What is Earthquake
Earthquakes (Latin terrae motus), also known as earthquakes or earthquakes (from the Latin Tellus, the Roman goddess of the Earth), vibrations are sudden, rapid and more or less powerful than the Earth's crust, caused by the sudden displacement of rock mass underground . This shift is generated by the tectonic forces that act continuously in the earth and causes the release of energy at a point inside the Earth, said hypocenter, hence, a series of elastic waves, known as "seismic waves, propagating in all directions, even within the Earth itself, the place of the earth's surface vertically dell'ipocentro place is called the epicenter and is usually the most affected.
The earth's surface is slowly but constantly moving (see tectonic) and earthquakes occur when the resultant voltage exceeds the capacity of the rock material to bear. This condition must be very often the boundaries of tectonic plates in which the Earth's lithosphere can be subdivided. The earthquakes that occur at plate boundaries are called earthquakes interplacca, less frequent ones that occur within the lithospheric plates are called intraplate earthquakes.
Each day on Earth thousands of earthquakes occur, and only a few dozen are perceived by the population and most of them cause little or no damage. The average duration of a shock is well under 30 seconds for the biggest earthquakes, however, can be up to several minutes. The elastic waves that propagate in an earthquake are of different types and in some cases can result in a predominantly horizontal or vertical movement of the ground (or shock wave sussultatory). An earthquake may be accompanied by loud noises that may remind roars, rumbles, lightning, shooting sequences, etc., and these sounds are due to the passage of seismic waves and the atmosphere is more intense near the epicenter
Some earthquakes, especially the strongest, are also accompanied, preceded or followed by natural phenomena such as lightning or flashes (lights telluric), sudden changes of the magnetic field, electric or local radiation (emission of radon); interference to radio communications , nervousness of the animals, changes in the level of ground water or coastal volcanic activity. All these events are reflected in the observations and testimonies, and have been studied and confirmed by scientific research that has come to the explanation of each of them, even if in the absence of unanimous agreement, are not recognized or de facto measures actually taken on against the forecast.
Earthquakes can cause severe damage and high loss of life through a series of destructive agents, the principal of which is the violent movement of the soil with consequent stress of building structures pose, possibly accompanied by other effects such as flooding (eg tsunami or failure of dams), land subsidence (landslides, landslides or liquefaction), fire or leakage of hazardous materials. In any earthquake, one or more of these agents may act to cause serious damage and many casualties. Earthquakes are natural events by far the most powerful on earth, earthquakes can release more energy in seconds to thousands of atomic bombs, usually measured in terms of seismic moment. In that regard, just think that an earthquake can move in a few seconds of rock volumes of hundreds of cubic miles. The single event that has seen more deaths in the last thousand years is the Shaanxi earthquake (China) 1556, magnitude 8.3, because of which killed 830,000 people . The one with the highest magnitude, however, is the Earthquake of Valdivia (Chile) in 1960, which reached 9.5. Some earthquakes may, like that of Japan in 2011, moved a few inches of the Earth's axis.
Hypocenter And Epicenter
Earthquakes of magnitude greater are usually accompanied by minor events (and not necessarily less destructive) that follow the main shock and define replication (often called incorrectly aftershocks). When multiple events occur simultaneously or nearly so, it may be induced earthquakes (the earthquake triggered the fracturing of another rock that was already close to the critical point of rupture). The source of the earthquake is generally distributed in an inland area of the earth's crust - in the case of earthquakes the most devastating this can be an extension of the order of a thousand miles - but it is usually possible to identify a precise point from which the seismic waves have originated, this is called the "hypocenter" was originated here and the movement of pre-existing fracture (fault) or the sudden generation. The vertical projection dell'ipocentro the Earth's surface is instead called the "epicenter" and is usually the point where you experience the most damage.
There are three types of seismic waves:
Compression waves or longitudinal(P)
P waves are rocking the rock back and forth, in the same direction of wave propagation. They thus generate "compression" and "rarefaction" in the subsequent material in which they propagate. The speed of propagation depends on the elastic properties of the material and its density. Since the P waves propagate more rapidly, are also the first (P = Primary) to reach the seismometers, and therefore be recorded by seismographs. The earth's crust such waves travel at a speed that can reach up to 10 km per second. These seismic waves longitudinally through all materials.
