![]() ![]() According to epicenter distance the earthquakes are teleseismic (>3000 km), regional (700-3000 km) and local (<700 km) ones. The earthquake mechanism is defined by moment tensor. The earthquakes are defined by various parameter, the most important such parameters are epicenter coordinates, hipocenter depth, magnitude, intensity distribution and focal mechanism. The graphical expressions of the data registered by seismometers are called seismograms (Fig. Seismometers register the mechanical ground movements, thus, they record signals of seismic waves. The surface waves due to ground particle polarisation are devided into Reyleigh and Love waves. What is more, P and S waves experience refraction and reflection from the boundaries of the inner sheres of the Earth thus generating more phases of the seismic waves. ![]() Depending on the polarisation of the movement of the ground particles the body-waves are devided into longitudinal or primary (denoted P), and transversal or secondary (denoted S), because the latter ones arrive later to the receiver. The seismic waves which propagate in the volume of the Earth are called body-waves, and surface waves propagate on the surface of the Earth. There are several types of seismic waves. The earthquakes are studied while registering, analysing seismic waves and their propagation in the Earth. In the cases the source of the earthquakes lay beneath the sea-floor or ocean-floor the tsunamis can be generated which endanger the coastal areas, for example due to Mesina earthquake in 1908 the tsunami of 12 meters hight was generated, due to Sumatra earthquake in 2004 the coastal area of the Indian ocean have been sweped away, and tsunami generated in 2011 due to Tohoku earthquake damaged Fukushima NPP (Fig. ![]() The most dangerous for people are falling buildings which collapse due to hard ground vibrations. The most dangerous are the seismic waves which are generated during an earthquake and propagates from the seismic source (the propagation depends on focal mechanism and polarisation), thus, generating the vibration of the ground. The earthquakes occur due to several reasons: movement of the tectonic plates, volcanic eruptions, glacial isostasy, meteorite impacts, large landslides and antropogenic activities (due to change of distribution of large masses, i.e. All earthquakes might be severe and dangerous. Intraplate earthquakes usually take place in the zones of tectonic weakness, what is more, they are not so frequent as the tectonic stresses accumulate during longer period of time. The earthquakes take place both on the boundaries of the tectonic plates and inside of them, because the tectonic stresses accumulate and can be transmitted at great distances (Fig. Due to instant displacement large amount of energy turns into heat which results in rock melting, and only a small amount of energy (about 10-5) radiated as seismic (acoustic) waves the process is called the earthquake. The majority of epicentres of the earthquakes are located on the boundaries of the tectonic plates, but some of them are located inside the tectonic plates as well.ĭue to interaction of the tectonic plates the tectonic stresses exceed the frictional forces in some places, thus, instant displacement between the plates might occur (Hook‘s law). Seismicity in the world for year 1964-2008 according to International Seismological Centre (ISC). Moving tectonic plates interact among themselves, and on their boundaries the processes or orogeny, volcanism, magmatism and earthquakes are presented (Fig. Totally there are 7 large and several tens of smaller tectonic plates on Earth. Surface of the Earth consists of solid, brittle and moving tektonic plates.
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