As a result of sudden displacement and ruptures in earth crust there are mighty elements - earthquake. Destructive force of earthquakes is most appreciable in cities.
Earthquakes belong to the most destructive natural powers. The mightiest earthquake can be more powerful than the nuclear bomb dumped to Hiroshima in 1945 in 10 thousand times.
Sometimes during earthquake the earth underfoot reminds a deck of a vessel during sea rolling. Depending on force of earthquake sites of a terrestrial surface become covered by small ripples or, on the contrary, heel sharply and filled up every which way. On occasion at earthquake it seems that on a terrestrial surface there are waves. Eyewitnesses of tremors of 1906 in San Francisco told that saw terrestrial waves in height to metre. When pushes it have stopped, found out that the break of earth crust (Dignity-andreas) along which there were pushes, was displaced on 6 with superfluous metres.
The Majority of earthquakes lasts some seconds, but on occasion duration of tremors exceeds minute. For example, earthquake of 1906 in San Francisco lasted only 40 seconds, and the pushes which have shaken Alaska on January, 24th, 1964, 7 minutes were not appeased: three of them have led to considerable destructions.
In many cases after the main push of earthquake go the subsequent, their force gradually fades. These residual pushes, which geologists name aftershokami, displacement and the deposits result, the rocks lifted by earthquake and also can cause a huge damage. In 1985 earthquake by force has fallen upon the Mexican capital Mexico City in XI points on a scale of Merkalli. Next day has occurred aftershok which force has reached X points. As a result of these two blows of underground elements 10 000 persons were lost nearby, and the city remained to lie in ruins.
Earthquakes arise deeply in earth crust bowels. The external cover of our planet consists of tectonic plates being in movement. The majority of large earthquakes occur in terrestrial depths, at edges of tectonic plates when these plates change the position not gradually, and it is sharp, under the influence of force which presses on their edges, breaks rock and shifts sites terrestrial repeat. The collected energy is liberated in the form of tremors of various capacity. Earthquake consequences depend on its force, depth of tremors and character of a terrestrial surface which can be opened wide by a precipice, rise or form hollows. In mountain district of earthquake lead to avalanches and landslips, sometimes even clay soils on gentle slopes of hills start to flow down downwards, like a volcanic lava. Friable sandstones and alumina under the influence of earthquake can get a liquid consistence and turn to unsteady breeds. Such, in particular, it was observed on Alaska in 1964.
At moretrjasenijah there are huge waves - a tsunami. On ocean open spaces such waves are hardly distinguishable, they rush on a water table with a speed of 790 km at an o'clock. As approaching coast they are slowed down, but grow up. The sea recedes from coast then to bring down on them some shaft enormous (to 20 metres) waves.
After moretrjasenija 1755 the 17-metre shaft of water has fallen upon the Portuguese capital Lisbon. The subsequent pushes have led to collapses and fires. Three quarters of constructions of a city has failed, pogrebja under fragments of 60 000 persons.
The Surface of the Earth stays in constant movement. Though catastrophic earthquakes on the consequences happen not so often, experts in earthquakes - seismologists - annually register about a half-million of pushes.
Seismologists measured force of earthquake In the past (i.e. Quantity of energy liberated by it) under the Richter scale named in honour of the American scientist, put the method into practice in 1935. Now they use a scale of Merkalli is more often. It was developed by the Italian seismologist Juseppe Merkalli in 1902.
At earthquake the shock wave moves from epicenter - points of a terrestrial surface over a site of terrestrial thickness where there are pushes. Distinguish some types of waves. Waves of compression, the R-wave, or longitudinal waves, force particles of breeds to fluctuate like a spiral spring. R-waves cause fluctuations of particles along a direction of distribution of a wave by alternation of sites of compression and depression in breeds. Waves of shift, a S-wave, or cross-section seismic waves, force particles of breeds to fluctuate to perpendicularly direction of distribution of a wave like a vibrating guitar string. The third type tolchkovyh waves - superficial, or L-waves which force a terrestrial surface to go ripples and cause the strongest destructions.
seismically active zones
Scientific steels to designate on cards seismically active sites of a terrestrial surface still before have understood the nature of earthquakes. Tremors can occur everywhere where rocks move along earth crust breaks, but strong earthquakes, as a rule, arise in accurately certain zones. Most often they happen in volcanic areas - for example, in the Pacific volcanic belt.
