Monday, December 7, 2009

Earthquake Facts: 4 Facts on Earthquakes

Earthquake history can be traced back to 1177 B.C. There are several Earthquake facts that run a chill down the spine. The Earthquake damage varies as per the conditions. Earthquake prediction is not very reliable.
A sudden release of energy in the earth's crust causes seismic waves and results in an earthquake. This is caused due to rupture of geological faults, landslides, volcanic activity, mine blasts and nuclear experiments. Thus, a natural phenomenon or human-caused event creates seismic waves.

Earthquake facts

The region below the earth's surface where the rupture of the fault starts is called as a 'hypocenter'. The point directly above the hypocenter on the earth's surface is called as the 'epicenter'. The calculated value of the earthquake size is called as the 'magnitude'. The measure of the shaking due to the earthquake is called as the 'intensity'. Earthquakes do occur in cold, hot or rainy climate and so, the concept of 'earthquake weather' has not been established. A majority of earthquakes have been recorded at a depth of less than 80 km i.e. 50 miles below the earth's surface. The deepest earthquakes have taken place at plate boundaries where the earth's crust is subducted into the mantle. These take place 750 km below the surface. Most of the earthquakes take place along plate boundaries. For example, the boundary between the Pacific plate and the North American plate or the huge Pacific plate. As earthquake waves have a frequency less than 20 Hz, these cannot be detected by humans. The world is subjected to 500,000 detectable earthquakes per year. Of these, 100,000 can be felt, while 100 cause actual destruction. The largest recorded earthquake in the world took place in Chile on May 22, 1960. The magnitude of this was 9.5 (Mw). In the United States, the largest recorded earthquake took place on Prince William Sound, Alaska on March 28, 1964. The magnitude of this was 9.2. The deadliest earthquake in the world hit central China in 1556. About, 830,000 people were killed. Another deadly earthquake hit Tangshan, China in 1976. About, 250,000 people were killed.

Earthquake damage

The effects of an earthquake are most prominent in a wide region near the epicenter. Faults that reach the surface cause surface ground cracking. Vertical and horizontal displacements of several yards are commonplace. Microearthquakes have slight periodic movements called fault creep. The nature of the ground decides the range of earthquake vibration and consequent destruction of a region. In unconsolidated surface material like river deposits or poorly compacted fill, the vibration have more amplitude and last longer. The bedrock areas are less susceptible. Structures in densely populated urban areas cannot bear tremendous shaking and have to suffer immense damage. L waves can give rise to damaging vibrations in buildings and other structures and this may lead to uncontrollable fire.

Earthquake history

In China, an earthquake has been recorded in 1177 B.C. In Europe, 580 B.C. was the year when an earthquake was felt. In North America, New Madrid, Missouri was hit by a great earthquake in 1811-1812. It has been guessed that this measured 8 on the Richter scale. A very demolishing earthquake took place in the United States at San Francisco in 1906. More than 700 people were killed in this. In 1964, a doubly potent earthquake hit Alaska. This has less casualties owing to low population density.

Earthquake prediction

The above term means a conjecture that an earthquake of a particular magnitude would affect a particular region at a specific time. There is some proof that earthquakes can be predicted with good accuracy of time and space. However, the prediction techniques are not found reliable and reproducible. So, these are not accepted by seismologists. People try to link the probability of an earthquake with the aid of seismicity patterns, electromagnetic fields, irregular clouds, animal demeanor, water level in wells and hydrogen gas content of ground water. An earthquake prediction has to include the area, magnitude, time window, physical basis and estimate of probability.

Earthquake maps that indicate the average number of earthquakes per year per 12,300 square kilometer of a particular magnitude are drawn. Maps are classified as per varying depths of the earth's surface.

By Abhay Burande


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