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Earthquake (continued) Many smaller events occur scattered throughout the Japanese islands, caused by the overall compression of the whole region. Although these small events are energetically minor when compared to the great offshore earthquakes, they are often more destructive, owing to their greater proximity to popula- tion centers. Although most earthquakes occur on or near plate boundaries, some also occur, infrequently, within plates. These earthquakes, which are not related to plate boundaries, are called intraplate earthquakes, and can sometimes be quite destructive. The immediate cause of intraplate earthquakes is not understood. Some of them can be quite large. Three of the largest earth- quakes known to have occurred in the United States were part of a sequence of intraplate earthquakes which took place in the Mississippi Valley, near New Madrid, Missouri, in 1811 and 1812. Another intraplate earthquake, in 1886, caused moderate dam- age to Charleston, South Carolina. In addition to the tectonic types of earthquakes described above, some earthquakes are directly associated with volcanic activity. These volcanic earthquakes result from the motion of underground magma that leads to volcanic eruptions. Sequences Earthquakes often occur in well-defined sequences in time. Tectonic earthquakes are often preceded, by a few days to weeks, by several smaller shocks (foreshocks), and are nearly always followed by large numbers of aftershocks. Foreshocks and aftershocks are usually much smaller than the main shock. Volcanic earthquakes often occur in flurries of activity, with no discernible main shock. This type of sequence is called a swarm. Size Earthquakes range enormously in size, from tremors in which slippage of a few tenths of an inch occurs on a few feet of fault, to the greatest events, which may involve a rupture many hundreds of miles long, with tens of feet of slip. Accelerations exceeding 1 g (acceleration due to gravity) can occur during an earthquake. The velocity at which the two sides of the fault move during an earthquake is only 1–10 mi/h (10–100 cm/s), but the rupture front spreads along the fault at a velocity of nearly 5000 mi/h (3 km/s). The earthquake's primary damage is due to the generated seismic waves, or sound waves which travel through the Earth, excited by the rapid movement of the earthquake. The energy radiated as seismic waves during a large earthquake can be as great as 1012 cal (4.19 × 1012 joules), and the power emitted during the few hundred seconds of movement as great as a billion megawatts. The size of an earthquake is given by its moment: average slip times the fault area that slipped times the elastic constant of the Earth. The units of seismic moment are dyne-centimeters. An older measure of earthquake size is magnitude, which is proportional to the logarithm of moment. Magnitude 2.0 is about the smallest tremor that can be felt. Most destructive earthquakes are greater than magnitude 6; the largest shock known was the 1960 Chile earthquake, with a moment of 1030 dyne-centimeters (1023 newton-meters) or magnitude 9.5. It involved a fault 600 mi (1000 km) long slipping 30 ft (10 m). The magnitude scale is logarithmic, so that a magnitude 7 shock is about 30 times more energetic than one of magnitude 6, and 30 × 30, or 900 times, more energetic than one of magnitude 5. Because of this great increase in size with magnitude, only the largest events (greater than magnitude 8) significantly contrib- ute to plate movements. The smaller events occur much more often but are almost incidental to the process. The intensity of an earthquake is a measure of the severity of shaking and its attendant damage at a point on the surface of the Earth. The same earthquake may therefore have different intensities at different places. The intensity usually decreases away from the epicenter (the point on the surface directly above the onset of the earthquake), but its value depends on many factors and generally increases with moment. Intensity is usually higher in areas with thick alluvial cover or landfill than in areas of shallow soil or bare rock. Poor building construc- tion leads to high intensity ratings because the damage to structures is high. Intensity is therefore more a measure of the earthquake's effect on humans than an innate property of the earthquake. Effects Earthquake shaking produces many different effects. Although the fault motion that produced the earthquake is sometimes observed at the surface, often other earth movements, such as landslides, are triggered by earthquakes. On rare occasions the ground has been observed to undulate in a wavelike manner, and cracks and fissures often form in soil. The flow of springs and rivers may be altered, and the compression of aquifers sometimes causes water to spout from the ground in fountains. Undersea earthquakes often generate very long-wavelength water waves, which are sometimes called tidal waves but are more properly called seismic sea waves, or tsunami, which can be extremely damaging to coastal cities and ports. Prediction Earthquake prediction research has been going on for nearly a century. A successful prediction, specifying the time, location, + ward ' s science