It may be pointed that the intensity and effects of earthquake are not determined by the magnitude of seismic intensity as determined by Richter scale but are decided on the basis of quantum of damages done by a specific earthquake to human lives and property. An earthquake becomes hazard and disaster only when it strikes the populated area. The direct and indirect disastrous effects of earthquakes include deformation of ground surfaces, damage and destruction of human structures, such as buildings, rails, roads, bridges, dams, factories, destruction of towers and cities, loss of human and animal lives and property, landslides, floods etc.
The shock produced by earthquakes particularly in those hilly and mountainous areas which are composed of weaker lithologies and are tectonically sensitive and weak cause slope instability and slope failure and ultimately cause landslides and debris falls which damage settlements and transport systems on the lower slope segment.
Example: The Peruvian earthquake in may 1970. Due to this shock the huge masses of falling ice dislodged thousands of tonnes of rock mass from the Huascaran mountain and thus, covered a distance of 15km within few minutes and buried many buildings and killing about 25,000 people.
Earthquakes inflict great damage to human structures such as buildings, roads, rails, factories, dams, bridges etc. It may be pointed out that in the ground surface composed of unconsolidated geomaterials, such as alluvium, colluvium, artificially infilled and levelled depressions, swamp deposits reclaimed through the dumping of coarse sands and city garbage, the vibration of earthquakes last longer and the amplitudes of seismic waves are greater than in the structures of consolidated materials and bedrocks. Thus, the earthquake cause more damage in the unconsolidated areas than the solid structured area.
Example: Two major earthquakes of Bihar- Nepal border in 1934 and 1980 killed 10,700 people and damage 25,000 houses respectively.
Earthquakes have their worst effects on towns and cities, because the earth tremor of higher magnitude shack the ground to such an extent that large buildings collapse and men and women are buried under large debris and rubbles of collapsed structural materials of buildings, ground water pipes are bent and damaged and thus, water supply is totally disrupted, electric poles are uprooted and electric and telephone wires and cables are heavily damaged causing total disruption of electric supply, road blocks etc.
Example: Kolkata city was severely damaged due to severe earthquake of 11 October, 1737, as thousand of buildings were damaged and 3,00,000 people were killed.
It may be pointed out that it is not the intensity of earthquake alone which matters more as regards the human casualties but it is the density of human population and houses which matter more in terms of human deaths and loss of property.
Example: the Kangra earthquake of India in 1905 recorded 8.6 magnitude on Richter scale but it could cause deaths of only 20,000 people where as 1976 Tang-shan earthquake of China measuring 7.8-8.1 on Richter scale killed 7,50,000 people.
The strong vibrations caused by severe earthquake strongly shake the buildings and these strong oscillations cause severe fires in houses, mines and factories because of overturning of cooking gas cylinders contact of live electric wires, displacement of other electric and fire related appliances.
Example: The house wives were cooking their lunches in the kitchens when disastrous killer earthquake struck in the vicinity of Tokyo and Sagami Bay in 1923.
Severe earth tremors and resulted vibrations caused by severe earthquakes result in the deformation of ground surface and faulting activity.
Example: The Alaska earthquake of 1964 caused displacement of ground surface upto 10-15m.
The seismic waves, caused by the earthquakes travelling through sea water, generate high sea waves and cause great loss of life and property.
Example: The Kutch earthquake of june 16, 1819, generated strong tsunamis which submerged the coastal areas. Tsunamis occur per year along the fire ring of Pacific ocean.
Read about Orogeny,