In process of perfection of methods of definition and exact detection of places of tremors there was possible more reliable mapping of seismically active zones. As a result scientists have received fuller picture of seismic activity of the Earth.
The Seismology has been put on originally scientific basis in 1960th years when scientists have started researches by definition of small underground nuclear explosions after the contract on prohibition of tests of the nuclear weapon. Experts have established many stations of seismic supervision. The cards of seismic activity made subsequently have shown that earthquakes occur along lines of oceanic ridges, round ocean hollows, on sites of breaks of earth crust and near to young mountain and volcanic files more often.
The obtained data has confirmed the theory that 100-kilometre external earth crust and its firm geological cloak consist of eight basic tectonic plates lying on a surface of a semi-fluid internal cover - astenosfery. Constant activity astenosfery sets tectonic plates in motion. They approach, face, disperse, linked etc.
Movings of tectonic plates occur slowly, and seldom have constant character. Sometimes, apparently, that throughout long time of tectonic displacement in general is not present. The rock friction about the same breed keeps plates on a place. When growing force of movement starts to exceed durability of breed, tectonic plates suddenly and sharply start moving, and on an earth surface pushes are felt.
Nobody can precisely predict Movement, when there will be an earthquake. Careful mapping and tracing of underground activity have allowed scientists to define seismically dangerous sites of a terrestrial surface and to put forward hypotheses about rate of large earthquakes. Many powerful earthquakes had an effect small, but accurately distinguishable previous pushes, and the Russian scientists have proved that changes in speed of distribution of insignificant tremors often involve powerful earthquakes. Seismologists believe that researches of minor alterations conducted now in a geographical surface and local on character of fluctuations of a magnetic field of the Earth will allow to receive and other signals about approaching earthquakes.
Some features in behaviour of animals can prompt to the person that earthquake comes nearer. Feeling inevitable terrestrial pushes, dogs howl, the horse can incur, and birds uneasily describe circles in the sky. In 1975 inhabitants of one Chinese city have paid attention to strange behaviour of animals and in time have left the houses - in some hours there was an earthquake.
In seismically dangerous zones scientists regularly take samples of water from wells and chinks. To ruptures of underground breeds can precede lomka their crystal structure when through the formed cracks gas gets to underground waters (and therefrom - in chinks) radon. Increase of level of the maintenance of radon in well water speaks about possibility of fast earthquake.
Liberation elektricheski the charged gases can precede Earthquake. Such gases can have a characteristic luminescence. Scientists also have found out that before earthquake over a break line gaseous hydrogen - in the quantities exceeding usual ten times can be allocated.
Special devices - strain gauges - help to find out any deviations in force of pressure on lines of breaks and huge cracks resulting these processes which can last on some kilometres underground and on its surface. Inklinometr, similar on plotnitsky level, defines earth crust movings, and placed under a break line wire gauges notify on any unusual movements in terrestrial thickness.
The Most exact device for detection of coming nearer earthquake is the seismograph.
Reduction of consequences
Any of the described methods does not exclude an error, but each of them has allowed mankind to understand the nature and character of earthquakes better. Besides, scientists study possibilities of reduction of scales of caused destructions. Some experts believe that destructive consequences of shifts of earth crust can be avoided by means of the small artificial pushes made by explosive materials.
In separate areas small earthquakes have been prevented at the expense of flooding of breaks. In particular, filling with sewage of deep chinks near to Denver (the State of Colorado, the USA) has caused some insignificant pushes in earlier seismically quiet area, having prevented, natural seismic processes along a break the Dignity-andreas in the next California. Architects of Russia, America and Japan give particular attention to designing of reliable, seismically strong buildings. Collapses during earthquakes of building designs leads to awful human victims. Hence, constructions in seismically dangerous areas should be deprived an excessive decor and chimneys and, to be erected on the special bases reducing amplitude of seismic fluctuations on third.