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Earthquakes

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Major earthquake damage was inflicted upon north-west Turkey on August 17, 1999.
 
Earthquakes are the most expensive natural disasters humans face. A strong earthquake in the wrong place at the wrong time can cause hundreds of billions of dollars of damage and cost tens of thousands of human lives. In the span of three months (from August to October 1999) three earthquakes occurred globally with magnitudes exceeding 7. The impacts of these three varied widely; one affected a large city, crumbling buildings and taking lives, while another jostled a remote desert wasteland, and caused only minimal damage to peripheral towns. They illustrate the differences between an earthquake that occurs in a remote region of the planet versus one that occurs near a large city.

On August 17th, 1999, a moment magnitude 7.4 earthquake shook the area of Izmit, Turkey, which caused 3 to 6.5 billion dollars damage with over 17,000 fatalities. It struck in a portion of Asia Minor that has experienced 7 major tremblors greater than 7.1 since 1939 along the North Anatolian fault. In 1939, the Erzincan earthquake, a magnitude 7.9, was credited with 30,000 deaths.
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Westward progression of large-magnitude earthquakes in northern Turkey throughout the century.
 
Large earthquakes occur in this area for a reason; Anatolia (a portion of Asia Minor located in Turkey) is trapped between the northward migration of the African Plate (in what is called a suture zone) and the northward movement of the Arabian Plate relative to the Eurasian Plate. These plate motions "squeeze" the Turkish microplate. The resulting "stress" is relieved when motion occurs along major faults, causing earthquakes. Major earthquakes along the North Anatolian fault show a disturbing trend. Since 1939, the major earthquakes have shown a general westward migration. The next major city west of Izmit is Istanbul, a city with many ancient historic structures and over 8.5 million inhabitants.
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A 7.6 earthquake epicenter was registered near the small town of Chichi, Taiwan on September 20, 1999, not far from the capital Taipei.
 
On September 20, 1999, another major population center was hit by an earthquake. This time, the quake struck Taiwan with a magnitude of 7.6 registered by the U.S. Geological Survey. At least 2,100 people were killed and 12,000 houses destroyed. Like earthquakes in Asia Minor, this one occurred for a reason; Taiwan is located above a subduction zone, where the Philippine Plate is being subducted under the Eurasian Plate. Motion along this large fault creates major earthquakes while melting of plate material helps generate island arc volcanism. Like Anatolia, this is an area that experiences frequent earthquakes, with a 7.8 occurring in the same general area as recently as November 14, 1986. Major earthquakes and volcanoes along the Pacific rim have lead many researchers and teachers to call this area the "Ring of Fire".
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Mercalli-Scale map of greater Los Angeles. People from all over Southern California reported the Hector Mine earthquake in terms of the shaking felt and local damage done.
 
In contrast to the two previous earthquakes, the Hector Mine quake occurred in a remote area. This earthquake, which was rated at 7.0, occurred early in the morning on October 16, 1999. It occurred east of the city of Los Angeles, in a remote part of the Mojave desert along a fault that had been previously thought inactive. The fault ruptured along a 40 kilometer stretch, causing up to 4.7 meters of right ward motion (~15 feet). Because of the potential of a catastrophic earthquake in the Los Angeles area, the region has been incredibly well instrumented. For this reason, the Hector Mine earthquake is one of the best monitored major earthquakes in history, with excellent information about the quake (and local damage) being reported within hours of the quake while most of Los Angeles was sleeping. Like the Anatolian and Taiwan quakes, the Hector Mine earthquake occurred for a reason. The general area of coastal southern California is located where the Pacific Plate is sliding northwards relative to the North American Plate, in what is called a transform fault. This creates a series of faults that are part of what is named the San Andreas Fault system. Major movement along this system (on average 10 centimeters/year) creates the potential of a very large earthquake. Had an earthquake as strong as the Hector Mine quake occurred even 100 kilometers westward, the damage would have been far greater, adversely impacting a huge metropolitan center of nearly 10 million people.
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A travel-time map of the globe, showing the "P"-wave propagation after the September 7, 1999 earthquake in Greece. The red lines and corresponding numbers show how many hours after the quake a seismograph there would detect the distant tremblor.
 
Earthquakes that happen in one place can be detected by seismographs in other parts of the world. The waves from the epicenter of a major earthquake propagate outward as surface waves. In the case of compressional waves, the energy released in this fashion radiates from the focus under the epicenter and travels all the way through the globe. Seismologists, who study the distribution of these waves, can map such propagation. The further away a point on the earth is from the focus of a quake, the longer the time it will take for a seismograph station at that point to detect the far-away quake. The compressional waves are also named "P"-waves. Due to the nature of the earth's interior, "P"-waves can be refracted and reflected as they encounter differing density layers between the core and the mantle. As a result, seismographs in some areas on the other side of the world opposite the epicenter of a major earthquake feel nothing. Such an area on the surface of the earth is called a shadow zone, encompassing the area between the black lines marked "103°" and "120°" on the above map.
 
Although it is easy to think, given the large number of quakes that have occurred recently, that the number of earthquakes is increasing over time, long term records suggest that this is not true. What is changing is our ability to measure even small earthquakes, and thus fewer go unrecorded. However, what is changing is the size of cities, and the places people choose to live. As cities grow larger, the potential of a major quake striking a place where there are buildings and people goes up. As more and more remote areas are populated, even remote earthquakes like Hector Mine, are starting to be felt.

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