Earthquakes are tremors and vibrations of the Earth's surface caused by natural causes (mainly tectonic processes) or artificial processes (explosions, filling of reservoirs, collapse of underground cavities of mine workings).
The main cause of an earthquake is the rapid displacement of a portion of the earth's crust as a whole at the time of plastic deformation of elastically stressed rocks in the earthquake source. Most sources of earthquakes occur near the Earth's surface.
The displacement itself occurs under the action of elastic forces during the discharge process - a decrease in elastic deformations in the volume of the entire plate section and displacement to the equilibrium position. An earthquake is a fast (on a geological scale) transition of potential energy accumulated in elastically deformed (compressible, shear or stretched) rocks of the earth's interior, into the vibration energy of these rocks (seismic waves), into the energy of changes in the structure of rocks in the earthquake source. This transition occurs at the moment when the ultimate strength of the rocks in the earthquake source is exceeded.
It is believed that the root cause of earthquakes is global geological and tectonic forces arising from temperature irregularities within the planet.
Most often, earthquakes occur on the outskirts of tectonic plates. All earthquake epicenters for 35 years of the XX century are marked on the map, and thus we can clearly see the outlines of plates on our planet
How many earthquakes occur per year? ten? 20? 50? No, from the news we learn only about the most destructive of them, but in fact, more than a million (!!!)
Of course, the number of recorded fluctuations is increasing every year, but only because the number of earthquake observation points on the planet is also increasing and instruments for recording them are improving. For example, 100 years ago, only about 40 earthquakes were recorded per year.
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According to the scientific classification, earthquakes are divided into 3 groups according to the depth of their occurrence:
• "normal" - from 30 to 70 km, • "intermediate" - from 70 to 300 km, • "deep focus" - over 300 km.
How many of you remember the "earthquake in Moscow" on May 24, 2013? Then these were the echoes of an earthquake in the Sea of Okhotsk, practically in another part of the world. But since it was a "deep focus" earthquake, it reached across all of Russia to Moscow itself. Considering that the speed of a seismic wave can reach 8 km / s, then it can shake up the entire territory of our huge country in 12-15 minutes.
At the moment, scientists know the following mechanics of the occurrence of an earthquake: friction along a fault prevents rocks from sliding, as a result of which energy is accumulated. When the stress reaches a critical point exceeding the friction force, a sharp rupture of the rocks occurs with their mutual displacement, and the accumulated energy, being released, causes wave oscillations of the earth's surface in all directions from the source like sound waves. Earthquakes can also occur when rocks are collapsed into folds when they crack, forming faults.