Earthquake

What is an earthquake?

An earthquake is what happens when two blocks or plate or the earth suddenly slip past one another. The surface where they slip is called fault plane. The location below the earth's surface where the earthquake starts is called hypocenter, and the location directly above it on the surface of the earth is called epicenter.

More than a million times a year, the earth's crust suddenly shakes during an earthquake. Most of the world's earthquakes are fairly slight. A mild earthquake can feel like a truck passing, it can destroy roads and buildings, and also cause the sea to rise in huge waves.

Earthquakes often happen near volcanoes and young mountain ranges: at the ridges of the earth's plates.

What causes earthquakes and where do they happen?

The earth has four major layers: the inner core, outer core, mantle, and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece, it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving aroung, sliding past one another and bumping into each other, we called it as a tectonic plates and the edge of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keep moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

 

   

Earthquakes are caused by disturbances in the interior of the earth and other causes.

1. Tectonic Movements: The disturbances inside the earth are called tectonic movements.These forces bring about changes on the earth surface and physical features like mountains, plateaus and rift valleys are formed.Most disastrous earthquakes are caused by tectonic forces .Tectonic forces create tension and pressure and the stress begins to build up inside the earth. When the stress tends to be more than what the rocks of the earth can bear, the rocks are broken and displaced from their state of equilibrium. It is known as faulting. The energy accumulated during faulting is released.This release of energy gives rise to mighty waves.These waves originate from a point called Focus in the interior of the earth and then spread out in all directions.On the surface whatever comes into their contact begins to vibrate.The chief cause of earthquakes felt often in California in USA is often the San Andreas Fault found there.
2. Volcanic Eruptions:The volcanic eruptions are often very violent and cause vibrations in the earth crust. Sometimes the vent of a volcano is blocked temporarily and explosive eruption takes place suddenly causing tremors in the earth crust.The Krakatoa that erupted in 1883 became the cause of a violent earthquake there.
3. Other Reasons:The roofs of underground caves sometimes give way and release great force to cause minor tremors in the earth crust. Nuclear explosions also release massive energy to cause tremors in the earth crust.

Why does the earth shake when there is an earthquake?

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the sharp edges of the faults and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth's surface, they shake the ground and anything on it, like our houses and us.

How are earthquakes recorded?

Earthquakes are recorded by instruments called seismograph. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes to, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

           

A short curving line that doesn't curve very much means a small earthquake, and a long curving line that curve a lot means a large earthquake. The length of curve depends on the size of the fault, and the size of the curve depends on the amount of slip.

Earthquake Damage

In general, great loss of life during an earthquake can be avoided. It is often not the earth's shaking that kills people, but falling buildings, particulary poorly constructed ones. Landslides and tsunami is also cause a lot of damage. During an average earthquake, it is best to stay indoor in a doorway or under sturdy table. 

Fire poses a great danger following an earthquake. Gas leaks and oli spills can lead to large fires. Loose rock and debris may be dislodged by an earthquake and cause landslides. Landslide too may be triggered by the ground shaking. Mudflows, or lahars, can result from rain or snow mixing with loosened soil.

Effects of Earthquakes

Earthquakes are less advantageous and more harmful to human. Damage done is chiefly in following respects:

1. Loss of Property: Severe earthquakes reduce to rubble human structures ranging from huts to palaces and single storey to multi storey buildings.Even pipelines laid under the ground and railway lines are damaged or displaced.The best example of this type of damage is Koyana earthquake in 1970.
2. Loss of Life: Earthquake tremors of a few seconds takes the lives of thousands of people. Many people have been rendered homeless or suffered injuries in various ways.
3. Changes in the path of rivers: On account of the impact of earthquakes,sometimes rivers also change their path.Consequently, when floods come they play chaos with people's lives.
4. Tsunami:The earthquakes in the sea generate massive waves called Tsunami in Japanese language.It sometimes rises to the height of 20-25 metres. It causes great damage to life and property of people living in coastal areas as well as to tourists.Tsunami caused by an earthquake in the sea near Sumatra on 26th Dec,2004 hit south east Asian countries including India and Sri Lanka.There was heavy damage in these countries.
5. Mud Fountains: On account of earthquakes of high intensity,warm water and mud fountains also burst.
6. Cracks in Earth Crust: Earthquake cause cracks in earth's crust anywhere in fields,roads,parks and even hills.They are thus rendered useless.The San Andreas fault in California ,U.S.A. was created in a similar manner.

   

  

What is a tsunami? 

A tsunami is a large ocean wave usually caused by an underwater earthquake or a volcanic explosion. Tsunamis are not tidal waves. Tidal waves are caused by the forces of the moon, sun, and planets upon the tides, as well as the wind as it moves over the water. With typical waves, water flows in circles, but with a tsunami, water flows straight. This is why tsunami cause so much damage!