The measurement of earthquakes
The history of scales
The ancient Greek already tried to measure the earthquakes. Jacopo Gastaldi, a mapmaker from Piemont, worked out the most formerly intensity scale in 1564. He did it to measure the big earthquake of Nizza's vicinity in 1594. In the XVII-XVIII. centuries, the scales generally had four grades, but in the XIX. century, the geologists described the destructive power of the earthquakes with ten grades. On the first place, Kitaibel and Tomcsányi made isoseismic map from among Hungarians in 1810.
One prototype of the earthquake-scales nowadays used is the scheme with 10 grades made by Rossi and Forel in 1883, and Mercalli's scale (also has 10 grades), which came into being in 1897. Cancani, who also connected the maximum values of ground movement's acceleration to each grade, expanded the latter to 12 grades in 1904. After that Sieberg continued the completion with the analysis of effects and with the description of building damages, which was accepted by an international scheme in 1917, and since then it was used all over the world, and called its full name: Mercalli-Cancani-Sieberg scale. Experts use the updated version of the Mercalli scale, the MSK '64, which was worked out in 1964.
The Japanese - who are often struck by earthquakes, because of the position of their country, and because of the geological conditions - made their own scale in 1900: the seven graded Omori scale, however they modified it also; by decreasing it to six grades. The Omori scale is suitable to process, measure especially destructive quakes. The first grade is for "perceptible", but harmless, the sixth is for "destructive quake".
While the Mercalli type grade and the preceding scales are all classified by environmental damages and losses caused by the earthquakes, until the Richter scale brought a completely different approach. In 1935, C. Richter American seismologist introduced the notion of extent and the magnitude, which are based on instrumental recordings, and he created his own scale on the basis of the earthquake's evolving energy. The earthquake's power in conformity with the Richter scale is determinable with a seismogram, whose recorded during the quake. In this way the largest earthquake till now had the magnitude of 8,9, which is equal to about 1018 joule. It is impossible to have a stronger earthquake than that, because the earth's crust isn't able to accumulate more energy than the Richter scale's 8,9th grade. (The nuclear bomb, which was dropped at Hiroshima, had the energy of 6x1013 joule. So, there was 16667 times more energy evolved in the largest earthquake than the detonation of Hiroshima's nuclear bomb.)
The effects of the quakes are also estimated by the measure of occurrences and changes caused on the surface of the earth. The intensity scales are based upon this. These effects depend on evolved kinetic (total-)energy in the deep and also on:
What's the difference between the intensity and the magnitude?
The intensity is the measure of the earthquake's effect on people and their buildings in a specific place, while the measure of the magnitude was determined by the evolving energy in the course of the quake. Recently the intensity is measured on the 12 degreed European Macroseismic Scale (EMS). This is the corrected version of the formerly used Mercalli scale and MSK scale. When the intensity-value is bigger than five, it is possible, that the quake damages the buildings. The Richter scale is for measuring the magnitude. Theoretically, it doesn't have lower and upper limits, but in practice, there is a limit value, and there is no earthquake, which is bigger than it. The known larger earthquakes, which have the magnitude around M=9, while the earthquake motion risen by a fallen brick is about M=-2. (The magnitude scale is logarithmic, so the value of the magnitude may be negative!) If a quake's magnitude is greater with one than an other, then it means 30 times bigger energy.
Charles F. Richter's scale
The Richter scale determinates the measurement of earthquakes, the so-called magnitude. Charles F. Richter, the American seismologist introduced the notion in 1935. The scale describes the earthquake's strength with numbers between 0 and 9. The scale also continued, but since they started to use it in the 1930s, fortunately there haven't been quakes larger than 8,9 magnitude. We get the earthquake's magnitude, if we measure the largest amplitude in micron (10-6 m) on a seismogram, which created by a standard seismograph 100 kilometres away from the earthquake's epicentre, and we take the natural (ten based) logarithm of the quake.
Finally, let's see the observable effects of the earthquakes assigned to each magnitude:
Magnitude of 1-2
The quake can only be observable with a seismograph or other device.
More than 500000 earthquakes occur yearly.
Magnitude of 3-4
The earthquake is hardly perceptible. Chandeliers swing, smaller losses could happen.
The number of these quakes is between 10000 and 100000 yearly.
Magnitude of 5-6
The earthquake is strongly perceptible. Objects fall from the shelf, the walls crack.
The number of these quakes may between 20 and 200.
Magnitude of 7-8
The quake causes very heavy damage. Houses and bridges collapse. Roads are ruined.
We should calculate with 10 quakes from this size.
Mercalli-Cancani-Sieberg's (MCS) scale
Mercalli-Cancani-Sieberg's 12-graded scale has come into general use in Europe. Besides the description of the occurrences (the effects on the buildings from the 6th grade) there are measurable acceleration-intervals, which are given in mms-2 assigned to each degree.
For those, who shudder at these recondite mathematical calculations, there is the Mercalli-Cancani-Sieberg scale based on earthquakes' external effect, with the following degrees:
Medvegyev-Sponhauer-Karnik (MSK) scale
In the socialist countries, the MSK-64 scale was accepted. The individual grades' - which number are also 12 - characterisation is extended to the phenomena of nature, to the buildings and to the senses and the environment of people. Strongest earthquake in Hungary until now, had the grade of 8-9.
European Macroseismic Scale (EMS)
First of all the sudden loosening of the accumulated tensions in the course of larger mass' reorganisation (mountains and hills are ascending, basins are descending) causes quakes. The Working Party of European Seismological Board worked out the European Macroseismic Scale (EMS) based on many previous experiences, which first version was made in 1992, in 1998 they modified it in Luxemburg and since then they use it in the European countries.