Prof Dhammika A. Tantrigoda
Department of Physics, University of Sri Jayewardenepura
Nugegoda, Sri Lanka
Dreadful memories of the tsunami that ravaged several coastal cities of Sri Lanka claiming many innocent lives on the early hours of 26 December 2004 is still haunting the minds of many of us. This powerful tsunami, which devastated several South Asian countries, originated off the west coast of Sumatra. According to local and international news agencies, the tsunami has claimed well over 150 000 lives causing unprecedented damage to property. It has been generated as result of a massive Earthquake of magnitude 9 on the Richter scale. According to the United States Geological Survey, this is the fourth largest earthquake in recorded history, the largest being the great Chilean Earthquake that took place in 1960, with a magnitude of 9.5 on the Richter scale.
Tsunami is a train of sea waves triggered off due to a sudden collapse of the ocean floor. This normally happens as a result of earthquakes taking place at shallow depths below the sea floor. Tsunamis can also be caused by volcanic eruptions and falling of large boulders into the water. Violent eruption of Krakatoa volcano in 1883 caused sudden collapse of the sea floor leading to a massive tsunami, which claimed a large number of human lives. Tsunamis are sometimes referred to as tidal waves. This is a misnomer, as tsunamis have nothing to do with tides that are caused by the gravitational attraction of the sun, moon and other planetary bodies. The word tsu-nami has a Japanese origin and it means harbour wave (“tsu” means harbour while “nami” means wave). Tsunamis have enhanced effects in harbours and other U or V shaped water inlets and this could have been contributed towards the Japanese origin of the word.
Origin of Earthquakes
Let us now see how earthquakes that trigger tsunamis are originated. The thin outermost part of the earth (first 50 to 100 km) is known as the lithosphere and it consists of several large detached tile like segments and several other such smaller segments. These segments are known as lithospheric plates or simply plates. Plates “float” on a region called asthenosphere, which consists of rocks that have transformed into an extremely “thick” or viscous material, which can flow with very slow speeds. All the plates are moving relative to each other at very slow speeds in a complicated manner. Earthquakes can be observed in most plate margins, especially at the vicinity of plate margins known as transform faults and subduction zones. At a transform fault two plates move passing each other horizontally. One such plate margin is in California in western USA. This is known as San Andreas Fault and many powerful earthquakes have been generated at this fault. At a subduction zone a heavy oceanic plate goes under a relatively light continental plate (figure 1). Descending oceanic plate tries to drag along some of the adjacent continental plate resulting strains in both plates. So the subduction does not proceed smoothly and continuously; it proceeds with jerks and each jerk is responsible for an earthquake. The oceanic plate on which most of the Indian Ocean is lying on is plunging down (subduct) under Indonesia and the recently observed magnitude 9 earthquake took place at this plate boundary.
No one exactly knows the mechanism that triggers earthquakes as they happen deep down in the earth. However, we can build models to explain how earthquakes occur in just as we build models to explain atomic and nuclear phenomena. The elastic rebound model is one such model that has been built to explain the origin of earthquakes that takes place at a transform fault. It is useful to study this model as it gives a very good insight into how earthquakes originate. As discussed earlier, at a transform fault two plates move passing each other almost horizontally. Due to frictional and other forces each plate is trying to stop the motion of the other that result in deforming both plates. This is somewhat similar to two gigantic rubbers glued to each other trying to move in opposite directions parallel to the two faces that have been glued. As a result of relative motion of the parts of the rubbers that are away from the glued boundary they get deformed and are in a state of strain. The figure 2.b shows the way in which two plates can undergo deformation in this manner. There is a limit to which “glued” rocks can withstand deformation and once this limit is passed, rocks in that region snap releasing huge amounts of energy. This is how the elastic rebound model explains the origin of an earthquake. Normally the whole boundary of “glued” plates does not get dislocated in one instance. Only the rocks in a certain region of the boundary get dislocated and this has been illustrated in figure 2c. If the extent of the dislocation is large the release of elastic energy is also large and the earthquake is classified as one having higher magnitude. Once the main shock occurs, other parts of the glued regions can also snap and release energy and these events are known as after shocks. This explains how several small earthquakes that were reported to have taken place at the same plate boundary occurred after the massive earthquake of 26th December. After shocks are normally not powerful as the main shock. Sometimes a small release of energy can take place before the main shock known as foreshocks. Dislocation of rock units over an extensive region on the plate will take place in an earthquake. However, compared to the size of the whole plate boundary this region can be well approximated to single point. This point is known as the focus of the earthquake. The point directly above the focus on the surface of the earth is known as the epicentre of the earthquake.
When an earthquake takes place basically two types of waves collectively known as “body waves” transmit the energy outwards. Once these waves reach the surface their interference with each other and other phenomena will lead to the formation of another type of waves known as “surface waves”. Unlike body waves surface waves have higher amplitudes and almost all the physical damage due to an earthquake is due to the effects of surface waves. How the body waves and surface waves are generated and how they travel and also how the whole earth vibrates like a giant bell after an earthquake is a fascinating problem in physics and in applied mathematics. Some of the concepts in physics and mathematical tools developed to solve this problem have been successfully used in formulating some of the concepts in advanced branches of contemporary physics such as quantum mechanics and nuclear physics.