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More than one million earthquakes occur on theearth every year. A large majority of earthquakes will go unnoticed by most people because of their slight magnitude. On the other hand, some earthquakes are very strong and cause considerable damage. According to the distance from the earthquake epicentre, we have local earthquakes (in the region of the quake), near earthquakes (less than 1.000 km away), remote earthquakes (roughly up to 10.000 km), and so on. Concerning the magnitude, the earthquakes are described as slight, moderate, great, and very great (global catastrophe).
In the past, people believed that an earthquake was a sign from gods, bringing punishment or announcing the end of the world. However, earthquakes are natural movements of theearth's surface. When they occur on the ocean floor, they are called ocean or submarine earthquakes. Ground motion is produced by seismic waves, which transmit the tension generated in the interior of the earth. The tension is generated for many years as a result of gradual deformation of rocks, and finally produces fracturing of a part of the earth's mantle.
The place where the shocks originate is called hypocentre. Directly above the hypocentre, on the surface, is the epicentre. Concentric vibrations spread from the hypocentre throughout the earth's body. These are the earthquakes or seismic waves. "Seismos" is a Greek word meaning shock.
The strongest shocks are found near the epicentre. They are measured and evaluated by instruments and observation. The most important instrument for measuring and studying earthquakes is the seismograph. It registers the generated waves.
In order to obtain the most exact results, a seismograph (after its installation) should have a minimal contact with the earth's surface, otherwise it may register permanent earth movements. The seismographs are therefore isolated from underground movements by being hung on springs or joints. The distance from the hypocentre is determined using seismic travel-time curve.
This is done by compiling the data concerning the moment of the arrival of various types of waves at various times recorded by a number of seismographic sites. The distance from the epicentre is then calculated based on the available information. Today, all this is done using the most advancedcomputer technology.
Once the epicentre is located, the intensity of the quake is measured in units of magnitude. This scale was developed in 1935 by a Californian seismologist, Charles Richter. Measuring of the intensity of the earthquake on the basis of the observed shocks, which provides information concerning its subjective force, is done using Mercalli scale.
When we look closely at regions where many earthquakes have occurred in recent years, we see that region which is most prone to earthquakes is at the edges of thecontinental plates of South and North America, the region of the Pacific Ocean, South Asia, and southern Europe. The north of Europe, the interior of Australia, and Africa, as well as some oceans (with the exception of the ocean ridges) are almost free of earthquakes. As yet, there is no single explanation for shocks occurring in plates, where the earth's crust is considered stable. It is assumed that the stress on the edges of these plates generates pressure in the middle of these formations, which may cause earthquakes at "weak points."
Earthquake hypocentres are most common in the depths of 0 to 70 kilometres. Less common are earthquakes having hypocentres in great depths, of up to 700 kilometres. These occur mainly where due to slow drift of the tectonic plates the crust of plate edges is pushed down, as is the case, for example, of the South American andes.
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