Earthquake is a phenomenon generally considered in the context of volcanic activity. An earthquake is the shaking or trembling of the earth, caused by the sudden movement of a part of the earth’s crust.
The chief cause of the earthquake shocks is the sudden slipping of rock formations along faults and fractures in the earth’s crust.
This happens due to constant change in volume and density of rocks due to intense temperature and pressure in the earth’s interior. Some quakes originate at depths as great as several hundred kilometres and in such cases the tremors are too weak to reach the surface or cause much damage.
The actual shifting of the land at the time of an earthquake occurs only in a narrow zone on either side of the fault-line. In such a case, the main zone of shock and consequent destruction is linear because the vibrations originate in the line of fracture. A sudden slipping of even five to fifteen metres along a line of fracture 80 to several hundred kilometres long can cause a very severe earthquake.
Volcanic activity also can cause an earthquake but the earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth’s crust. Some minor earthquakes are caused by the collapse of roofs of cavities, mines or tunnels.
Focus 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 epicentre. On the earth’s surface, the maximum damage is caused at the epicentre(Fig.1.38).
There are annually 8,000-10,000 earthquakes in the world, which comes to about an earthquake every hour. Actually, there are many more undetected, because there are no stations to record them over the oceans covering a very large surface of our earth.
The vibration of earthquakes which can be felt by human beings last from a few seconds to several minutes. Generally, the greater the intensity of the shocks, the longer they last. The average duration of shocks of sufficient intensity to produce much damage is perhaps from one to two minutes.
Earthquake waves travel ordinarily at the rate of about 5 to 8 km per second through the outer part of the crust but travel faster with depth.
A line connecting all points on the surface of the earth where the intensity of shaking produced by earthquake waves is the same.
Effects of Earthquakes:
Earthquakes cause landslides, damming of rivers, depressions which form lakes. Formation of cracks or fissures especially in the region of the epicentre is common. 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. An earthquake may also lead to change in surface drainage and underground circulation of water. More devastating features of earthquakes are fires and seismic waves 0tsunamis).
Distribution of Earthquakes:
The Pacific Ring of Fire accounts for about 68 per cent of all earthquakes and these are closely linked with the phenomenon of plate tectonics. 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. The sharpest break in relief in this belt is the cause of earthquake.
The mid-world mountain belt extends parallel to the equator from Mexico across the 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 remaining 11 per cent of the shocks are recorded outside these two belts. Only a few occur along the fracture in African lakes, Red Sea and the Dead Sea zone. For geographical distribution of earthquakes. For an account of earthquakes in the Indian sub-continent, Physical Aspects in Indian Geography section.)