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Earthquakes are considered one of the most deadly natural catastrophes that can affect human life.

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Earthquakes Earthquakes are considered one of the most deadly natural catastrophes that can affect human life. Most often, a quake occurs in earthquake-prone zones where two tectonic plates meet, split, or slip by one another; the type of plate contact determines whether the earthquake will be shallow or deep. During the movement of these plates, intense forces overcome the friction between the plates. If the plates become "locked together," forces build up and eventually must give away - with the plates lurching into new positions and creating an earthquake. Other earthquakes form in association with volcanic regions, where the buildup of heat and pressure often triggers smaller tremors and localized quakes. The focus is the point under the Earth's surface where the earthquake energy is released. The point on the surface just above the focus is called the epicenter. Most earthquake foci occur no more than 62 miles (100 kilometers) below the surface. Earthquake waves, or seismic waves, travel out from the focus of an earthquake in all directions. Primary waves (P-waves) move in a back-and-forth direction; they are the fastest seismic waves, reaching the far side of the globe in 20 minutes. The waves travel though the Earth's molten core. Secondary waves (S-waves) move in a side-to-side direction; they are slower than P-waves and can move only through solids, stopping at the Earth's central molten core. ...read more.


The difference in time between the first and second (primary and secondary) waves is measured, and an empirical factor is added (which takes into account the fact that the waves become weaker as they travel away from the focus) to determine the magnitude of the quake. Richter Number Increase in Magnitude by a Factor of 10 1 1 2 10 3 100 4 1,000 5 10,000 6 100,000 7 1,000,000 8 10,000,000 Comparison of Richter Magnitude and Energy Released Richter Number Approximate Energy Released (Amount of TNT) 1. 170 grams 2. 6 kilograms 3. 179 kilograms 4. 5 metric tons 5. 179 metric tons 6. 5,643 metric tons 7. 179,100 metric tons 8. 5,643,000 metric tons Famous Earthquakes Famous earthquakes are usually arranged by their magnitude on the Richter scale. To compare, the average tornado can rumble with a Richter scale magnitude of 4.8, and a 1-megaton nuclear bomb can measure 7.5 on the Richter scale. The highest magnitude earthquake recorded to date - at about 9.6 (although the magnitude is debated) - was the 1960 Chilean earthquake, in which a fault 621 miles (1,000 kilometers) long slipped 33 feet (10 meters); the largest deep quake on record was on June 9, 1994, beneath a remote area in the Amazon rain forest of northern Bolivia, with a Richter scale magnitude of 8.3. ...read more.


Aftermath of a Modern Volcano From June 14 to 16, 1991, on the Philippine island of Luzon, Mount Pinatubo erupted, resulting in a stratospheric cloud that contained roughly twice as much sulfur dioxide as the one produced by Mexico's El Chich� in 1982. But even more amazing was the fact that the cloud circled the Earth within the short span of 3 weeks. The potential effect was evident for months after the eruption: Mount Pinatubo's release of sulfur dioxide resulted in unusually brilliant sunsets in different parts of the world. Scientists predicted even greater effects from this eruption - that the volcanic particles in the air would reflect radiation from the Sun and cause a reduction in surface temperatures. As a result, the Northern Hemisphere would have colder than normal weather. The decrease in solar energy would decrease the wind strength along the equatorial belt. If the winds died down, the surface temperature of the Pacific Ocean would rise - contributing to the development of El Ni�, the anomalous current of warm water that flows north along the west coast of South America. Did the volcano create such problems? Scientists do not totally agree. But in the years following the eruption of Mount Pinatubo, El Ni� conditions persisted longer than usual; there was a major blizzard in March 1993 - one of the worst of the century - along the East Coast of the United States, and other weather oddities occurred. ...read more.

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