EARTHQUAKES

Earthquakes are tremors which are produced by the passage of vibratory waves through the rocks of the earth. If you throw a stone in a pound of still water, a series of concentric waves are produced on the surface of water. These waves spread out in all directions from the point where the stone strikes the water. Similarly, any sudden disturbances in the earth’s crust may produce vibrations in the crust which travels in all directions from the point of disturbance.

Earthquakes and volcanoes are endogenic forces while weathering and agents of sculpture are exogenic forces. Some are sudden forces and bring about sudden changes. Earthquakes and volcanoes are examples of sudden forces. There are other forces which work slowly and take longer duration of time to show their effects.

FOCUS AND EPICENTRE

The place of origin of an earthquake inside the earth is called its focus. The shock waves travel in all directions from the focus. On the earth’s surface, the shaking is strongest near the epicenter.

There are several types of earthquakes which affect different parts of the world at different times. Some of the common types of earthquakes are briefly described.

Tectonic Earthquakes- There are the most common earthquakes and

generated by sliding of rocks along a fault plane.

Volcanic Earthquakes- As the name indicates these earthquakes are caused

by volcanic eruptions and are quite devastating. However, these are confined to areas of active volcanoes.

Collapse Earthquakes- In areas of intense mining activity, often the roofs

of underground mines collapse and minor tremors take place. These are called collapse earthquakes.

Explosion Earthquakes- When a chemical or nuclear explosion takes

place, ground shaking occurs and an earthquake is known as explosion earthquakes.

Reservoir induced Earthquakes- Such earthquakes are caused by the pressure of water collection in large reservoir. The koyna earthquake was cause by koyna reservoir.

Measuring Earthquakes- The earthquakes are measured in terms of their magnitude and intensity.

 The earthquakes magnitude is related to the energy released during the quake and is recorded by an instruments called Seismograph. It is measured on Richter scale. This scale was developed by Charles Francis Richter. The magnitude is expressed in absolute numbers, 0-10.

EFFECTS OF EARTHQUAKES

Earthquakes cause heavy loss to life and property. The maximum damage is caused near the epicenter of the earthquake. Its tremors were felt all over the subcontinent including India, Pakistan and Nepal. It measured 7.9 on the Richter scale. About one lakes people lost their lives. Sometimes earthquakes disturb the existing rocks strata and block the flow of Water River. The river water submerges the surrounding low-lying areas causing severe floods. The floods in the caused by an earthquake. Earthquakes occurring in sea, result in high waves in the sea water and cause heavy damage to ships. Such seismic sea waves are called Tsunamis in Japanese. Tsunamis which hit 11 countries. It was caused by a severe earthquake measuring 8.9 on the Richter scale and occurring of the coast of Sumatra (Indonesia).

Faults, thrusts and folds are often associated with earthquakes. An earthquakes disrupted several country roads on the floor of a broad valley in Japan during the earthquake of 1891. A tremendous fault occurred during the California earthquake of 1906, running for hundreds of kilometers along the coast. Earthquakes are responsible for land-slides in mountainous areas which causes heavy damage.