Annually in the world there are about one million earthquakes. Really strong fluctuations of earth crust happen approximately every two weeks. Often in various media it is possible to meet a formulation: "Earthquake measuring 5.5 …". However, what is behind this statement?
In 1935 the American seismologist Charles Richter offered the classification of earthquakes based on assessment of the energy which is marked out in epicenter of tremors. The size characterizing energy is called magnitude of earthquake. Magnitude is a dimensionless size, the maximum value on the Richter scale is 10.0.
For measurement of impact of an earthquake on the environment use an intensity scale. Most often use four scales of assessment of intensity of an earthquake:
1. Medvedev's scale – Shponkhoyera – Karnika (MSK-64); 2. European macroseismic scale (EMS); 3. Merkalli's scale (MM); 4. Scale of the Japan Meteorological Agency (JMA).
Scales of MSK-64, EMS and MM have twelve degrees, and JMA seven scale. The intensity of an earthquake is determined by external signs of destructions. On MSK-64 scale the tremors measuring 3 points mean the weak earthquake accompanied with easy rocking of the hanging objects. This description practically coincides with the description of intensity of an earthquake in 3 points on scales of EMS and MM, and approximately there correspond to intensity 1-2 points on a scale of JMA.Sopostavlenie of intensity and magnitude of earthquake is made in the course of observation and collecting of statistical data. The approximate ratio of sizes for the centers of the earthquakes which are at a depth of 30-70 kilometers can look as follows: the earthquake in 6 points on any of 12 a ball scale approximately corresponds to magnitude 2.8-4.3 on the Richter scale. For example, the magnitude of the Great Chinese earthquake which center was at a depth of 32 kilometers was about 8.0 on the Richter scale that corresponds to 11 points on twelve a ball scale. The description on a scale of intensity of EMS looks as follows: "Devastating. Practically all buildings are completely destroyed". So strong earthquakes happen approximately once a year, however the large role is played by arrangement of epicenter therefore many pushes remain unnoticed.