Seaquake in Southeast Asia

On December 26th 2004 one of the biggest natural disasters in history occurred in Southeast Asia: a seaquake in the Indian Ocean caused an enormous tsunami hitting many countries. Over 200.000 people did not survive the consequences of this devastating tidal wave and hundreds of thousands lost all their belongings. As to magnitude this disaster can’t be compared with the flood disaster in Holland in 1953, but nevertheless people who witnessed that flood will undoubtedly have been reminded of that time on December 26th. Once again it became clear how devastating the power of the sea can be. All of a sudden the word tsunami became a frequently used and notorious concept. But what is a tsunami? And how does such a tidal wave originate? Have there been other big tsunamis in the past? Is a tsunami possible in Holland?

Tsunami

Tsunami is the Japanese word for ‘harbour wave’. A tsunami is a series of waves caused by an earthquake, a volcanic eruption or the impact of a meteorite. These waves aren’t caused by wind or the position of the sun, moon or planets and have nothing to do with tides, contrary to normal waves.

Origin

The surface of the earth consists of different plates moving along each other. Normally these movements happen very slowly (a couple of centimetres per year) but when these plates suddenly start rising or falling due to the fact that they grate over each other, collide with each other or slide under each other, an earthquake will occur. The boundaries of the plates run mainly under the sea. In the sea the energy of the trembling ground will be transferred to the above lying water. And this will cause a so-called seaquake.

A seaquake can result into a tsunami but that’s not always the case. The tremor must have a magnitude of 7.5 on the Richter scale to cause a tsunami. Such a large magnitude usually only occurs when a oceanic plate slides under a continental plate. This is called subduction.

When water absorbs the energy of the earthquake, a wrinkling of the water surface above the epicentre of the quake arises. The seaquake often covers a large area, as a result of which this wrinkling has a lot of force. Gravity makes sure that the vertical force of the wrinkling is converted into horizontal movements. Because of this the wrinkling splits into several waves, moving from the epicentre into the direction of deep water or the shore. This effect can be compared with the wrinkled circle of a stone thrown into a pond. The wave above the deep water moves away from the epicentre at a high speed. Meanwhile the wave heading for the shore, is slowed down due to the resistance of the seabed. In general we can say: the deeper the ocean, the higher the speed of the wave and consequently the shallower the ocean, the lower the speed of the wave. In deep water one often doesn’t notice a tsunami because the waves are low, have a great length and have a long interval. However, when the tsunami comes closer to the shore, the so-called ground effect appears. The front of the wave is slowed down by the resistance of the rising seabed while the back of the wave still has maximum speed. Because of this the wave is crushed together so to speak, as a result of which the water can only go up and the wave becomes even higher. The final meters before the wave reaches the shore, the resistance increases because the water becomes shallower. The front of the wave is slowed down while the back rolls on, which makes the wave even higher. Because the wave gets higher, suction at the front comes into being. The seawater between the tsunami and the shore is pulled up. The coastline sometimes retreats for hundreds of meters. When the wave, with a possible altitude of thirty meters, finally reaches the shore, it devastates everything on its way. When the wave is raged out, the water retreats back into the sea in an absorbing way. After the first tsunami wave, often others will follow. These are normally less powerful.

Consequences

Because the tsunami consists of several waves and because of the combination of the force of the wave itself and the sucking power back into the sea, the consequences of a tsunami are often enormous. That was also the case with the tsunami in Southeast Asia. There was an enormous amount of victims, millions of people became homeless and the economy and nature were damaged considerably.

Men

There will never be a precise number of victims caused by the tsunami of December 26th 2004, because a lot of people are missing, never to be recovered. The official amount of victims now stands at 289.260, the estimated amount of missing persons included. By far most victims fell on the Indonesian island of Sumatra. Especially the northern province of Atjeh was closely situated to the epicentre of the seaquake. Other countries that were hit are Sri Lanka, Thailand, India, Malaysia, Myanmar, Bangladesh, the Maldives and a number of countries in eastern Africa. Because many tourists were present at the time of the disaster, there are many European and American victims. In total twenty seven Dutchmen died in the tsunami, ten are still missing. Many inhabitants of the stricken countries who survived the disaster lost everything: family, friends, their homes, sometimes the entire village was swept away. They have to start all over.

