The Science Behind Earthquakes - Video

March 2010

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• Timed Text format (captions only)

Haiti is extremely close to the edge of a tectonic plate, and tectonic plates move against each other, which can cause an enormous amount of geological force to build up in such locations. In other words, there is a geological strain along the length of two plates.

In the case of a magnitude 7 earthquake, it isn’t exactly two plates, but a large fault where geological forces have built up over hundreds of years. When these forces accumulate over a long period of time, they overcome the resistance of the fault, and it’s at that moment that a major sliding event occurs. If the amplitude of the sliding is relatively small, the earthquake will be relatively small on the Richter scale. But when the sliding continues for several tens of seconds over a long portion of the fault surface, the vibrations and, of course, the resulting damage will be that much greater.

And after a magnitude 7 earthquake like the one on January 12, particularly given that it occurred near the surface, we expect that there will be aftershocks. Aftershocks are smaller earthquakes that occur after a major one. The magnitude of the aftershocks varies. They can be quite minor, and they can even be fairly strong. The largest aftershocks are generally around magnitude 6. So they are large enough that they can even cause additional damage after a major earthquake.

In addition, this earthquake was relatively close to the surface, so the populated areas affected were very close to where the energy was released. As a result, the already considerable damage has been made worse because the vibrations associated with these aftershocks have often toppled buildings that were already damaged by the main shock.

The earthquake was very destructive first because it was very strong. Magnitude 7 on the Richter scale means an earthquake that is a thousand times stronger in terms of energy released than the atomic blast over Hiroshima, so an enormous amount of energy was released.

So, to sum up, that’s what caused so much damage in the populated areas: the strength of the earthquake, the fact that it was so shallow and the fact that buildings had not been constructed to withstand intense vibrations.