Sometimes sudden vibrations of the earth's crust originate from the interior underneath and move upwards. These give rise to surface trembling of the ground, which is called earthquake.
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The point in the interior of the earth from where the vibrations or earthquake waves originate is called the focus of the earthquake. The waves radiate upwards in cones of waves known as underground or body waves. They arrive the surface in concentric circles where they are called surface wave, is known as the epicentre of the earthquake.
You must be wondering what causes these vibrations that gives rise to the earthquake. Like other tectonic (i.e. folding, warping, faulting and vulcanicity) the vibrations are caused by the collision of underground tectonic plates.
That is why all these processes often occur together in the regions of tectonic subduction. Earthquake are also caused by vulcanicity , and movement of the earth's crust along faults, like the San Andreas fault in U.S.A.
The regions of tectonic subduction where these processes concentrated, lie along belts in the earth called tectonic belts. The most prominent of these belts is the tectonic belt that surrounds the Pacific ocean, known as the circumpacific fiery ring or the Pacific ring of fire.
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Japan and Alaska are in this ring, and are noted for their frequent earthquake and volcanoes. So are California (USA), Peru and Chile (South America) and Indonesia (Asia). In Africa, the Atlas region of North Africa (notably Morocco and Algeria) and the rift valley region of East Africa are prone to earthquakes, while minor tremblings, called earth tremors.
Earthquakes have a number of disastrous effects which may vary with the intensity of the vibrations. (Just imagine what will happen if the ground under your feet suddenly started trembling.
The trembling alone, no matter how slight, would give you great anxiety and discomfort, you may even lose your balance and fall.
When prolonged and severe, an earthquake gives rise to great damage to lives and property. A recent example is the 2011 Tohoku earthquake which hit the east coast of Japan led to the death of 18,184 persons. The Peru earthquake of 1970 killed 50000 persons and rendered 100000 homeless. So it is in many other regions where earth quake occur.
Earthquake also fracture rocks anad cause them to be displaced either vertically or horizontally.
Vertical displacement may raise the sea floor, like the Moroccan sea floor from 400m deep to only 15m deep.
Horizontal displacement causes wide open chasms and landslides on the earth's crust. Such wide chasm resulted from the San Francisco earthquake of 1906 which destroyed most part of the central part of that city.
Earth quake gives rise to tsunami which are destructive to lives and properties, like the Tohoku earthquake which struck Japan in 2011 leading to tsunami which cause Japan huge loses and even led to the failure of the Fukishima nuclear plants.
The worst known earthquake is the Chile earthquake in 1960. The ground rose and fell in waves, objects thrown in the air, great chasm and land displacement were caused and sea waves were generated which drown hundreds of thousands of people.
However, in this modern time the advent of technology has minimized the fatalities in earthquake disasters. With advance countries like Japan installing earthquake and tsunami warning systems to forewarn its citizens of earthquakes and Tsunami. Also the building of earthquake proof building have also gone a long way in helping during earthquake disasters.
https://www.soest.hawaii.edu/GG/ASK/earthquakes.html
