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What is an 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.  At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground.  Any activity within the earth that creates vibrations can be called an earthquake, whether it's a natural phenomenon or an event caused by humans — something that generates seismic waves.  Earthquakes are caused mostly by the rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.  Earthquakes send waves through the earth and this means that places many miles from the centre of an earthquake can be affected by these waves.

Measuring an Earthquake

The size of an earthquake can be measured with a seismometer, also known as a seismograph.  The measurements can be recorded using the Richter scale or the Modified Mercalli scale.  These methods use numbers to represent the size of earthquakes, from 1 to 10.

Earthquakes can be recorded by seismometers up to great distances, because seismic waves travel through the whole Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas), whereas the felt magnitude is reported using the modified Mercalli scale (intensity II-XII).

Seismic stations watch for earthquakes, the number of stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, mainly because of the vast improvement in instrumentation, rather than any increase in the number of earthquakes.

Charles F. Richter, former President of the Seismological Society of America and the originator of the Richter scale, said in an article published in the December 1969 issue of Natural History magazine: "One notices with some amusement that certain religious groups have picked this rather unfortunate time to insist that the number of earthquakes is increasing. In part they are misled by the increasing number of small earthquakes that are being catalogued and listed by newer, more sensitive stations throughout the world".

In order to determine the likelihood of future seismic activity, geologists and other scientists examine the rock of an area to determine if it appears to be "strained". Studying the faults of an area to study the build-up time (the it takes for the fault to build up stress sufficient for an earthquake) also serves as an effective prediction technique.  Measurements of the amount of pressure which collocates on the fault line each year, time passed since the last major temblor, and the energy and power of the last earthquake are made.  Together the facts allow scientists to determine how much pressure it takes for the fault to generate an earthquake. Though this method is useful, it has only been implemented on California's San Andreas Fault.

What Causes an Earthquake?

The surface of the Earth is divided into pieces called "tectonic plates". These plates move, and where tectonic plates meet it is called a fault. When the plates rub against each other along a fault, they usually do so smoothly, but when these plates do not move smoothly, earthquakes result.  In some parts of the world earthquakes occur near the edges of these plates.

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip.  Normal faults occur mainly in areas where the crust of the earth is extending, reverse faults in areas where the crust is shortening. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other.  Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as an oblique slip.

Earthquakes can also be induced.   Four main activities contribute: constructing large dams and buildings, drilling and injecting liquid into wells, and coal mining or oil drilling.  Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan Province; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.  The largest earthquake in Australia's history was also induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas.  The earthquake was spawned from a fault which reactivated due to the millions of tonnes of rock removed in the mining process.

The Size and Frequency of Earthquakes

Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.  Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5.

The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.  In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend.

Where do Earthquakes Occur?

The distribution of earthquakes around the Earth's surface is called seismic activity.  Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate.   Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.

In the United Kingdom, there is low seismic activity it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years.

Aftershocks and Fore Shocks

An aftershock is an earthquake that occurs after a previous earthquake, the main shock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is re-designated as the main shock and the original main shock is re-designated as a fore shock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.

Earthquake clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time.  These earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.

Earthquake swarms

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 seismic activity at Yellowstone National Park.

Earthquake storms

Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.

Effects of Earthquakes

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures.

Ground rupture is a visible breaking and displacement of the Earth's surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.

Earthquakes, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.

Earthquakes can also cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake.

Soil liquefaction can also occur when water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink.  For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.

Earthquakes can cause tsunamis that can travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.  Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.

Flooding can be a secondary effect of earthquakes which may cause dams to collapse, or landslips that dam rivers.

Other secondary effects of earthquakes are disease, lack of basic necessities, higher insurance premiums, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings.

Earthquakes can also precede volcanic eruptions, which cause further problems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).

Earthquake-proofing

Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: earthquake engineering, earthquake preparedness, household seismic safety, seismic retrofit (including special fasteners, materials, and techniques), seismic hazard, mitigation of seismic motion, and earthquake prediction.

Some countries, such as Japan or parts of a country like California in the United States, experience a lot of earthquakes.  In these places it is a good practice to build houses and other buildings so they will not collapse when there is a quake. This is called seismic design or "earthquake-proofing".

The ability of a building to withstand the stress of an earthquake depends upon its type of construction; shape, mass distribution, and rigidity. Different combinations are used. Different shapes of buildings such as square, rectangular, and shell buildings can withstand earthquakes far better than skyscrapers. To reduce stress, a building's ground floor can be supported by extremely rigid, hollow columns, while the rest of the building is supported by flexible columns located inside the hollow columns. A different method is to use rollers or rubber pads to separate the base columns from the ground, allowing the columns to shake parallel during an earthquake.

To help prevent a roof from collapsing it can be made out of light-weight materials. Outdoor walls can be made with stronger and more reinforced materials such as steel or reinforced concrete. During an earthquake flexible windows can help as they hold the windows together so that the glass doesn't shatter.

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.

Earthquake Beliefs, Myths and Legends

In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he would strike the ground with a trident, causing this and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.  But the Greeks did attempt to explain quakes in a more scientific way: From the time of the Greek philosopher Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to "air (vapours) in the cavities of the Earth".  Thales of Miletus, who lived from 625-547 (BCE) was the only documented person who believed that earthquakes were caused by tension between the earth and water.  Other theories existed, including the Greek philosopher Anaxamines (585-526 BCE), who believed that short incline episodes of dryness and wetness caused seismic activity. The Greek philosopher Democritus (460-371BCE) blamed water in general for earthquakes. And Pliny the Elder called earthquakes "underground thunderstorms".

In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison would drip on Loki's face, forcing him to jerk his head away and thrash against his bonds, causing the earth to tremble.

In Japanese mythology it is Namazu, a giant catfish, who causes earthquakes. Namazu lives in the mud beneath the earth, and is guarded by the god Kashima who restrains the fish with a stone. When Kashima lets his guard fall, Namazu thrashes about, causing violent earthquakes.

I have even heard somewhere that in the Koran it states that mountains were set on the earth so that the earth would not shake, and that an increase in earthquakes was a sign of the impending Armageddon, and in the Bible it says, "And there were voices, and thunders, and lightnings, and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake. and so great".  (Revelation 16-17)

Earthquakes in Popular Culture

In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.  Fictional earthquakes tend to strike suddenly and without warning.  For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998).  A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection, After the Quake, depicts the consequences of the Kobe earthquake of 1995.

The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works.   Jacob M. Appel's widely-anthologized short story, A Comparative Seismology, features a con artist who convinces an elderly woman that an apocalyptic earthquake is imminent.  In Pleasure Boating in Lituya Bay, one of the stories in Jim Shepard's Like You'd Understand, Anyway, the "Big One" leads to an even more devastating tsunami.

In the film 2012 (2009), solar flares (geologically implausibly) affecting the earth's core caused massive destabilization of the earth's crust layers. This creates destruction planet-wide with earthquakes and tsunamis, supposedly foreseen by the Mayan culture and a myth surrounding the last year noted in the Mesoamerican calendar - 2012.

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