Transverse or shear waves (S)
S waves, or waves "secondary", move the rock perpendicular to their direction of propagation (shear wave). They are slower than P waves, traveling in the Earth's crust at a rate between 2.3 and 4.6 km / s. S waves can not propagate through fluids because they do not resist shear.
Surface waves (R and L)
Surface waves, unlike what you might think, do not manifest themselves in the epicenter, but only at some distance from it. These waves are the result of the interplay of P waves and S waves, and are therefore very complex. Surface waves are the ones that cause the most damage.
Rayleigh waves, also known as R-waves, particles move in elliptical orbits a second vertical plane along the direction of propagation, as is the case for waves in water.
The waves of Love, also known as L waves, the particles move rather perpendicular to the direction of propagation (such as S-waves), but only in the horizontal plane.
Causes Of Earthquakes
The locations of the plates is assumed to be the mechanism of earthquakes which follows. Secondary cause is the movement of magma within a volcano, and can be indicators of an imminent eruption. In very rare cases, earthquakes have been associated with the accumulation of large bodies of water in the basins of dams, as the Kariba Dam in Zambia, Africa, and the injection or extraction of fluids from the crust (the Rocky Mountain Arsenal ). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure.
Types Of Faults
Earthquakes occur on fractures or cracks in the earth's crust known as faults where earthquakes that accumulates mechanical stress induced by tectonic movements. There are different types of faults divided according to the relative movement of the portions adjacent to the tectonic fracture and angle of the fault plane. The process of formation and development of the fault and the earthquake itself is known as faulting.
Predictability Of Earthquakes
Early nineteenth century we have studied the correlations between changes of the height of local ground water and gravity, as well as the emission of radon, but sadly the current state of knowledge have not yet been developed that allow models to highlight useful signals the effective prediction of an earthquake or of its possible characteristics, intensity and spatiotemporal location.
Radon is formed by the radioactive decay of radium and, being a noble gas, does not combine with other elements and chemical compounds, so much of the radon that is formed inside the rocks trapped in them. If you experience a sudden movement or cracking of the rocks, as occurs during or immediately before an earthquake, the radon content in depth emerges on the surface, where, however, it is already present to some extent, by increasing the local concentration with sudden peaks or so-called " drafts . Hoping to create a system of short-term forecast of earthquakes and reliable, several studies are under way, for this research using a network of radon detectors, properly distributed on the surface of the affected areas.
The Haicheng earthquake of February 4, 1975 was historically the first and only earthquake expected, but in this case the signals were so intense and regularly made to leave no doubt as to the proximity of the event.
The predictability of earthquakes in Italy has been the subject of discussion and controversy outside the scientific, following the earthquake of April 6, 2009 Eagle, on the occasion of the tragic event, the press reported with emphasis on the news that Giuliani Giampaolo , a laboratory technician, in the weeks before the quake had supported the hypothesis of the imminence of a catastrophic shock, this would occur, he said, in March, roughly in the same region, and he based his analysis on 'sudden increase in radon emissions, but using forecasting methods that were not considered severe by the scientific community.
Another hypothesis for the prediction of an earthquake that was proposed by Raffaele Bendandi, a pseudo-self-taught scientist, according to which the earthquakes like the tides are due to the influence of the Moon and other planets on the Earth's crust.
On the study of seismic precursors of electromagnetic origin is actively engaging the Italian Amateur Radio Association (ARI) providing stations to listen to low frequency electromagnetic emissions in the ELF (Extremely Low Frequency).
During the Cold War, the P waves have been studied to control the nation that nuclear tests. Each of the two blocks was studying the progress of nuclear blocs, thanks to the use of seismometers, to the point that the nuclear test (underground or into the atmosphere) were used by both the U.S. and the USSR as a sort of warning - or indirect communication - against the enemy
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