Earthquakes are tremors which are produced by the passage of vibratory waves through the rocks of the earth. The place of origin of an earthquake inside the earth is called its hypocentre or focus. The point on the earth’s surface vertically above the focus is called the epicentre. The shock waves travel in all directions from the focus. On the earth’s surface the shaking is the strongest near the epicentre. A line joining the places of equal earthquake intensity is called isoseismic line. Lines joining places which experience the earthquake at the same time are known as homoseismal lines.
Intensity of an earthquake is an evaluation of the severity of ground motion at a given location and is measured in terms of the damage caused to life and property. The scale used to measure it is Modified Mercalli Scale (MMS). Magnitude of an earthquake is related to the energy released at the earthquake centre. It is measured with the help of Richter Scale which was devised by Charles F. Richter.
Following table shows the combination of intensity and relation to epicentre and to the wave paths radiating magnitude of earthquake from the focus of an earthquake. Roman numerals indicate intensities.
Table 5.1 Intensity and Magnitude of Earthquakes
|Mercalli Intensity||Description of Characteristic Effects||Richter Magnitude|
|corresponding to highest|
|I||Instrumental||Detected only by seismographs|
|II||Feeble||Noticed only by sensitive people||3.5|
|III||Slight||Like the vibrations due to a passing lorry; felt by people at rest,||to 4.2|
|especially on upper ors|
|IV||Moderate||Felt by people while walking; rocking of loose objects, including||4.3|
|V||Rather strong||Felt generally; most sleepers are awakened and bells ring||4.8|
|VI||Strong||Trees sway and all suspended objects swing, damage by||4.9-5.4|
|overturning and falling of loose objects|
|VII||Very strong||General alarm; walls crack; plaster falls||5.5-6.1|
|VIII||Destructive||Car drivers seriously disturbed; masonry ssured; chimneys fall;||6.2 to 6.9|
|poorly constructed buildings damaged|
|IX||Ruinous||Some houses collapse where ground begins to crack, and|
|pipes break open|
|X||Disastrous||Ground cracks badly; many buildings destroyed and railway lines||7..0-7.3|
|bent; landslides on steep slopes|
|XI||Very disastrous||Few buildings remain standing; bridges destroyed; all services||7.4-8.1|
|(railways, pipes and cables) out of action; great landslides and ods|
|XII||Catastrophic||Total destruction; objects thrown into air; ground rises and||>8.1|
|falls in waves.|
Distribution of Earthquakes
The distribution of earthquakes is more or less similar to that of volcanoes. The earthquakes mostly occur in weak crustal areas of the earth.
About 68% of world’s earthquakes are observed along the coasts of the vast Pacific Ocean. This is known as the ‘the ring of fire’. This is the area of intense volcanic activity also. This area is closely linked with the region of crustal dislocations and volcanic phenomenon. 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. It causes sharpest break in relief which becomes a cause for the earthquakes.
Mid-world Mountain Belt
Nearly 21% of the world’s earthquakes occur in the mid-world mountain belt. It extends parallel to the equator from Mexico across Atlantic Ocean, the Mediterranean Sea, from Alpine-Caucasus ranges to the Caspian, Himalayan mountains and the adjoining lands. This zone has folded mounta ins, large depressions and active volcanoes.
The remaining 11% of the shocks are recorded outside these two belts. Only a few occur along the fracture in African lakes and Red Sea.