Geology And Geophysics

Earthquakes



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Earthquakes result from the strain caused by the movement of the plates that make up the earth's outer shell; earthquakes are directly associated with the activities that occur at ocean ridges, trenches, etc. What happens is that when the earth's surface cannot cope with the strain it breaks up and releases tremendous amounts of energy in the form of sudden violent shocks.

Earthquakes cause fissures in the ground; also, landslides and avalanches are triggered off; sometimes the courses of rivers are diverted. An earthquake causes destruction all round; it can wipe out an entire community in minutes. Coastal earthquakes generate giant waves which can cause destruction thousands of miles away. The loss of lives and destruction of property that occured in Armenia in Soviet Russia were unimaginably high and point to the magnitude of the devastating effects of earthquakes.

The point at which an earthquake occurs is called the focus or hypocentre. According to seismologists - scientists who study earthquakes - there are three types of earthquakes. Earthquakes are classified on the basis of the focus below the earth's surface. The three types of earthquakes are shallow earthquakes, intermediate ones, and deep ones. Shallow earthquakes take place at depths of zero to seventy km, intermediate ones at the depth of 70- 300 km, and deep earthquakes below 300 km. It is the shallow earthquakes which cause most of the damage at the earth's surface. There are about three times as many intermediate shocks as the deep ones, and about ten times as many as shallow ones. The deepest earthquake known to have caused damage was the one in Romania in 1940, it killed about a thousand people. Its focus was about 160 km deep.

The point on the surface of the earth directly above the focus is called the epicentre. Earthquake epicentres are largely concentrated in narrow belts which coincide with the boundaries of the earth's plates. The most intense earthquake activity is round the margin of the Pacific. About 75% of all shallow earthquakes, 90% of intermediate earthquakes, and almost all deep earthquakes take place in this belt. It covers the west coast of South America, the west coast of North America, the Caribbean, parts of Asia, Indonesia, New Zealand and the Antarctic . There is yet another belt which spreads through the Alps and North Africa, along the northern shores of the Mediterranean, eastwards through Iran and the Himalayas, then through southern China to join the Pacific belt.

Earthquakes are recorded and measured by seismographs. Seismographs are instruments which amplify ground motion so that even the weakest waves can be detected at very great distances. When an earthquake occurs seismic waves emitted by the shock of fracturing rock radiate in all directions from the focus. It is these waves that make it possible the recording of earthquakes.

From the amplitude. or size, of the recorded seismic waves, seismologists determine the magnitude of the earthquake. Earthquake magnitude is a measure of the seismic energy released at the focus; it is graded according to the Ric hter Scale. The Scale was devised by a Californian seismologist, Charles. F. Richter, and so it has been named after him.

Before the magnitude Scale was devised in 1935, seismologusts could only measure earthquake intensity. This is based on the degree of damage caused at the earth's surface, and on people's reaction to the shock. Magnitude is an absolute measurement of an earthquake's strength, but the intensity diminshes with the distance from the focus.

An earthquake in a sparsely populated region will do less harm then one in a city. A city which has shock-resistance buildings will suffer less damage than a technologically less advanced city. An earthquake at night, when many people are asleep in low-rise buildings, will kill fewer people than one occuring during the day, when many people are at work in high rise office blocks. The most dangerous place to live in terms of earthquake risk and possible loss of life is the Mediterranean-Middle East region. The amount of destruction and death caused by an earthquake depends not only upon the magnitude of the shock but also on where and when it takes place.

People living in areas prone to earthquakes take all possible measures to minimize loss of life and property. The Japanese have thus learnt to live with earthquakes. If the earthquake in Armenia in the USSR had occured in Japan, the losses would have been much less because Japan, highly prone as it is to earthquakes, has taken all the possible measures and precautions to minimize the losses caused by earthquakes.

There is no foolproof method to predict earthquakes. Until the 60's the only people in the USA concerned with predicting earthquakes were astrologers and mystics. Now scientific attempts at prediction are made, and are preferred to pseudo-scientific efforts. In Japan earthquake prediction has been a respectable pursuit since the great Tokyo earthquake of 1923, in which over 99,000 people died.

The science of earthquake engineering is still a neglected area in many countries. Prediction and control are still some way from becoming reliable methods for coping with earthquakes. For the time being their effects can be lessened by careful siting of communities and construction of shock resistant buildings. If due attention is paid to seismological aspects while designing buildings and dams in areas exposed to earthquakes, the people living in these areas can survive any major earthquake as proved by the Japanese.

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