Earthquake

An Earthquake is shaking of the ground, caused by geologic (tectonic) processes in the Earth. Earthquake may cause tsunami and destructions of constructions; trigger the falls in mountings and awake the volcanos.

Magnitude
Earthquake is usually attributed to the mutual movement of the relatively hard layers, that cause deformations, stress of the ground. When this stress become strong enough to cause the crack. The energy of deformation is released as sound wave with low frequency, from a hundred of miliHertz to few Hertz. The corresponding misplacement of the ground is earthquake.

The earthquakes are characterized with magnitude, often the Richter's scale is used. Various definitions of this scale are used by various authors, in TORI the magnitude \(M\) is determined the maximal acceleration \(a\) of the ground:
 * \( M=\log_{10}(a/g)+10\)

where \(g\) is acceleration of the free fall. Weak earthquakes of magnitude 1-2 are detectable only with special devices. The humans usually feel the earthquakes of magnitude \(M\!>\!4\). The strong earthquakes of magnitude 8-9 cause destructions and dangerous tsunamis. In principle, the constructions could be designed to resist earthquakes of magnitude up to 10. The stronger earthquake with shakes of low frequency would bring the whole building (together with the ground it is built on) to the air, but such earthquakes were not recorded in the human history.

Effects of earthquakes
Even \(10\%\) of variation of acceleration (magnitude 9) are sufficient to cause the disasters, as it happened in Japan 2011 March 11. Many buildings and roads were destroyed. The tsunami washed out cities below the historic stones with records Do not build homes below this point . The same tsunami is reported to damage the equipment of the Fukushima nuclear plant, causing the Fukushima disaster.

Warnings
In Japan, within minutes since a strong earthquake, the info about seismic activity and tsunami warning appear. Often, a strong earthquake is followed by tsunami within a halhour, and the human from the coast are adviced to get altitude at least several meters above the sea level.

Origin of earthquakes
The origin of earthquakes is attributed to the natural, geological processes; it is assumed that the human cannot affect them. In principle, the synchronized underground (underwater) explosion of several bombs of class Tzar-bomba could cause effects similar to an earthquake and tsunami;  some primitive estimates for such a hypothesis are suggested in the articles Who contaminates Japan and Russia used the nuclear weapon in 2011, but these hypothesis look as a kind of conspiracy theory rather than as a serious scientific concept with strong predictive ability. The conspiracy theories are unavoidable at the lack of the official information about important events.

Conclusion
Earthquakes are characterized with magnitude \(M\). Weak earthquakes are often, strong are rare. Usually an earthquake happens as series of chokes, waves, and magnitude \(M\) is not only parameter that determines the ability to cause tsunami and destructions. Also, the destructive ability of the ground waves depends on their frequency pattern and their polarization; the horizontal displacements (and accelerations) are more destructive than vertical. The special science, seismology deals with origin and effects of the earthquakes.

See also:

 * tsunami
 * Fukushima disaster