Earthquakes

"An earthquake is a sudden shaking of the ground. They generate sismic waves which can be recorded on a sensitive instrument called a seismogragh Today's high-technology, digital seismographs record ground shaking over a larfe band of frequencies and seismic amplitudes, today's seismometers are called broadband because they are able to sense ground motion over a wide range of frequencies. In the Beginning of the 1960s, significant strides were made in the study of earthquakes and the Earth's structure with the deployment of the World-Wide Seismographic Station Network (WWSSN). This network consists of over 120 seismographs in 60 countries."(1)

Most earthquakes tectonic and occour when the frictional stress of gliding plates.(2)

"When people think of earthquakes, they often picture the ground cracking open, roads and bridges buckling and breaking apart, buildings collapsing, and people being injured or killed. Such disasters frequently result from earthquakes, although many earthquakes are so small that people can barely feel them. What causes earthquakes? Why are some earthquakes worse than others? These and many other questions have been answered by //**seismologists**//, the scientists who study shaking motions of the Earth. 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. An earthquake occurs because of geologic forces inside the Earth. These forces build up slowly and eventually become so strong that they cause rocks to break underground. When this happens, tremendous energy is released suddenly in the form of motion that spreads out in all directions from the break, causing the ground to shake and move. This sudden release of energy and movement is what makes an earthquake so destructive. When Earth was first formed, radioactive materials, such as uranium, potassium, and thorium, began to decay naturally in the planet's interior. This process of radioactive decay slowly heated up the rocky material deep inside the planet. Since heat attempts to rise, this heated material has been moving slowly toward the Earth's surface. The movement is very slow — less than an inch or so each year. But over hundreds of millions of years, this slow movement adds up to distances of thousands of miles. The sudden release of energy from an earthquake sends out several different shaking movements, or seismic waves. The word "seismic" comes from a Greek word for "shaking." Some of these seismic waves travel over the surface of the earth and are called surface waves. Others, called body waves, travel down through the earth's deep interior before returning to the surface. Thousands of seismometers are now operating at stations all around the world. When an earthquake occurs somewhere on Earth, many of these will record the motions of the sound waves, shear waves, and surface waves that are produced. By measuring the size of these waves and the times at which they reach each seismic station, seismologists can determine where the earthquake occurred and how large it was. More than a hundred years ago, when people began studying earthquakes scientifically, they needed a practical way of describing the strength of an earthquake's shaking motions. They began by describing the pattern of damage to buildings and making maps to show different levels of damage at different places. In the 1930s, the American seismologist Charles Richter studied thousands of seismograms of earthquakes that had occurred in southern California. He realized that it would be useful to have a numerical scale for comparing the size of earthquakes that went beyond describing them as just large or small. Richter knew that he would have to take two things into account in devising such a scale: the distance from the epicenter and the great difference in size of ground motion between small and large earthquakes"(3) There are three kinds of faults, all of which move in different ways to create an earthquake. At a //normal fault,// one side of the fault drops below the other side. At a //thrust fault,// one side moves up and over the other. At a //horizontal,// or //slip-strike, fault,// one side moves sideways past the other.(4)
 * What happens when an earthquake starts?**
 * How do you locate an earthquake?**
 * Intensity**
 * Magnitude**

An earthquake is one of the most terrifying phenomena that nature can whip up. We generally think of the ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception instantly, and often with extreme violence. (5)

(1) [|http://www.thetech.org/exhibits/online/quakes/] (2) [|http://www.livescience.com/earthquakes/] (3) [|http://www.ask.com/bar?q=what=are=tectonics&page] (4) [|http://www.worldbook.com] (5) []