Introduction
Earthquake is a natural calamity of such a type that it never lives opportunity and scope to people to save their lives and escape. People are caught unaware and the catastrophe is so sudden and and the frequency so uncertain that there is not much scope to caution people before hand.
There are earthquakes almost every year causing large-scale damage and devastation. On the basis of the statistics, it has been estimated that earthquakes take an average yearly toll of 14000 lives and cause damage to extensive property. But, it should be borne in mind that all the earthquakes are not of equal strength and that the extent of damage depends on the degree of acceleration with which the ground rocks during the earthquake.
As we know, earthquakes are due to the sudden release of energy stored in the rocks beneath the earth’s surface. The seismic waves that result from such release of energy cause the main or major Shock, besides there are occasional minor disturbances that take place subsequent to the main or major shocks called the after shocks. The after-shocks prevail for a long period and can cause sufficient damage to the structures already weakened by the major shocks.
Damage by earthquakes varies with the strength of the earthquake, local bedrock, type of building construction and life-support systems.
Effects of the earthquakes
(i) Dueto the vibration of the ground, buildings, bridges, dams, poles and posts and fences etc. may be slightly or heavily damaged and people are hit by falling debris from buildings. Railways are buckled and twisted.
(ii) Earthquakes also cause changes in the geological structure of an area. There may be both vertical as well as horizontal displacement of rocks causing development of slopes or scraps and sometimes fissures and open cracks.etc. It may also destroy the road communication, and tear apart the water pipes and gas pipes etc.
(iii) Landslides and subsidence of land also take place during an earthquake. Sudden subsidence of the land near sea or lake cause flooding and drownings. In the Peruvian earthquake of 1970, million tons of ice, snow, rock and boulders movin: at a tremendous speed (estimated at 480 km per hour) buried the town of Yungay and all its inhabitants.
(iv) Ground water and its movement gets disturbed by earthquakes; besides, the courses of streams and rivers change, new springs develop andin certain favourable conditions sand dykes may also develop.
(v) Fire is a usual problem associated with earthquakes, due to broken gas and water mains and fallen electrical wires.
(Vi) Tsunamis An important secondary effect of a major earthquake is the seismic sea wave, or tsunami as is known to the Japanese, An earthquake below the sea-floor generates seismic sea waves which often have catastrophic consequences. They usually devastate the costal regions.
They are caused usually by earthquakes below the sea-floor, submarine landslides or volcanic explosions.
During an earthquake, sometimes there is a sudden subsidence or upheaval of the sea floor, due to displacement of blocks. Accordingly, all the water at the epicentre of the earthquake is lifted or dropped for an instant giving rise to a sea wave of several hundred kilometers long but only a few metres high in the open sea. This sea wave moves at a speed of 750 to 800 kms per hour. Even though the speed of the wave slow down drastically as it moves through shallow coastal water, the height of the wave rises to 30 or 40 metres on approaching the coast.
This wave of water, as a very large and fast wave, hits the shore but because of its extremely long wave length it does not withdraw quickly as the normal waves do. Its long duration and great height cause great damage to the entire coast and many deaths by drowning in low lying coastal areas. It is thought that coastal flooding which occured in Japan in 1703, with an estimated life loss of 1,00,000 persons, may have been caused by seismic sea waves. In the year 1952, a strong earthquake with the epicentre at the northern tip of Kuriles trough gave rise to a Tsunami that reached the shores of Kamchatka and Kuriles and caused enormous destruction.
Source
A Text book of Geology, By G.B Mohapatra