What is earthquake? Full details on earthquake

 What is earthquake? 

An earthquake is simply understood as shaking of the Earth's surface caused by rapid movements of the Earth's rocky outer layer. An earthquake (also known as a quake, tremor or temblor) takes place because of a sudden release of energy in the Earth's crust that creates seismic waves. The study of earthquakes and the waves they create is called seismology (derived from the Greek word 'seismos' that means, 'to shake') and seismologists are those scientists who study earthquakes. Earthquakes, or seismic tremors, go on occurring at a rate of several thousands per day around the world. A worldwide network of seismographs (machine that record movement around the Earth) detects about 1 million small earthquakes per year and thousands of them everyday. But only the earthquakes measuring more than 4 in magnitude on the Richter scale and experienced by us. the measurement of seismic waves with seismograph in a pattern of graphic show on two-dimensional paper is called the seismograph. 

In the last 500 years, earthquakes around the world have killed several million people . For example, over, 270,000 people died in the 2004 Sumatra earthquake (of 9.15 magnitude), more than 87,000 people died in the 2008 China earthquake (of 7.8 magnitude) and 2,30,000 people died in the 2010 Haiti earthquake (of 7 magnitude). Chile has a dubious distinction of being the country of high seismicity and this country already experienced the largest earthquake (9.5 magnitudes) in 1960 ever recorded on earth. Nepal also is an earthquake-prone country. 

The earthquake prone areas in the world: The most important earthquake belt is the circum-pacific  belt, which includes coastal region around the Pacific Ocean- for example, New Zealand, New Guinea, Japan, the Aleutian islands, Alaska, and the western coasts of North and South America. It is estimated that 80 percent of the energy presently released in earthquakes comes from those whose epicenters are in this belt. Therefore, a belt earns the sobriquet of "Pacific Ring of Fire". A second belt known as the Alpide belt passes through a Mediterranean region eastward through Asia covering Himalayan mountain system Indo-China join ultimately the Circum-Pacific Belt in the East Indies. The energy released in earthquakes from this belt is about 15 percent of the world total. 

Causes of an Earthquake 

An earthquake is a naturally originating sudden tremor or movement of the earth's  crust. There are two main causes of earthquakes: 

Firstly, earthquake caused by volcanic eruptions, they are very common in areas of volcanic activity (like Asia Pacific Regions also known as "Ring of Fire") where they either precede or accompany or succeed eruptions. When extraordinary levels of pressure develop due to deposition of acidic lava on the mouth of the volcano, the volcanic explosions is of devastating nature producing an earthquake of destructive magnitude. When Krakatoa (Indonesia, between Java and Sumatra) exploded in 1883, the explosion was heard over 5000 km Away in Australia and the shockwaves produced a series of tsunami (large waves),one of which was over 36m high killing over 36,000 people. On the other hand, volcanoes producing free flowing basic lava do not cause earthquakes because the vent remains open allowing the release of pressure all freely and constantly. Since pressure does not accumulate inside the volcano, violent explosions do not happen and therefore no earthquakes takes place. 

Secondly, earthquakes cause by 'plate-tectonic activity'; the majority of earthquakes worldwide are of its type. Tectonic earthquakes take place when the crust accumulates "strain" and eventually moves. The earth has a rigid outer layer, known as lithosphere having a thickness of about 100km (60 miles). This lithosphere is broken up into about dozen large and several small plates. Himalayan Mountain-range made up of both Indian Plate and Tibetan Plate. According to geologists, Indian and Euro-Asian plates comes in motion. Euro-Asian plate is also known as the Tibetan plate. These plates move relative to each other, typically at rates of 5 to 10cm (2 to 4 inches) per year, and interact at their boundaries, where they either inverge or diverge or slip past one another. Such interactions are responsible for most of Earth's seismic and volcanic activity, although earthquakes and volcanoes are not wholly absent in plate interiors. 

Hypocenter and Epicenter 

An earthquake is the effect just as concentric waves created after a stone falls into the water. Imagine a situation when a stone is thrown into the water. After the stone hits the water level, a series of concentric wave as mentioned either moves upwards from the center. Exactly the same events occur in an every earthquake when a sudden movement within the curst or mantle creates concentric shock waves moving out from that point. Geologists and geographers call this point as origin of the earthquake - the focus or the hypocenter. Since this is often deep below the surface and difficult to map, the location of the earthquake is fixed at a point of the Earth surface directly above (or vertically above) the focus i.e. the hypocenter. This point located on the earth surface lying vertically above the hypocenter (actual origin of the earthquake below the earth surface) is called the epicenter. 

Effects of earthquake: 

A series of earthquakes with a 5.6 magnitude collapsed houses and triggered landslides in a mountainous part of south-western China on September 7, 2012, killing at least 50 people and damage caused by this earthquake was preventing rescuers from reaching some outlying areas, and communications were disturbed. There are several effects of earthquake whic are mentioned below: 

1. Shaking, and Ground Rupture 
2. Landslides and avalanche 
3. Fire, flood and Diseases 
4. Solid liquefaction 
5. Tsunamis etc.