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An earthquake is shaking or trembling of the earth that is volcanic or tectonic in origin. A broadly satisfying explanation of the majority of earthquakes can be given in terms of what is called plate tectonics. The basic idea is that the Earths outermost part also called the lithosphere consists of several large and fairly stable slabs of solid and relatively rigid rock called plates (Bravely 154). These plates continuously move pushing and rubbing against each other.
Since the earliest of time people have been looking for an explanation for earthquakes. The people of Maori in New Zealand believed the god of earthquakes, Ruaumoko is said to have pressed into the earthquake as his mother turned face downward while feeding him. According to the legend, he has been growling and spitting fire ever since. Aristotle believed that mild earthquakes were caused by wind escaping from caves within the bowels of the earth and severe shocks were by gales that found their way into great subterranean caverns (Matthys 87). However, with modern technology we found that what actually causes earthquakes is tectonic plates which on average move only two inches per year they are driven by convection currents which is the upward movement of heated particles rising from the earths molten core. As the plates are driven against each other one will try to and eventually will slip underneath the other.
When plates move quickly an earthquake is the result. Before the availability of instruments capable of a quantitative measure of their magnitude, earthquakes were classified according to their intensity. In 1931 a seismologist came up with twelve degrees of intensity ranging from I (felt only by a few) to XII (total damage) this was called a Modified Mercalli scale. However according to Dr. Pignataro Even with such scales, the subjectivity of the observer renders their use less then perfect, since today even experienced seismologists sometimes assign different intensities to the same earthquake. If the magnitude of an earthquake is to be compared worldwide, a measure is needed that does not depend (as does intensity) on the density of the population and type of construction.
A strictly quantitative scale that can be applied to earthquakes in both inhabited and un habited regions was originated in 1931 by Charles Richter in California. Because the size of earthquakes varies enormously, the amplitudes of the ground motions differ by factors of thousands from earthquake to earthquake. It is therefore most convenient to compress the range of wave amplitudes measured on seismographs using some mathematical device (Bolt 118). The first earthquake recorder described in any detail was an artistic device invented by the Chinese scholar Chang Heng about 132 A. D.
The instrument was a seismoscope: unlike a seismograph, it did not give the complete time history of earthquake shaking but simply the direction of the principal impulse due to the earthquake. Along seacoasts, another disaster may follow large earthquakes. The sudden offset of a major fault under the ocean floor moves the water as if it were being pushed by a giant paddle, producing powerful water at the oceans surface. These water waves spread out from the vicinity of the earthquake source and move across the ocean until they reach a coastline (Van Rose 100).
References to the devastation of tsunamis can be found throughout history. The earliest description is of a damaging sea wave near the north end of the Aegean Sea in 479 B. C. One of the worst tsunamis in history hit the eastern coast of Honshu following a great earthquake centered out at sea on June 15, 1896. The seismic sea wave washed onto nearby land as much as 25 to 35 meters above high tide level.
Entire villages were engulfed. More then 10, 000 houses were washed away, and 26, 000 people were killed. Within the last 200 years alone, about 3000 tsunamis have produced fatalities. While the evidence is that most great water waves are caused by fault rupture there are also other causes. One example is a submarine landslide, such as occurred in Safari Bay in Japan in the devastating earthquake of 1923 these under water landslides may be triggered by nearby earthquakes. Sometimes a landslide or avalanche of soil and rock on a mountain into a bay, lake or even reservoir can produce a deadly wave.
A famous landslide-induced sea wave occurred in Alaska after a large earthquake in 1958. Water waves rushed to the opposite shores of the bay as far as 500 meters. Earthquakes and volcanoes often, but by no means always, accompany each other. A volcanic eruption is at a point of weakness in the earths crust. It starts when a movement, often due to the shaking of an earthquake, opens a crack in the crust: a volcano is nothing more then a vent in the earths crust. One good example of a volcano was Mt.
St. Helens. In March 1980 a few earthquakes near Mt. St.
Helens began a sequence that led to an eruption two months later. By the end of March seismographs recorded hundreds of earthquakes each day. By mid-April, a bulge was seen developing on the peak of the mountain. May 8, 1980 the (USGS) predicted the earthquake would be soon and began evacuating the area.
Many residents refused to go such as Harry Truman who said, If I got out of here, I wouldnt live a damn day. Because it was their home for many years and they didnt believe it would erupt. On May 18, 1980 a huge landslide. Then an incredible blast, compared to a six-megaton bomb, blew the entire north face of the mountain off hurling ash, boulders and ice in the air. The explosion was so large that as far away as North Dakota ash was seen.
In Spokane Washington, 700 miles away the city was covered in ash and dust. It was pitch black at 5 pm, streetlights were out and 3 feet of ash sat in the streets. April 18, 1906, at 5: 12 am in San Francisco, a section of rock snapped along the San Andreas Fault at the speed of 1. 9 mi / sec . The break spread quickly along the fault southward and northward.
As this rupture in the rocks grew, seismic wave radiated out through the earth, shaking the ground surface across a wide area of Nevada and California (Matthys 3). The estimated magnitude was 8. 25, and lasted 40 seconds, but the shaking continued for 10 minutes doing the greatest damage. Pavement buckled, brick houses were severely damaged, sewer lines were broken and streetcar tracks were bent into wave-like forms. What didnt the earthquake destroy was destroyed when gas and water mains ruptured, sending geysers shooting up through broken pavement and drowning some residents who were pinned in the wreaked building. Fifty fires burned in the business district but, no sirens rang because, the emergency battery shattered at the first shock. (Matthys 108). As the city counted their losses they found over 500 dead, 5, 000 injured and 100, 000 homeless.
Alexander Made a San Francisco citizen said what wasnt divested by the earthquake was divested by the aftermath.
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Research essay sample on Earths Crust San Francisco
