What is an earthquake?
Earthquake is a trembling or shaking movement of the earth's surface that is volcanic or tectonic in origin. The shaking starts as a result of sudden movements of the earth crust or in the upper mantle. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The force of the earthquake is estimated in the grades of seismic scores.
The upper crust consists of about a dozen of huge blocks – tectonic plates, which move under the influence of convectional currents rising from the high-temperature mantle. In the picture you can see how the plates move toward each other. The tension is being accumulated because of the rock resistance in the place of the break. The tension in the earth's crust increases until it becomes more than the solidity of the rocks. Then the beds of earth rocks are ruined and move abruptly. This is called “a fault”.
Vertical faults cause sudden rises and falls of the rocks. This movement is usually several santimeters, but the energy, thrown out with the movement of billions tons of rock, is enormous. The tension accumulated in the point of the fault is lessened.
Earthquakes are often described as instant event, but the fault continues for some time. For example, the earthquake in San-Francisco in 1906 lasted for 40 seconds. The Great Earthquake in Alaska in 1964 lasted 3 minutes.
The point where the fault starts is called “a focus” or “a hypocenter” of the earthquake. The point on the earth’s surface right above the focus is called “an epicenter”. Here the power of the shocks is the strongest. The focus can be located on different depth, therefore the earthquakes are divided into deep-focused (300-700 km deep), middle-focused (55-300 km) and low-focused (less than 60 km).
In the earthquake focus the rocks move and tremble. They push and move neighbor rocks, which move other rocks. In this way acoustic waves appear. They are called “seismic waves”. There are the following types of waves here: compression, fault and surface waves.