EARTHQUAKES

EARTHQUAKES :-

Any sudden movement of a portion of the earth's crust due to a natural cause, which produces a shaking or trembling is known as an earthquake.

Causes of Earthquakes: The chief cause of earthquakes is the sudden slipping of the portion of the earth's crust past each other along fractures or faults. The movement of the molten rocks underneath the surface produces strains which break the rocks apart. Another cause of earthquake is volcanic activity. A violent or explosive eruption often causes the earth in its vicinity to quake. Earthquakes are' often common in most volcanic areas. Minor causes of earthquake are sudden landslides, submarine slides and collapse of cavern roofs. An underground stream can, by erosion. dig out large caverns in the body of the earth. This can later cause the roofs of these avers to collapse.

The place of origin of an earthquake inside the earth is called its focus. The point on the earth's surface vertically above the focus is called the epicenter. Observations have

shown that most of the earthquakes originate at a depth from 50 to 100 km and only single earthquake occurs at still greater depths. The shock waves travel in all directions from the focus. On the earth's surface, the shaking is the strongest near the epicenter. That is why the greatest amount of destruction is caused near the epicenter.

The earthquakes arc studies by a special branch of geology known as Seismology. The instrument recording the shock waves is called the Seismograph.

Effects of Earthquakes: Sudden movements under the earth cause violent earthquakes which are often very destructive. At times, they cause landslides and damming the rivers. Sometimes, they cause depressions' forming lakes. Formation of cracks or fissures in the region of epicenter are commonly noticed in the crust. These deep fissures are sometimes many kilometers long and the buildings, people and animals fall into them. Sometimes water, mud and gases are ejected from beneath the fissure. The gases may ignite the air and water, and mud may flood the surrounding area. Larger areas also subside or sink during very severe earthquakes. Landslides occur during earthquakes in highlands. An earthquake may also lead to change in surface drainage and underground circulation of water. Crystal displacements may close up the crevices along which water was coming out or they open up new ones. That is why we notice a disappearance of springs in some places and their appearance in others as a result of strong shocks.

Faults, thrusts and folds are. associated with earthquakes. It is of interest to mention a thrust of the strata that disrupted several country roads on the floor of a broad valley in Japan during the earthquake of 1891.

Perhaps more devastating are firs and a seismic; sea waves (called Tsunamis in Japanese) which are originated by earthquakes. Instances are not lacking to show that hundreds of thousand of people have fallen victims not directly to earthquakes, but to the fire, flood and sea waves which follow them.

Distribution: About 68 per cent of all earthquakes are observed in the vast region of the Pacific Ocean as a 'ring of fire' and is closely linked with the region of crystal dislocations and volcanic phenomenon. Chile, California. Alaska, Japan, Philippines, New Zealand and the Mid ocean areas have had many minor and major earthquakes in this belt. Mountains here run along the border of continents and nearly parallel to the depressions in oceans. It causes sharpest break in relief which becomes a cause for the earthquakes.

Around 21 pet cent of them occur in the Mid-world mountain belt extending parallel to the equator from Mexico across Atlantic ocean, the Mediterranean sea from Alpine Caucasus ranges to the Caspian, Himalayan mountains and the adjoining lands. This zone has folded mountains, large depressions and active volcanoes.

The earthquakes in India are at present mainly confined to the Himalayan region and its foothills. They are also felt in the Ganga Valley. But the earthquakes in Koyna Dam region in 1968 and Latur in 1993 in the Deccan Table came as a surprise. This region was otherwise considered to be free from earthquakes. Scientists believe that while in the former case, the reservoir caused cracks in the rocks, in the latter case the movement of the Indian plate might have been the cause.