|
Events that occur at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes. The Richter scale has been invented by a California seismologist named Charles F. Richter who devised in 1930 a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes.
Earthquakes occur basically every day on Earth, but the majority of them are minor and cause no damage (magnitude 5). Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage.
Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively.
The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. It is usually possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" and usually proves to be the point at which the fault slip was initiated.
The position of the focus is known as the "hypocenter" and the location on the surface directly above it is the "epicenter". The fault may slip well beyond its epicenter, though. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are.
Earthquakes, especially those that occur beneath oceans or seas, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.
There are four types of seismic waves that are all generated simultaneously. They arrive in the following order: first the body waves P-waves (primary or pressure waves) then S-waves (secondary or shear waves), next the surface waves (Love waves) then Rayleigh waves.
Causes
Most earthquakes are powered by the release of the stresses that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere. The most severe of these earthquakes are located along compressional and translational plate boundaries. Deep focus earthquakes are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition at depths greater than 600 km. Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Finally, earthquakes (in a broad sense) can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity. Another possibility to explain earthquakes is related to gas movement in the earth's interior, mainly methane.
Major earthquakes
The World Almanac tells us that there were only 21 earthquakes of major strength between the years 1000 and 1800. But between 1800 and 1900 there were 18 major earthquakes. In the next 50 years, between 1900 and 1950, there were 33 major quakes, and between 1950 and 1991 there were 93 major earthquakes, almost tripling the number of the previous half century, and claiming the lives of 1.3 million people around the world.
January 17, 1994, an earthquake measuring 6.6 on the Richter scale causes $30 billion worth of damage in Los Angeles. Seismologists predict that California's imminent "big one" could be 50 times more powerful.
January 17, 1995, Over 5000 people are killed and 26,000 injured in the Great Hanshin quake in Japan. The port city of Kobe is devasted, its infrastructure destroyed, and over 300,000 people made homeless. Japanese scientists at the Earthquake center in Tsukuba predict a much stronger quake measuring over 8 on the richter scale could hit the Tokyo area in the very near future.
List of past earthquakes
http://en.wikipedia.org/wiki/List_of_earthquakes
|
