Earthquakes

Earthquakes are violent tremors of the earth's crust which originate naturally and below the surface sending out a series of shock waves.

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 faultline. 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.

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.
The vibrations 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.

An isoseismic line is an imaginary line connecting all points on the surface of the earth where the intensity of shaking produced by earthquake waves is the same.

Magnitude and intensity are the two ways in which a quake's strength is generally expressed. The magnitude is a measure that depends on the seismic energy radiated by the quake as recorded on seismographs. The intensity, in turn, is a measure that depends on the damage caused by the quake. It does not have a mathematical basis but is based on observed effects.
A quake's magnitude is usually measured in terms of the Rich~er scale. The Richter scale is logarithmic, Le., the difference between magnitude 4 and 5 is one-tenth of that between 5 and 6.
For measurement of the intensity of an earthquake, the Modified Mercalli Intensity Scale is used.

Aftershocks are earthquakes that often occur during the. days and months that follow some larger quake. Aftershocks occur in the same general region as the main shock and are believed to be the re~;ult of minor readjust­ment of stress at places in the fault zones.

Seismic waves spread out from the seismic focus. The waves are of three main types. P (pri­mary, or push) waves are compressional, and can pass through any medium: they are the first waves to be recorded on a seismogram. S (secondary or shake)
waves are distortional waves: they cannot be transmitted by liquids. L (surface or long) waves travel along the surface of the earth and are re­corded after the P and 5 waves. Earthquakes can cause vertical and hori­zontal displacement of parts of the crust. They can cause the raising or covering of parts of the sea-floor, and landslides. Earthquakes occur when there is sudden displace­ment of rock strata along lines of weakness in the earth's crust and also during volcanic eruption.

The two main areas of earthquakes in the world are (i) around the Pacific Ocean along a belt of volcanoes known as the ring of fire, and (ii) from the middle of Asia through the Mediterranean Sea to West Indies; the former is the more active of the two.