Earthquake

Principal characteristics

The point of origin of a seism is called Hypocentre or hearth. It can be between surface and less 700 km for the major events. One more often speaks about the epicentre of the seism, which is the point of the surface of the Ground which is with the vertical of the hypocentre.

Three categories of earthquakes

An earthquake is a more or less violent jolt of the ground which can have three origins: rupture of a Fault or a segment of fault (tectonic seisms); intrusion and degasification of a magma (volcanic seisms); explosion, collapse of a cavity (seisms of natural origin or due to the human activity). In practice one classifies the seisms in three categories according to the phenomena which generated them:

the seisms Tectonique S are most frequent by far and devastators. Most of the tectonic seisms occur in extreme cases of the plates, where there exists a slip between two rock mediums. This slip, localized on one or more Fault S, is blocked during the inter-seismic periods (between the seisms), and energy accumulates by the elastic strain of the rocks. This energy and the slip is abruptly slackened at the time of the seisms. In the zones of Subduction, the seisms represent half of the destroying seisms of the Ground, and they dissipate 75 % of the seismic energy of planet. It is the only place where one finds seisms deep (of 300 with 645 kilometers). On the level of the médio-oceanic dorsal , the seisms have surface hearths (0 with 10 kilometers), and correspond to 5 % of total seismic energy. In the same way, on the level of the great faults of Setback, take place of the seisms having hearths of intermediate depth (of 0 with 20 kilometers on average) which correspond to 15 % of energy. The relaxation of accumulated energy is not generally done in only one jolt, and it can occur several readjustments before finding a stable configuration. Thus, one notes Réplique S following the principal jolt of a seism, decreasing Amplitude, and over one duration going from a few minutes to more than one year. These secondary jolts are sometimes more devastators that the principal jolt, because they can make collapse building S which had summers only damaged, whereas the helps are with work. It can as occur a more powerful counterpart still as the principal jolt whatever its magnitude. For example a seism of 9.0 perhaps followed by a 9.3 several months counterpart later even if this event remains extremely rare.
the seisms of volcanic origin result from the accumulation of magma in the magmatic Chambre of a Volcan. The seismographs then record a multitude of microseisms (Trémor) due to ruptures in the compressed rocks or to the degasification of the magma. The magnitude is calculated starting from the various types of seismic waves by taking account of parameters as the distance to the epicentre, the depth of the hypocentre, the frequency of the signal, the type of Sismographe used, etc the magnitude is not a scale but a function continues logarithmic curve. The scales of intensity comprise noted degrees of Roman numbers, of I with XII for the most known scales (Mercalli, MSK or EMS). Among the various scales, one can quote:
the scale Rossi-Forel (also noted RF),
the scale Medvedev-Sponheuer-Karnik (also noted MSK),
the scale of Mercalli (noted MM in its modified version),
the scale of Shindo (震度) of the Japanese meteorological agency,
the European macroseismic scale (also noted EMS98).

The relations between magnitude and intensity are complex. The intensity depends on the place of observation of the effects. It generally decrease when one moves away from the epicentre because of the attenuation introduced by the geological medium crossed by the seismic waves, but of possible effects of site (echo, local amplification for example) can disturb this average law of decrease.

Various types of seismic waves

Notes and Sources

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