Friday, 6 May 2011

Earthquake

Earthquake


An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe.

The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude.


The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.

Release of energy waves called seismic waves in the crust of earth, leads to the creation of a natural disaster called earthquake. Earthquakes can be recorded using an instrument called as seismometer. Richter scale is used to represent magnitude and intensity of earthquakes. Those earthquakes which show reading of 3 on Richter scale are usually not harmful as such, but earthquakes which show reading of 7 or more are extremely harmful. So, more the scale of reading more harmful is the earthquake.

The origin point of seismic waves is called epicenter of earthquake. If epicenter is located near the sea then it can cause the creation of Tsunami waves. Earthquakes lead to severe shaking of earth’s surface, landslides etc. Earthquakes are generally results of natural phenomena like geological stress, volcanic eruptions, landslides etc but earthquakes can also be induced by some human activities like mine blasts and nuclear experimentation etc. the first rupture point of earthquake is called focus hypocenter and epicenter is the point at ground level exactly above the hypocenter.



Naturally occurring earthquakes and earthquake storms

Those earthquakes which occur due to some natural phenomena are generally more intensified and harmful. These Tectonic earthquakes can occur just anywhere in earth where enough elastic strain energy is stored to conduct fracture propagation at a fault plane. There are basically 3 types of faults, which cause creation of the earthquakes- reverse, strike-slip and normal. Reason of a certain displacement decides the intensity of the earthquakes. Earthquakes are classified as extreme destruction source of nature because an exact warning can’t be extracted about earthquakes. Even sometimes earthquakes can hit a region in a series. In this situation one new earthquake gets triggered due to displacement, shaking or stress induction by previous strike of earthquake. This situation is called earthquake storm. This is extremely detrimental for human life and property.

Impacts of earthquakes

Impacts of earthquakes can be classified into various classes-

Ground rupture- It is the main result of an earthquake strike. Shaking of ground causes severe damage to the buildings or other structures on the ground including houses etc. Shaking of ground at a particular place depends upon the distance of that place from the epicenter. Severe shaking of ground causes destruction of all the buildings of a city and many people die by burring into the building materials.

It can’t be avoided because earthquakes strike a place all of a sudden. So this shaking of ground compels construction engineering to develop buildings which are resistive to the strikes of earthquakes. The branch of engineering deals which deals with anti-earthquake construction is called Earthquake construction.

Landslides- Earthquakes causes instability of land results into landslides; this claims many lives at the effected zone.

Fire- Earthquake causes breaking of electrical power lines or gas supply lines which causes incidents of fires. Water lines also got ruptured and decreased pressure makes it impossible to control the spread of fire. In earthquake of San Francisco in 1906, more deaths happened because of fire as compared to earthquake itself.

Soil liquefaction- When severe shaking occur then soil or sand loses their strength for a temporary period and gets converted from solid to liquid. This liquefaction causes sinking of buildings, bridges etc.

Tsunami and floods- when epicenter of an earthquake is located near sea, then the traveling of seismic waves below the sea causes generation of Tsunami waves, which can travel at a speed of 600-800 kilometers per hour. For creation of Tsunami earthquakes are needed to be highly intensified. In general earthquakes havingreading of less then 7.5 on the Richter scale are not able to generate Tsunami waves. But stronger earthquakes can are comfortable enough to create Tsunamis. Sometimes when earthquake triggered landslides fall into sea, it leads to the creation of Tsunami waves.

Earthquake Construction

It is defined as the branch of the modern architectural engineering, having major concern of making structures which can withstand a severe earthquake shock. This branch of engineering is earning major concerns by countries like Japan, Chine, Turkey, Algeria and other countries near boundaries of tectonic plates. Because these areas of world are much more susceptible to strikes of earthquakes and major steps are necessary to avoid the destruction of human life and infrastructure.

Earthquakes Construction Techniques

Japan is first country of world to develop special techniques of earthquake construction, but now a days these techniques are common in other countries also. These days a construction is believed to be incomplete if it’s not properly entitled with earthquake precautions. In earlier days we were not available with modern facilities of engineering and technology. So during those times engineers construct a structure with a hope and wait for the strike of earthquake to test their precautionary measures of construction. But these days we are now available with the shake-tables, affixed with large motor utilities and computer monitoring techniques. This helps us to develop precise models which can be named as anti-earthquake construction models. Modern construction materials like reinforced concrete and concrete can also help but a special set of guidelines is necessary to do a construction with these flexible materials.

It has been observed that these materials are not comfortable to withstand with lateral forces or stresses caused by earthquakes. So steel reinforcing bar called rebar should also be used in constructions. All the joints i.e. where columns meet with the beams should be carefully tied using rebar. Brick infill should be avoided. Concrete of good quality should be used in a construction and anchor bolts should be used for providing strength to the construction. In spite of construction guidelines different countries issue a different set of rule book for the construction in that particular country. This rule book is according to distance of the country from the boundaries of tectonic plates. These rule books restrict construction in that zones which are very much prone to the earthquake strikes.
Moreover a residential structure must be attached to the foundation of the structure by means of the bolts to restrict the sliding of the main foundation during shakings. As earthquake engineering is developing day by day, more prominent structures are available to the human populations which are safe and efficient in severe earthquake strikes. Research in the field is still going on to develop 100 % efficient guidelines for anti-earthquake constructions.

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