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Earthquakes
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Earthquakes are the shaking, rolling or sudden shock of the earth’s surface. They are the Earth's natural means of releasing stress. More than a million earthquakes rattle the world each year. The West Coast is most at risk of having an earthquake, but earthquakes can happen in the Midwest and along the East Coast. Earthquakes can be felt over large areas although they usually last less than one minute. Earthquakes cannot be predicted - although scientists are working on it!
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Earthquakes are natural events that occur as part of the geological processes that form the earth's mountains, oceans, valleys, and plains. Over the last 100 years or so, scientists have learned a great deal about these processes, including earthquake's causes, how to measure them, and where they occur. Through studying earthquakes, scientists have learned much about the earth itself.
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Earthquakes can cause buildings and bridges to collapse, disrupt gas, electric and phone lines, and often cause landslides, flash floods, fires, avalanches, and tsunamis. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and are followed by vibrations of gradually diminishing force called aftershocks. The underground point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is measured by the Richter scale and the Mercalli scale.
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Earthquakes are caused by the movement of the earth's tectonic plates. Earthquakes occur where the earth's plates meet along plate boundaries (see plate tectonics page for more infomration on this).For example as two plates move towards each other, one can be pushed down under the other one into the mantle. If this plate gets stuck it causes a lot of pressure on surrounding rocks. When this pressure is released it produces shock waves. These are called seismic waves. This is an earthquake.
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Earthquakes in the Northeast U.S. cannot be associated with specific known faults, as opposed to the typical seismic activity evident in California. While California earthquakes typically occur at, or near the conjunction of two of the Earth's major tectonic plates ("inter-plate" activity), earthquakes in New England are far from plate boundaries. These earthquakes are referred to as "intra-plate" activity.
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Earthquakes tend to be concentrated in narrow zones. There are 7 major crustal plates on earth, about 80 km (50 miles) thick, all in constant motion relative to one another. They move at between 10 and 130 mm (from less than one half to 5 inches) per year.
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