Earthquakes

Earthquakes are sudden disturbances within the Earth’s outer layer, the crust manifested at the surface by a shaking of the ground. The shaking is what causes the destructiveness of an earthquake. There are six kinds of known shock waves that cause earthquakes.

Earthquakes are caused by the sudden release of energy within some regions of rocks on the earth. The energy involved in the earthquake depends on the type of earthquake. The four kinds of energy are elastic strain, gravitational potential, chemical reactions, and motion of bodies. The release of elastic strain, is the most important, though. It is the only kind capable of storing enough energy to create a major earthquake. This is called a tectonic earthquake.

Earthquakes can also be formed by human activities. Such as injecting fluids into deep wells, filling in new reservoirs, the underground detonation of atomic explosives, and the excavation of mines. One example of this was in the city of Denver in 1962. They had begun to experience earthquakes for the first time in its history. It was then discovered that the quakes had occurred during the same time as waste fluids had been pumped into deep wells in an arsenal in the city. Shortly after the pumping stopped the earthquakes stopped.

A third kind of earthquake can take place. It is called a volcanic earthquake. It usually occurs before a volcano is about to erupt. They occur when magma fills the chambers beneath the volcano, and when the chambers swell the rupture it can cause lots of small earthquakes. On the big island of Hawaii seismographs can register up to 1000 earthquakes a day before a major eruption.

What happens during earthquakes?

I have already told you that what causes and earthquake, but what happens during a powerful earthquake is pure horror. During an earthquake with a Richter scale power of 7 and up, many structures will collapse, including apartment buildings and offices with many people in them. Also the ground will be badly cracked with gaping holes. For those who have experienced earthquakes (not me) I’m sure it is hard to describe.

What happens after an earthquake?

Moving around after and earthquake can be very dangerous. There may be many aftershocks, and they should be expected. In any major earthquake, the fault rupture does not relieve all of the accumulated strain at once. In some cases the frequency of aftershocks after a major earthquake may be 1,000 a day. In rare occasions, a major earthquake will be followed by another one at the same focus within the same day, such as in multiple earthquakes. In most cases, though, the first earthquake will be the strongest, and the number of aftershocks per day decreases in time. Also, the number of aftershocks is roughly inversely proportional to the time since the occurrence of the largest earthquake of the series.

What are foreshocks and earthquake swarms?

Foreshocks are small earthquakes that occur prior to a larger one, as in aftershocks occur prior to one. Most earthquakes do not have foreshocks, but there are in a few exceptions.

A tremendously large number of earthquakes that occur during a relatively brief period of time are called earthquake swarms. One example of an earthquake swarm was in Japan between August of 1965 and 1967, when there were hundreds of thousands of earthquakes. Some of which were of moderate size (up to a magnitude of 5). On one day alone, April 17, 1966, there were 6,780 small earthquakes.

 The earthquake scales: Mercalli and Richter

There are two commonly used scales in the measuring of earthquakes. The two are the Mercalli scale, introduced by Italian seismologist Giuseppe Mercalli at the turn of the 20^th century, and the Richter scale, introduce by American seismologist Charles Francis Richter. The Mercalli scale measures intensity of and earthquake and is based on eyewitness accounts of damage. On the other hand, the Richter scale is used to measure magnitude, the energy released at the focus of the quake. The Richter scale is a logarithmic one. That is, a category 5 quake is 10 times as powerful as a category 4 quake, 100 times as powerful as a 3 quake, and 1,000 times as powerful as a category 2.

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