Earthquakes are usually classified on the following bases:
(a) Cause of origin;
(b) Depth of focus; and
(c) Intensity and magnitude of earthquake.
(a) Cause of Origin: On the basis of the causes of earthquake, they are classified as:- (i) Tectonic and (ii) Non-tectonic earthquakes. The non-tectonic earthquakes are mainly of three types due to surface causes, volcanic causes and collapse of cavity roofs. The non-tectonic earthquakesdue Volcanic to surface causes Earthquakes or Denudation earthquakes
(b) Depth of focus : As we know, the instrument designed to detect seismic waves is called seismometer and the seismograph is a seismometer to record the earth vibration. This record of earth vibration is known as seismogram. It has now become possible to estimate the depth of focus of an earthquake by analyzing seismograms. On the basis of the depth of focus, earthquakes are classified as:
(ii) Shallow-focus earthquakes or normal earthquakes.
(iii) Intermediate-focus earthquakes, and
(iv) Deep-focus earthquakes.
(i) Surface-earthquakes : Surface-earthquakes are those in which the depth of the focus is less than 10,000 metres.
(ii) Surface-earthquakes : The earthquakes with the hypocentre at a depth of 10 to 50 kms are known as shallow-focus earthquakes.
(iii) Intermediate-focus earthquakes : When the earthquake is originated at a depth of 50 to 300 Kms, it is called intermediate- earthquake.
(iv) Deep-focus earthquakes : The deep-focus earthquakes or the plutonic earthquakes are those with hypocentres located at depths more than 300 kms. Majority of the deep focus earthquakes originate between 500 and 700 kms.
Shallow-focus earthquakes constitute about 85 percent of all the earthquakes and the intermediate and deep-focus earthquakes account for 12 and 3 percent respectively of all the earthquakes. Thus it is seen that the intermediate and deep-focus earthquakes together account for only 15 percent of the earthquakes.
(c) Intensity and Magnitude of Earthquakes
As we know, the tremors caused by earthquake may be so feeble and imperceptible that they can only be registered by highly sensitive instruments and may be so vigorous to cause large scale devastation. The strength of an earthquake can be measured either by its intensity or by its magnitude.
Intensity of an earthquake is a measure of the degree of damage and destruction it can cause. These effects can be observed without the help of any instrument. It is also a fact that intensity of an earthquake diminishes outwards from the epicenter.
Therefore places in which the earthquake has manifested itself with equal intensity are contoured with – a line known as ‘isoseismal’. Areas with one and the same intensity of earthquake restricted by isoseismal lines are known as isoseismal areas. There is a number of disadvantages in using intensity as a measure of the strength of a particular earthquake, some of the important disadvantages are as follows:
(i) The strength of an earthquake decreases with distance from its epicentre. Thus different degree of damage occurs at different distances for the same earthquake.
(ii) The degree of damage depends much on the geological characteristics of a particular area as well as the type of construction, population-density etc.
However, two scales of intensities are in vogue viz. (i) Rossi Forrel’s Scale and (ii) Mercalli Scale.
(i) Rossi-Forrel’s Scale
According to this scale, there exists ten distinct and well-defined intensities beginning from/ and ending withX of which / represents the most mild earthquake while X the most disastrous one. The scale is as follows:
III. Very Slight
Recorded by sensitive instruments only. Recorded by all seismographs and felt by experienced persons only. Felt by several persons at rest. It is strong enough for the duration and direction to be recorded. Felt by persons in motion. Moveable objects disturbed. Affects window doors, ceilings etc. General alarm, ringing of bells, disturbance of furniture and beds etc.
General awakening of persons from sleep, stopping of clocks, visible oscillation of trees etc. Overthrows moveable objects, general panic, fall of plaster from the walls without damage to the building etc.
Fall of chimneys and cracks in the walls of buildings. Partial or total destruction of buildings. General destruction of buildings rock-falls and landslides in mountai- ncous regions.
(ii) Mercalli Scale
This scale is developed by Mercalli, an Italian seismologist, after he made studies of the intensity and regional effects of earthquakes. The scale had at first ten divisions but later on it was modified to a scale of 12 degrees. The higher the number of intensity the greater is the damage. The scale is as follows:-
Name of the shock
Hardly noticeable. Detected only by instruments. Felt in some cases by people at rest. Delicately suspended objects may swing. Felt quite noticeably, like passing of a truck. Perceived by persons at rest. Felt by people in motion, windows and doors may start vibrating standing automobiles may rock noticeably.
Most sleepers wake up, plaster peels off in places, unstable objects move, bells ring.
Felt generally; people get panicky and run outdoors, cracks develop in the walls.
Slight damage to buildings, there are fissures in the walls and chimneys are broken.
Buildings are impaired, heavy furniture overturned, chimneys fall, persons <lriving motor car disturbed.
Buildings collapse, ground cracks conspicuously, underground pipes broken
Buildings are seriously damaged or destroyed, land-slides on steep slopes, rails bent, ground badly cracked.
Few building remain standing, bridges destroyed, numerous fissures appear on the earth’s surface, great landslides and floods.
Total destruction; waterfalls and lakes emerge; waves seen on ground surfaces; objects thrown into air; river channels change their course.