Earthquake is an unexpected and violent shaking of the earth due to natural or artificial causes. Tectonic plates shifting can be considered as the main natural cause. Among artificial causes, we can mention such processes as underground caves collapsing, mine workings, explosions, filling of water reservoirs, etc. Small shocks may also be attributed to the rise of the lava during volcanic eruptions.
Each year, there are about a million earthquakes on Earth, but the most are so weak that they pass unnoticed. Really strong earthquake that can cause extensive damage and destruction, occur on the planet about once every ten days. Most of them happens under the ocean, and therefore have no disastrous consequences (unless such an underwater earthquake causes a tsunami).
The cause of the earthquake is the rapid shifts of the earth’s crust at the time of the elastic deformation of strained rocks in the earthquake center. Most of earthquake occurs near the surface of the Earth.
Friction initially prevents sliding rocks along the fault. As a consequence, the force that causes the motion is stored in the form of elastic stress energy. When the stress reaches a critical point, which is above the friction force, there occurs a sharp rocks break with their mutual offset and the stored energy is released, causing the surface wave oscillations – earthquakes. Earthquakes can also occur in the folds of crumpled rocks, when the stress energy exceeds the tensile strength of rock and they split forming fault.
An earthquake generates seismic waves, which spread out in all directions from the earthquake center like sound waves. The initial point of rocks shifting is the focus or epicenter, the point on the Earth’s surface over the focus is the epicenter of earthquake. Shock waves spread out in all directions with decreasing of the intensity. The velocity of the seismic waves spreading can reach up to 8 km / s.
The depth of the epicenter is usually no more than 100 km, but sometimes it reaches up to 700 km. Sometimes the earthquake can occur near the Earth’s surface. In such cases, if the earthquake is violent, bridges, roads, houses and other buildings are torn apart and destroyed.
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