Definition (Encyclopedia of Global Resources)
Earthquakes occur following the rapid release of energy stored in rocks. Rocks beneath the Earth’s surface are continually subjected to forces in all directions. When the forces exceed the limits which the rocks can sustain, they respond by either folding or breaking. If the forces are relatively rapid and the rocks are brittle, then the rocks actually break. The result is a shaking of the ground. This shaking is most prominent on the Earth’s surface.
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Overview (Encyclopedia of Global Resources)
An earthquake first originates at a point called the focus, which is beneath the Earth’s surface. This fracture, which begins at a point, grows from a microscopic crack into a large fault which can extend for many kilometers. However, as mentioned, this fracture will be propagated only through brittle material. In other words, faults will not extend indefinitely into the Earth’s subsurface. Nor will they extend indefinitely through brittle material, because there will be a point where there is insufficient energy remaining to break rock far removed from the initial source of a fracture. Focal depths of earthquakes occur over a range of depths, extending from just below the Earth’s surface to a depth of approximately 700 kilometers. Below this great depth rocks are no longer brittle and thus cannot break.
In addition to the more obvious effects of seismic activity on the surface, earthquakes cause a considerable amount of subsurface activity. Seismic energy passing through brittle rock produces faults and cracks of varying sizes throughout the rock. These fissures serve as conduits for fluids, which can move through the rock much more readily than they could before the rock was broken. If the fluids contain dissolved minerals, these will be deposited in concentrated amounts. Such is the case when molten rock rises below the surface and is injected into cracks. Concentrated deposits of gold, silver, and other valuable metals are...
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Definition (Encyclopedia of Global Warming)
An earthquake is a shaking or trembling of the ground caused by energy coming from volcanic eruptions or from rocks breaking during the geologic process known as faulting. This process takes place as a result of differential movements of segments of the Earth’s crust known as plates. The plate tectonics theory has identified seven major crustal plates, as well as several minor plates, which are thought to be colliding with one another, separating from one another, or sliding past one another like books on a tabletop. The sudden movements of these plates, as they separate, collide, or slide past one another, are believed to cause most of the planet’s earthquakes.
Volcanoes can cause earthquakes too, but these earthquakes are generally minor and occur only during periods of heightened volcanic activity. Some additional causes of earthquakes include landslides that drop large quantities of earth suddenly on underlying surfaces, and human activities, such as blasting during surface or underground mining operations. The shock waves from underground nuclear tests can generate vibrations similar to earthquakes as well.
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Significance for Climate Change (Encyclopedia of Global Warming)
Concerns have been raised that climate change may lead to earthquakes in Alaska, or elsewhere, and these concerns may not be entirely unwarranted. Many glaciers and ice sheets are located in regions of past or continuing geologic activity, where crustal movements are still taking place. Removal of ice cover during periods of global warming might be sufficient to activate ancient faults in these areas and set off earthquakes of considerable magnitude. The rate at which the ice melts may not need to be rapid to cause such activity. Rocks can build up strains over a period of years without breaking at all, until the tension is suddenly released in a devastating shock. Scientists describe this process as elastic rebound. While the rocks on either side of a fault slowly slide past each other, the fault itself may remain “locked,” with no movement taking place. Meanwhile, the rock masses on both sides of the fault begin to stretch, in a fashion similar to the stretching of an elastic band. When the strain in the rocks becomes too great, the fault will break, and the rocks on either side will snap back to their original dimensions.
There is clear evidence that the removal of ice from the Earth’s surface following the ice ages is causing significant upward movement of the crust in several places. In parts of Scotland, Scandinavia, and Canada, the amount of rise has been dramatic. Interior Scandinavia has risen...
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Further Reading (Encyclopedia of Global Warming)
Anderson, David E., Andrew S. Gaudie, and Adrian G. Parker. Global Environments Through the Quaternary: Exploring Environmental Change. New York: Oxford University Press, 2007. Analysis of environmental changes during the last two million years of Earth history, with particular attention to the post-glacial uplift of Scotland, Scandinavia, and North America.
Hough, Susan Elizabeth. Earthshaking Science: What We Know and Don’t Know About Earthquakes. Princeton, N.J.: Princeton University Press, 2002. Excellent basic description of how the various kinds of plate movements can cause earthquakes and a helpful explanation of the elastic rebound theory.
Zeilinga de Boer, Jelle, and Donald Theodore Sanders. Earthquakes in Human History: The Far-Reaching Effects of Seismic Disruptions. Princeton, N.J.: Princeton University Press, 2005. Summarizes ways in which earthquakes affect human lives; includes an important section on “induced earthquakes,” which are caused when weight is added to the Earth’s crust.
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Earthquakes (Science Experiments)
Developing a theory
Detecting an Earthquake: How can movement of Earth's crust be measured?
Design Your Own Experiment
According to the ancient Greeks, occurred when the god Atlas shifted the weight of the world from one shoulder to the other. Other cultures believed that earthquakes were a sign of punishment. We now know that earthquakes are the shaking or trembling of the earth caused by underground shock waves or vibrations. Believe it or not, over a million earthquakes take place each year. Sometimes the trembling and shaking is gentle and hardly noticeable. Other times the motion is much more violent, causing cracks in the surface of the earth.
Huge blocks of rocks called make up Earth's outer shell, or crust. These plates fit together like a cracked egg shell. The plates push and pull on each other constantly. Sometimes this pressure causes a faultA crack running through rock as the result of tectonic forces., or a break in the rocks. Large pieces of these rocks, called , can...
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