The word Earthquake comes from the Latin terraemotus (earth in motion). Also called as an earthquake, it is a sudden movement or shaking of the earth's crust, produced by internal phenomena in some part of the Earth.
An earthquake originates when rocks under great pressure suddenly break, releasing the accumulated energy, which shakes the ground, spreading vibrations from its epicenter. Initially, stress may have little effect; But as they accumulate, rocks deform at faults and other weak points, eventually breaking apart. When this happens, the rock layers rebound and the accumulated energy is violently released in the form of a seismic shock.
Most tremors have been found to occur along the edges of tectonic plates, which are often also areas of volcanic activity.
The point or focus of origin of an earthquake is called the hypocenter, where seismic waves arise and propagate in all directions, causing the materials through which they are diffused to vibrate. The area of the earth's surface located directly above the hypocenter is called the epicenter and is the place where the earthquake forms with the greatest intensity.
The effects of an earthquake vary enormously, depending on their strength or intensity, the depth at which they occur, and the constitution of the soil and subsoil.
The intensity and occurrence of seismic movements are measured with devices highly sensitive to the vibrations of the earth's crust called seismographs. There are two basic types: one to measure horizontal movements (P waves); and the other, for vertical movements (S waves).
When the earthquakes are few intense they are registered only by the devices; On the other hand, when they are very intense they are destructive, causing great catastrophes, especially in constructions such as houses, buildings, roads and bridges. They also cause the loss of many human lives.
In order to better estimate seismic waves , arbitrary scales have been made, which show the destructive effects of various degrees. Such scales are the Richter, Sieberg, Omori, Cancani, Mercalli, and others. The best known are Richter's (magnitude grading) and Mercalli (intensity grading).
Attempts to predict when and where earthquakes will occur have met with some success in recent years. Currently, China, Japan, the former Soviet Union and the United States are the countries that support these investigations the most.
