Earthquakes

Earthquakes:

An earthquake is a major demonstration of the power of the tectonic forces caused by endogenetic thermal conditions of the interior of the earth. An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings apart and causing gaping fissures to open in the ground. The Richter scale devised by Charles F. Richter in 1935 measures the magnitude or intensity of energy released by an earthquake. Good Friday Earthquake of March 27, 1964 in Alaska (USA) measuring 8.4 to 8.6 on Richter scale is among the greatest earthquakes of the world ever recorded.

Causes of Earthquakes:

Earthquakes are caused mainly due to disequilibria in any part of the crust of the earth. A number of causes have been assigned to cause disequilibria in the earth’s crust such as volcanic eruptions, faulting and folding, gaseous expansion and contraction inside the earth, hydrostatic pressure of man-made water bodies like reservoirs and lakes, and plate movements.

(1) Vulcan City:

Volcanic activity is considered to be one of the major causes of earthquakes. Vulcan city and seismic events are so intimately related to each other that they become cause and effect for each other. Earthquakes follow each volcanic eruption and many of the severe earthquakes cause volcanic eruptions. The explosive violent gases during the process of Vulcan city try to escape upward and hence they push the crystal surface from below with great force and thus is’ caused severe earth tremors of high magnitude.

(2) Faulting and Elastic Rebound Theory:

The horizontal and vertical movements caused by end genetic forces result in the formation of faults and folds which in turn cause isocratic disequilibria in the crystal rocks which ultimately causes earthquakes of varying magnitudes depending on the nature and magnitude of dislocation of rock blocks caused by faulting and folding. The 1950 earthquake of Assam was believed to have been caused due to disequilibria in crystal rocks.

(3) Hydrostatic Pressure and Anthropogenic Causes:

Certain human activities such as pumping of ground water and oil, deep underground mining, blasting of rocks by dynamites for constructional purposes, nuclear explosion, storage of huge volume of water in big reservoirs etc. also cause earth tremors of serious consequences. The introduction of additional load through the construction of large dams and impounding of enormous volume of water in big reservoirs behind the dams cause disequilibria of adjusted rocks below the reservoirs.

(4) Plate Tectonic Theory:

The earth is composed of solid and moving plates having either continental crust or oceanic crust or even both continental oceanic crusts. The earth’s crust consists of 6 major plates (Eurasian plate, American plate, African plate, Indian plate, Pacific plate and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each other due to thermal convective currents originating deep within the earth. All sorts of disequilibria are caused due to different types of plate motions and consequently earthquakes of varying magnitudes are caused.

Effects of Earthquake hazardous: Earthquakes and their hazards are determined on the basis of the magnitude of seismic intensity as determined by Richter scale but are decided in the basis of quantum of damages done by a specific earthquake to human lives and property.

(i) Landslides:

Weaker landmasses and tectonically sensitive land margins cause landslides and debris falls, which damage settlements and transport systems on the lower slope segments.

(ii) Damage to Life and property:

Structures such as buildings, roads, rails, factories, dams, bridges suffer a huge damage thus causing a heavy loss of human life and property both. The vibrations of earthquakes last longer and the amplitudes of seismic waves are greater artificially in filled and leveled depressions, swamp deposits etc. than in the structures of consolidated materials and bedrocks. Two major earthquakes of Bihar-Nepal border in 1934 and 1988 explain the impact of earthquake disasters on human structures and human lives. The damage caused by the Bihar earthquake of 15 January 1934, measuring 8.4 on Richter scale, include 10,700 human deaths, landslides and slumping in an area of 250 km length and 60 km width, ruptures and faults in the ground surface etc.

(iii) Damages to Government Infrastructure:

Cities and towns are worst affected due to large concentration of human population, commercial complexes and residential areas. Due to collapse of large buildings there is greater loss of life and property. Due to collapse of buildings ground water pipes are bent and damaged thus water supply is disrupted, electric and telephone poles are uprooted and there is total disruption of power and communication. Other side effects are collapsed sewer system causing epidemics, roadblocks etc.

(iv) Fire Hazard:

Earthquakes strongly shake the buildings and thus strong oscillations cause severe fires in houses, mines and factories because of overturning of cooking gas cylinders, contact of live electric wires, churning of blast furnaces, displacement of other electric and fire related appliances.

(v) Landmass Deformation:

Severe earth tremors and resultant, vibrations caused by severe earthquakes result in the deformation of ground surface because of crusts and troughs in the ground surface and faulting activity.

(vi) Tsunamis:

The seismic waves, caused by the earthquakes traveling through seawater, generate high sea waves and cause great loss of life and property. Since the Pacific Ocean is girdled by the earthquakes and volcanoes tsunamis are more common in the pacific with a minimum frequency of 2 tsunamis per year.