Understanding Earthquakes Part 2

Why do some earthquakes lead to tsunamis?

by Dr Afroz Ahmad Shah

earthquakeSome of the past earthquakes, which have been followed by tsunamis, were of big magnitudes; for example, that of a Richter magnitude scale of 9.0 in Kamchatka, Russia (1952), of 9.1 in Andrean of Islands, Alaska (1957), of 9.5 in Chile (1960), and of 9.2 in Prince William Sound, Alaska (1964). However, the scale of destruction and the damage caused by these quakes were far less than the destruction which followed one on 26 December 2004 – the 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 230,000 people died and several millions more were affected.

Tsunamis, unlike the earthquakes on land, have one advantage: they give us time to evacuate because the waves take some time to travel until they have reached the coast and thus provides an opportunity to save lives. However in 2004, the warning system was a complete failure, particularly in far-off places such as Thailand, Sri Lanka, India and East Africa. There was no tsunami warning in place because people tend to forget about disasters of the past, and this has caused further loss to life. This happened again in September 2009, where an earthquake that produced a tsunami in southwest Pacific killed nearly 200 people, again pointing to the miserable failure of authorities to deliver a timely warning system.

When a big earthquake and the subsequent tsunami hit Japan on 11 March 2011, it offered us an opportunity to test our preparedness to tackle such disasters. However, the reality is that we again failed miserably to warn people in advance about the coming disaster, which caused a serious loss of life and property. Its source was close to the coast and therefore, the warning system did not help. Some 20,000 people lost their lives to the devastating tsunami. This happened in a country, which to a large extent is a well-prepared nation for earthquakes and tsunamis. But they hadn’t been expecting an earthquake and tsunami of the magnitude that occurred on 11 March 2011. This clearly suggests an immediate need to thoroughly understand and map all the seismogenic faults and more so, the megathrusts, so that earthquake hazards could be traced to their respective sources and necessary precautions could be exercised.

All of these earthquakes, like the one in March 2011, were megathrust events (see Figure 1), where one tectonic plate dives beneath another.

japanese waveWHAT IS A TSUNAMI?
A tsunami is defined as 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 create a total imbalance in water body and form a big tsunami. For example, the recent tsunami along the coast of Sumatra was caused by an earthquake offshore, where a fault line of some 1,000 miles has long ruptured. A volcanic eruption in or close to the ocean can also result in the formation of big waves. The eruption of Krakatoa two centuries ago caused a tsunami throughout Southeast Asia and the tremors travelled throughout the world. Likewise, a meteorite or landslide 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) of 5 to 20 seconds, and wavelengths of 100 to 200 meters. However, these numbers increase significantly during a tsunami, with periods in the range of 10 minutes to 2 hours and wavelengths of greater than 500 km. Due to this very large wavelength, a tsunami loses 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 slows down. The water behind, however, continues to push forward. The edge of the wave gets higher. As more fast moving waves push into the slowing wave front, the wave gets higher 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 a result of the friction along the megathrust faults (see Figure 1). For example, had there been no resistance to the continuous push, signifying a smooth journey of the Pacific plate underneath the North American plate, we would not have witnessed an earthquake or the tsunami. However, this is not the case with plates which are huge and heterogeneous bodies of rocks. When they rub against each other, a lot of friction is created that leads to fractures, which eventually becomes a fault or faults.

tsunami
Figure 1. Map shows the tectonic setting of Japan and the surroundings. There are four tectonic plates (Pacific plate, North American plate, Eurasian plate, Philippines Sea micro-plate). The plate boundaries are shown as red lines and the yellow arrows show the relative motion of the plates. The great earthquake and tsunami of March 11th resulted from sudden rupture along the portion of the megathrust fault below the area shaded in orange (just east of Japan). The largest yellow dot shows the epicenter of the earthquake – that is the place directly above where the megathrust first started to fail. The smaller yellow dot shows the largest aftershock to date. Smaller dots show lesser aftershocks in the first few days following the great March 11th earthquake.
Figure 2. Schematic side view of the subduction zone and source of the earthquake of March 11th. A tsunami was generated because the motion of the plates pushed and pulled on the ocean floor.

THE 26 DECEMBER 2004 ACEH-ANDAMAN EARTHQUAKE
The Indian/Australian oceanic plates, which are moving at a rate of 5 cm/year in a northeast direction with respect to the Sunda plate, subducted beneath the Sunda plate and the contact boundary thus formed, is called a megathrust fault (Sunda megathrust). On the 26 December 2004, a portion of the Sunda megathrust failed, which caused the earthquake and the associated tsunami. The rupture length was about 1,600 km, which caused a magnitude 9.2 earthquake. The strain energy (as elaborated in the previous Undertanding Earthquakes article, Scientific Malaysian Magazine Issue 2), which was stored since centuries ago, 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 a distance of about 5,500 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 Pakistan through India and Nepal, covering a distance of 2,500 km along the southern side of the Himalayan mountain range.

THE GREAT EAST JAPAN EARTHQUAKE OF 11 MARCH 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 (Figure 1). 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 11 March 2011 was initiated on one of the portions of this fault, which slipped along an area of the fault roughly 500 km long and up to 200 km wide, 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 to date. A number of smaller yellow dots show the aftershocks in the first few days, after the mega quake.

ABOUT THE AUTHOR:

Dr Afroz Ahmad Shah is a research fellow at the Earth Observatory Sciences (EOS), Nanyang Technological University, Singapore. He did his PhD in 2010 with Prof. Tim Bell in Structural and Metamorphic Research Institute (SAMRI), School of Earth & Environmental Sciences James Cook University, Townsville, Australia. He worked on tectono-metamorphic evolution of Precambrian rocks that lie in the foothills of Colorado Rocky Mountains, USA. He obtained an M.Tech. degree in engineering geosciences (2006) from IIT Kanpur India, where he worked with Prof. JN Malik on Active Tectonics of Himalayan foot hills, Nanital. Dr. Shah joined the EOS in 2010, and currently working with Prof. Kerry Edward Sieh on earthquake geology of New Guinea. He can be contacted at [email protected].



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