Measuring the released energy

In Italy, the energy that is released by an earthquake is often calculated using the “Mercalli scale” (actually it is a modified Mercalli-Cancani-Sieberg scale). It is one of the first scales that were elaborated in order to evaluate the intensity of seisms, when more precise instruments were still not available (the first version of the Mercalli scale dates back to mid 1800) even though it is still commonly used today. However this is not a real “measurement” of the energy that it is released. In fact it is based on an assessment of the damages caused by the earthquake. These certainly depend greatly on the intensity of the seism, but are also conditioned by geological factors (such as the type of rocks or the presence of loose sediments, the types of constructions that have been affected, population density, advance warning given, the time in which the earthquake occurred (it is well known that at night there is a greater number of victims), and also if the population is used to the experience of earthquakes (during the recent tsunami in South East Asia, many people were saved due to the  awareness of some Japanese tourists who, knowing the phenomenon well, advised those who were present to move away from the beach): a seismic event of the same intensity in a town with a high population density without antiseismic constructions will certainly cause greater damages than in a city with few inhabitants or built with suited antiseismic criteria. For this reason seismologists discourage people from using this scale, and prefer to use other types of scales that are more objective, as for example the Richter magnitude scale. This scale, which was adjusted by Richter and Gutemberg in the Forties, is obtained by measuring the maximum amplitude of the waves drawn by the seismographs. This amplitude, is used with suitable formulas to calculate the quantity of energy that was actually released by the earthquake. It is a scale that potentially does not have a maximum level because it only measures the intensity of earthquakes that have occurred: the highest level ever recorded was 9.5 in 1960 in Chile. However nothing prevents even greater earthquakes from taking place. The Richter scale is a logarithmic scale, therefore a degree of difference in magnitude is equivalent to  a wave amplitude that is approximately 10 times greater and energy that is released that is 30 times greater, two degrees are equivalent to an amplitude 100 times greater and an energy 900 times superior and so on. Therefore you can see how the energy of earthquakes can be very different and in some cases, can be frightful. To have a vague idea of the energy that is involved, it suffices to think that an earthquake of magnitude 9 has an energy of  2 x1018 joule, where the energy used every year in the entire USA amounts to 6×1019 joule!

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