Earthquakes have plagued our lives for as long as people have inhabited the earth. These dangerous acts of the earth have been the cause of many deaths in the past century. So what can be done about these violent eruptions that take place nearly with out warning? Predicting an earthquake until now has almost been technologically impossible. With improvements in technology, lives have been saved and many more will. All that remains is to research what takes place before, during, and after an earthquake. This has been done for years to the point now that a successful earthquake prediction was made and was accurate. This paper will discuss a little about earthquakes in general and then about how predictions are made.
Earthquake, "vibrations produced in the earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound."(Associated Press 1993) The vibrations can range from barely noticeable to catastrophically destructive. Six kinds of shock waves are generated in the process. Two are classified as body waves-that is, they travel through the earth's interior-and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are traveling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide.(Associated Press 1993)
Earthquake waves were observed in this and other ways for centuries, but more scientific theories as to the causes of quakes were not proposed until modern times. One such concept was advanced in 1859 by the Irish engineer Robert Mallet. Perhaps drawing on his knowledge of the strength and behavior of construction materials subjected to strain, Mallet proposed that earthquakes occurred "either by sudden flexure and constraint of the elastic materials forming a portion of the earth's crust or by their giving way and becoming fractured."(Butler 1995)
Later, in the 1870s, the English geologist John Milne devised a forerunner of today's earthquake-recording device, or seismograph. A simple pendulum and needle suspended above a smoked-glass plate, it was the first instrument to allow discrimination of primary and secondary earthquake waves. The modern seismograph was invented in the early 20th century by the Russian seismologist Prince Boris Golitzyn. "His device", using a magnetic pendulum suspended between the poles of an electromagnet, "ushered in the modern era of earthquake research." (Nagorka 1989)
"The ultimate cause of tectonic quakes is stresses set up by movements of the dozen or so major and minor plates that make up the earth's crust."(Monastersky Oct, 95) Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past another-as at the San Andreas Fault in California, North America's most quake-prone area-or is subducted (slides beneath the other plate). "Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 percent of the earth's seismic energy. They are concentrated along the so-called Ring of Fire, a narrow band about 38,600 km (about 24,000 mi) long, that coincides with the margins of the Pacific Ocean. The points at which crustal rupture occurs in such quakes tend to be far below the earth's surface, at depths of up to 645 km (400 mi)." (Monastersky Dec, 95) Alaska's disastrous Good Friday earthquake of 1964 is an example of such an event.
Seismologists have devised two scales of measurement to enable them to describe earthquakes quantitatively. "One is the Richter scale-named after the American seismologist Charles Francis Richter-which measures the energy released at the focus of a quake. It is a logarithmic scale that runs from 1 to 9; a magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times more powerful than a magnitude 5 quake, 1000 times more powerful than a magnitude 4 quake, and so on."(Associated Press 1992)
The other scale, introduced at the turn of the 20th century by the Italian seismologist Giuseppe Mercalli, "measures the intensity of shaking with gradations from I to XII." (Associated Press 1992) Because seismic surface effects diminish with distance from the focus of the quake, the Mercalli rating assigned to the quake depends on the site of the measurement. "Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale can be correlated with magnitudes 8 to 9 on the Richter scale."( Associated Press 1992)
Attempts at predicting when and where earthquakes will occur have met with some success in recent years. At present, China, Japan, Russia, and the U.S. are the countries most actively supporting such research. "In 1975 the Chinese predicted the magnitude 7.3 quake at Haicheng, evacuating 90,000 residents only two days before the quake destroyed or damaged 90 percent of the city's buildings. One of the clues that led to this prediction was a chain of low-magnitude tremors, called foreshocks, that had begun about five years earlier in the area." (Day 1988) Other potential clues being investigated are tilting or bulging of the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behavior. A new method under study in the U.S. involves measuring the buildup of stress in the crust of the earth. "On the basis of such measurements the U.S. Geological Survey, in April 1985, predicted that an earthquake of magnitude 5.5 to 6 would occur on the San Andreas fault, near Parkfield, California, sometime before 1993."(Day 1988) Many unofficial predictions of earthquakes have also been made. In 1990 a zoologist, Dr. Iben Browning, warned that a major quake would occur along the New Madrid fault before the end of the year. Like most predictions of this type, it proved to be wrong. "Groundwater has also played an important part in earthquake predictions. A peak in radon in the groundwater at Kobe, Japan 9 days before the 7.2 earthquake cause quite a stir. Radon levels peaked 9 days before the quake, then fell below the normal levels 5 days before it hit."(Monastersky July, 95)
In North America, the series of earthquakes that struck southeastern Missouri in 1811-12 were probably the most powerful experienced in the United States in historical time. The most famous U.S. earthquake, however, was the one that shook the San Francisco area in 1906, causing extensive damage and taking about 700 lives.(Nagorka 1989)
The whole idea behind earthquake predicting is to save lives. With the improvement in technology, lives have been saved. New ideas and equipment is starting to prove to be very helpful in predicting were and when an earthquake will strike. The time and research put into earthquake predicting has already started to pay off. It is only a matter of time before earthquakes will no longer be a threat to us.
Bibliography
Associated Press 1992, "The Big One: Recent Tremors May Be a 'Final Warning'"; SIRS 1993 Earth Science, Article 12, Aug. 30, 1992, pg. J1+.
Associated Press 1993, "Predicting the Effects of Large Earthquakes"; SIRS 1994 Applied Science, Article 17, Sept./Oct. 1993, pg. 7-17.
Butler, Steven 1995, "Killer Quake"; SIRS 1995 Earth Science, Article 47, Jan. 30, 1995, pg. 38-44.
Day, Lucille, 1988, "Predicting The Big One"; SIRS 1989 Earth Science, Article 5, Summer 1988, pg. 34-41.
Monastersky, R. 1995, "Electric Signals May Herald Earthquakes"; Science News, v. 148, Oct. 21 ,1995, pg. 260-1.
Monastersky, R. 1995, "Quiet Hints Preceded Kobe Earthquake"; Science News, v. 148, July 15, 1995, pg. 37.
Monastersky, R. 1995, "Radio Hints Precede a Small U.S. Quake"; Science News, v. 148, Dec. 23&30, 1995, pg. 431.
Nagorka, Jennifer 1989, "Earthquakes: Predicting Where Is Easy--It's When That's Tough"; SIRS 1990 Earth Science, Article 26, Oct.29, 1989, pg. E1-2.
No comments:
Post a Comment