Earthquake, Causes & Effects of Earthquakes, Distribution of Earthquakes Part – 4

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Earthquake

An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking the buildings apart. The earthquake is a form of energy of wave motion transmitted through the surface layer of the earth. All earthquakes are not of the same intensity. Some of them are very severe, others are very mild and still others are not even noticed. Only a few of them are major or strong earthquakes.

Though, earth experiences several earthquakes every day, however the frequency of the earthquakes varies largely from place to place. The network of seismographic stations all over the world records dozens of earthquakes every day. But, the occurrence of severe earthquakes is limited to a few regions. The instrument used for recording the earthquakes is known as seismograph. The point within the earth’s crust where an earthquake originates is known as the focus. It is also referred as the seismic focus. It generally lies within the depth of 60 kilometres in the earth crust.

The point vertically above the focus on the earth’s surface is known as epicentre. The impact of the earthquake is carried from the point of its origin by the earthquake waves. These earthquake waves originating from the focus travel in all directions. But their intensity is the highest at its epicentre. That is why the maximum destruction occurs at and around the epicentre. The intensity of vibrations decreases as one moves away from the epicentre in all directions.

Focus and Epicentre of An Earthquake

Focus and Epicentre of an Earthquake

Focus and Epicentre of An Earthquake

Causes and Effects of Earthquakes

Folding, faulting and displacement of the rock strata are the main causes of earthquakes. Some examples of this type of earthquakes are the Assam earthquake of 1950, the Bihar earthquake of 1934 and the San Francisco earthquake of California in 1906.

The violent volcanic eruptions put even the solid rocks under great stress. It causes vibrations in the earth’s crust which causes earthquakes to occur. But these earthquakes are limited to the areas of volcanic activity. An important example is the earthquake which continued for six days preceding the eruption of Mauna Loa volcano of Hawaii Island in 1868.

Minor earthquakes often accompany or are the result of landslides, seepage of water causing the collapse of the rocks of cavern or underground mines and tunnel. These are the least damaging earthquakes.

Violent earthquakes may themselves cause land-slides, damming of river course and occurrence of floods. An earthquake often forms cracks or fissures in the earth’s crust. They also cause the vertical and horizontal displacement of rock strata along fault line. They prove most catastrophic and devastating when they cause fires and seismic sea waves. Such tidal waves are called Tsunamis. These waves may wash away coastal cities. Buildings and bridges collapse causing death of thousands of people. Lines of transport, communication, and electric transmission get disrupted. The after effect of earthquakes is spread of epidemics like cholera.

Distribution of Earthquakes

The occurrence of earthquake is a phenomenon of almost every part of the world. But, there are two well-defined belts where they occur more frequently. These belts are the Circum-Pacific belt and the Mid-world mountain belt. The Circum-Pacific belt comprises the western coast of North and South America, Aleutian Islands, and island groups along the eastern coasts of Asia such as Japan and Philippines. It encircles the Pacific Ocean from end to end. The earthquakes in this belt are associated with the ring of mountains and volcanoes. It is estimated that about 68 % of earthquakes of the world occur in this belt alone. The second belt-extend from the Alps with their extension into the Mediterranean, the Caucasus and the Himalayan region and continues into Indonesia. About 21 % of total earthquakes of the world originate in this belt. Remaining 11 % occurs in the other parts of the world.

World Distribution of Earthquake Activity

World Distribution of Earthquake Activity

World Distribution of Earthquake Activity

In Brief, It Can Be Said That:

Landforms of different types present on the earth’s surface are result of the continuous work of both internal and external forces. Internal forces are responsible for creating inequalities in altitudes of different relief features. These forces originate in the interior of the earth. They are also known as endogenetic forces. These forces cause movements of the earth’s crust which are called earth movements. Slow movements bring slow and gradual changes in the relief features while sudden movements bring abrupt and rapid changes. Internal forces affect the earth into two way vertically and horizontally. When they affect vertically, they cause subsidence or upliftment of the earth’s crust. Contrary to this, when these forces affect horizontally or side to side, they result in folding and faulting of the rock strata.

Volcanoes are landforms marking the eruption of lava at the earth’s surface. The shape and size of volcano depends on the frequency of eruption, fluidity of lava and type of eruption.

Earthquakes are vibrations of the earth’s crust cause by the operations of the tectonic forces and volcanic activity. The volcanic activity is confined to three well defined belts of the world. The occurrence of earthquakes is also closely connected with two of these belts.