plate, we would have not witnessed an earthquake or the tsunami. However, this is not the case with plates, which are huge and heterogeneous bodies of rocks and when they rub against each other, it creates a lot of friction and therefore, leads to fractures, which eventually becomes a fault or faults.

The December 26, 2004, Aceh-Andaman earthquake

The Indian/Australian oceanic plates, which are moving at a rate of 5cm/year in a north-northeast direction with respect to Sunda plate, subduct beneath Sunda plate and the contact boundary thus formed, is called a megathrust fault (Sunda megathrust). On 26th of December, 2004, a portion of the Sunda megathrust failed, which caused the earthquake and the associated tsunami. The rupture length was about1600-km, which hosted a magnitude 9.2 earthquake. The strain energy (read my previous article on earthquakes), which was stored since centuries, was suddenly released along the fault causing the destruction. The megathrust faults resemble the thrust faults that are found on land but are very large in extent. For example, the Sunda megathrust runs south from Bangladesh, curving around the western and southern flanks of Sumatra, Java, Bali and eastern Indonesia to northwestern Australia, which stretches to about a distance of 5500 km. There are examples of megathrusts offshore of the Philippines, Taiwan, Japan and southeastern China. Similarly, there are megathrusts on land and the biggest one traverses from Pakistan through India and Nepal, which covers a distance of 2500 km along the southern side of the Himalayan mountain range.

The great East Japan earthquake of March 11, 2011

There are four tectonic plates in and near Japan, the Eurasian plate, the North American plate, the Pacific plate and the Phillippines sea micro-plate (Fig.). In the figure, the red lines show the faults that form the plate boundaries and the yellow arrows show the relative motion of the plates. The continuous push on the Pacific plate drags it down under the North American plate along the boundary called a megathrust fault. The resistance to its downward pull via friction builds up the strain energy along this boundary, which ultimately fails. The great earthquake and tsunami of March 11th was initiated on one of the portions of this fault, which slipped along an area of the fault roughly 500km long and up to 200km wide, it is shaded in orange in the figure. The big yellow dot shows the epicentre (the place directly above where the megathrust first started to fail) of the earthquake, while the smaller one shows the largest aftershock recorded till date. A number of smaller yellow dots show the aftershocks in the first few days, after the mega quake.


Afroz Ahmad Shah is a research fellow at Earth Observatory of Singapore, Nanyang Technological University, Singapore.

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