Ideas presented on World Environment Day 2007
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International Symposium on 30 Novmeber - 1 December 2007.
Information and trail map.
Arhat Mahinda's Message on 246 BC
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
Part 2 >
Richter Scale Magnitude of Earthquakes