Earthquake theory, Effects of Earthquake

Elastic Rebound Theory

The elastic rebound theory was purposed by the Professor H.F to explain the origin of the tectonic plates. According to this theory, the earth’s materials being elastic in nature can withstand a certain amount of the stress without going permanent deformation. When the stress exceeds the elastic limit, a crack or the fracture is developed where the frictional resistance along the fracture prevents the fractured blocks from being slipped off from each other and promotes the built up of the strain. The moment comes when the rocks can’t bear the more strain, then sudden dislocation of the rock blocks takes place where the energy stored in the system released which are sent through the earth in the form of the waves. These waves are known as the seismic waves that set up the vibration on the earth’s surface. The earthquakes also occur at the plate boundaries which give rise to the formation of the Himalayas (collision between Indian plate and the Tibetan plate).

Effects of the earthquake

The effects of the earthquakes generally include:

1. Shaking and rupturing of the ground

These shaking and the rupturing of ground is the main effect of the earthquake which can cause more or less the severe damage to the building or the ground. The effects of the shaking of the ground depend on the earthquake magnitude, the distance from the epicenter. The geological and the geomorphological conditions of the local area. The ground-shaking is measured by the ground acceleration.

The ground rupture is the visible breaking and displacement of the earth’s surface along the trace of the fault and may be of the order of few meters in case of the major earthquakes and these are very risks to the large engineering structures such as dams, bridges, and nuclear power stations.

2. Landslides and the Avalanches

The earthquake causes the vibration of the land and the loose geomorphic region can easily fall, mostly with the high pressure of the livestock and the heavy rain. The landslides are the major geologic hazard because they can happen at any time. Severe storms, earthquakes, volcanic activity, coastal wave attack and the wildfires can all produce the slope instability which can cause the falling of the mass of the land.

3. Fires

Following an earthquake, fires can be generated by the break of the electrical power or the gas line which causes the burning of the building structures and the properties and may also cause the loss of lives. The deaths in the 1906 San Francisco earthquake was caused by the fires than the earthquake itself.

4. Soil Liquefaction

The shaking, water-saturated granular material like sand temporarily loses its strength and transforms its form from the solid to the liquid. The soil liquefaction may cause the rigid structures like buildings or bridges to tilt or sink in the liquefied deposits which can cause the devastating effect of the earthquakes. For example, In the 1964 Alaska earthquake, many buildings were sunk into the ground by the soil liquefaction, eventually collapsing with them.

5. Tsunami

The long period sea-waves, long wavelength waves produced by the abrupt movement of the large volumes of the water are known as Tsunamis. In Tsunami, water flows straight instead of in a circle like the typical wave. Tsunamis can travel from 600- 800 kilometers per hour depending upon the water depth. The wave periods can vary from five minutes to an hour. The large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in the matter of the minutes. The Tsunami can also travel thousands of the kilometers across Open Ocean and the weak destruction on the far shores hours after the earthquake-generated to them.

Yet the earthquake above 7.5 magnitudes has caused the devastating Tsunamis.

6. Floods

The overflow of any amount of water that reach the land is known as the flood. The folds occur because the volume of the water within a body of water such as in a river or the lake exceeds the total capacity of the formation and as the result, the water exceeds from the normal perimeter of the water body. The floods may be the secondary effect of the earthquake.

7. Impacts on Humans

The earthquake may result in the disease, lack of basic necessities, loss of the life, damage to the general properties, roads, and bridges, electric poles, collapse of the buildings or the destabilization of the base of the buildings. The earthquake can also cause the volcanic eruption and the most significant effect is the death and loss of many lives during the earthquake.

Measurement of the strength of the earthquake

The strength of the earthquake can be measured by its intensity or the magnitude.

1. Intensity of the earthquake

The intensity of the earthquake is a measure of a degree of damage and destruction that the earthquake cause. This effect can be observed normally without any instrument. Generally, Ercalli scale is used to determine the intensity.

2. Magnitude of the Earthquake

Magnitude is the measure of the energy released during the earthquake and is determined on the basis of the amplitude of seismic waves recorded by the seismograph. The Richter scale is used to measure the magnitude of the earthquake which is the magnitude from 0 to 9 with higher numbers indicating smaller to the larger quakes. In this scale, the upper number presents the earthquakes ten times the previous quakes. Since the Richter scale is logarithmic, the difference between the two consecutive whole numbers on the scale means an increase of ten times. The largest earthquakes up to 6 on the Richter scale do not cause the serious damage.

Epicenter of the earthquake

The direct point above the earthquake’s focus, the point where an earthquake or the underground explosion originates is known as the epicenter. It is the location of the greatest damage. However, in some cases, the epicenter is the start of the much larger event. In this case, damage may spread over the larger areas with the greatest damage possibly occurring some distance from the epicenter. For example, In 2002, Denali earthquake in Alaska (7.9) the epicenter was at the western end of the rupture, but the greatest damage occurs about 330 km away at the eastern end of the rupture zone.

References

Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd.

Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Belland Howell Company, 1985.

Mahapatra, G.B.Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.