Economy

The tsunami also has consequences for the economy and nature of the stricken countries. Many buildings and roads have been destroyed and have to be rebuild. This will cost a lot of money. Furthermore the economy of those countries was to a great extent dependent on tourism. Particularly just after the disaster few tourists came to Southeast Asia because people were afraid. Because of this the economy was struck very hard. Fortunately the countries in Southeast Asia received help from other countries. Almost everywhere fund-raising drives were held to help the stricken countries.

Nature

The tsunami of December 26th 2004 caused a lot of damage to local nature, especially in the coastal areas. Furthermore a part of the coral reefs were damaged. Word goes that many animals felt something was about to happen and fled in time. Damage to nature didn’t end immediately after the tsunami: because a lot of people had to rebuild their houses, a lot of wood is necessary. Because of this more trees had to be cut down. Furthermore it is likely that people will eat more meat and eggs of endangered reptiles as a replacement for fish.

Tsunamis from the past

The tsunami of December 26th 2004 was, as far as is known, the heaviest and most powerful tsunami in the history of mankind. The most violent tsunami before 2004 dates back to 1775 when Lisbon was hit by a tsunami. Between 50.000 and 100.000 people lost their lives.

Below you will find a enumeration of other large tsunamis in recent history.

August 27th 1883: a tsunami as a result of a volcanic eruption kills 36.000 people on the islands of Java and Sumatra.
June 15th 1896: a seaquake near Japan causes a tsunami: 27.000 victims.
April 1st 1946: a tsunami caused by an earthquake in Alaska claimed 159 lives, most of them in Hawaii.
July 9th 1958: the heaviest tsunami in recent times, caused by an earthquake in Alaska, took only a few lives because the tsunami occurred in a deserted area
May 22nd 1960: an earthquake in Chile (8.6 on the Richter scale) causes a tsunami, killing about 1.500 people in Chile and Hawaii.
March 27th 1964: in Alaska a tsunami arises after an earthquake measuring 8.4 on the Richter scale: 120 people lost their lives.
August 23rd 1976: a tsunami in the southwest of the Philippines: 8.000 victims.
July 17th 1998: in Papua New Guinea 2.200 people are killed by a tsunami as a result of an earthquake measuring 7.1 on the Richter scale.

Prevention

A tsunami warning system exists in order to signal a tsunami in advance so people can flee to a safe place. This system consists of sensors in the seabed and in buoys on the water surface which can pass on information to satellites. The sensors measure the pressure of the water on the seabed. They are very precise and can even notice a tsunami wave of a centimetre at a depth of 6.000 meters. When a sensor measures a tsunami, a signal is passed on to the buoys on the water surface. From the buoys a signal is passed on to a satellite, which is connected to a station on land. From that station people of the coastal towns can be warned.

During the seaquake of December 26th 2004 no warning system was installed in the Indian Ocean. If there had been one, many lives could probably have been saved. However a disaster couldn’t have been prevented, because only coastal towns far away from the epicentre could have been warned on time by the warning system. For the people of Sumatra, which lies very close to the epicentre of the seaquake, a warning would very likely have come to late, because the water reached the island already after a few minutes.

A tsunami in Holland?

After all those frightening images of enormous tidal waves on TV, the first question that comes to mind is: can a tsunami occur in Holland? The answer is reassuring because it is very unlikely that Holland will be flooded by a tsunami. The nearest fault lines, which could create an earthquake, are situated in the Atlantic Ocean. If an earthquake would occur on these fault lines, they would be less fierce compared to an earthquake in the Indian Ocean for instance. The plates in the Indian Ocean slide over each other, whereas the plates in the Atlantic Ocean slowly slide away from each other. An earthquake in the Atlantic Ocean probably wouldn’t be powerful enough to create a tsunami. But you never know…If it does happen, a huge amount of land will be flooded because Holland is situated under and just above sea level. A tsunami could then roll tens of kilometres inland without any obstruction.