Research Project on Earthquake
General
An earthquake is a shaking or trembling of the crust of the earth caused by underground volcanic action or by the breaking and shifting of rock beneath the surface. The volcanic action and shifting rocks create strain which continues to build to a sudden release of pressure resulting in a shock wave. The vibrations produced in the crust can vary from barely noticeable to enormously destructive. Shock waves can be classified into two broad categories. Waves that send particles oscillating back and forth in the same direction as the waves are traveling are called primary. Primary waves, sometimes called compressional waves, travel through the earth beneath the crust. Secondary waves cause vibrations which move perpendicular to the wave. These waves travel on the surface of the earth and move much slower than primary waves. Thus, when an earthquake occurs, seismic centers throughout the world record primary waves before the secondary waves arrive.
Historical Overview
Earthquakes have captured the imagination of people living in earthquake prone regions since ancient times. Ancient Greek philosophers thought quakes were caused by subterranean winds, while others blamed them on fires in the bowels of the earth. About AD 130, a Chinese scholar named Chang Heng reasoned that waves must ripple through the earth from the source of an earthquake.
By 1859, an Irish engineer by the name of Robert Mallet proposed that earthquakes occurred by either a sudden movement of flexible materials which formed the earth's crust, or by their giving way and fracturing. In the 1870s, an English geologist called John Milne invented the forerunner of today's seismograph. The name comes from the Greek word "seismos," meaning earthquake. The modern seismograph was invented early this century by a Russian seismologist, Prince Boris Golitzyn. This device made possible the modern era of earthquake research.
Although earthquakes have occurred and affected mankind for as long as humans have been around, historical records are either lacking or considered unreliable before the middle of the 18th century. However, some records do exist and date major earthquake events back to 425 BC when one occurred off the coast of Greece resulting in an island named Euboea. Another occurred in Asia Minor about AD 17 and another leveled much of the Roman city of Pompeii in AD 63. During the Middle Ages, severe earthquakes struck England, in 1318; Naples, Italy, in 1456; and Lisbon, Portugal, in 1531.
Casualties from major earthquakes have resulted in the greatest disasters in history. The worst occurred in 1556 in the Shannxi (Shensi) Province in China and killed an estimated 800,000 people. During the 18th century, the Japanese city of Edo (site of modern day Tokyo) was struck and killed 200,000. Earthquakes in Sicily in 1693 and Lisbon in 1755 each killed about 60,000 people. In 1797, approximately 40,000 died when Quito, Ecuador was struck.
Historically, the most powerful earthquakes to strike North America occurred in 1811 and 1812 in southeastern Missouri. However, the most famous earthquake to strike in North America was the one that hit San Francisco in 1906, killing 700. The Northridge earthquake which struck Southern California during the early morning hours of January 17, 1994, was mild by comparison but resulted in 72 deaths and injured 11,846. The estimated 30 billion dollar loss makes it the most expensive disaster in the history of the United States.
Types of Earthquakes
All earthquakes may be classified as one of three types, tectonic, volcanic or artificial. Tectonic earthquakes are caused by movements of the plates which make up the crust of the earth. These earthquakes usually occur far below the surface of the earth (as much as 400 miles) at the boundaries of the plates, where they slide against one another causing a buildup of friction and pressure until a sudden release of energy results in an earthquake. Infrequently, these types of earthquakes can occur in the center of a plate when pressure builds and ruptures the plate. One such earthquake, which occurred around 1811, was powerful enough to change the course of the Mississippi River. Of the three types, tectonic earthquakes are the most devastating and account for almost half of the world's destructive earthquakes and as much as three-quarters of the earth's seismic energy. Volcanic earthquakes are generally much smaller and less intense than tectonic ones. They are caused when magma flows upward and fills the chambers beneath a volcano. The sides and tops of the volcano swell and tilt with pressure and result in numerous small earthquakes. Volcanic earthquakes are seldom destructive, but have particular interest because they are often the forerunner for volcanic eruptions. The eruption of Mount Saint Helens in Washington in 1980 is only one example.
Sometimes, earthquakes occur as a result of human actions such as when atomic explosives are detonated underground or when new reservoirs are filled. These artificial earthquakes are rare and relatively harmless when compared with the earthquakes caused by nature One such series of earthquakes occurred in the early 1960s near Denver, Colorado, when waste fluids were pumped into deep wells just east of the city. When the pumping stopped, so did the earthquakes.
Effects of Earthquakes
The most common damage and the greater amount of injuries caused by earthquakes is a result of falling objects. Much of the world's population live in densely populated urban areas which make them particularly vulnerable to building collapse and being struck by falling objects. Earthquakes can also topple bridges, towers and dams as well as trigger large landslides. Each of these present unique problems and can result in floods and fires which may be more devastating than the direct effects of the earthquake.
Besides the trembling and shaking of the earth caused by earthquakes, other consequences can result in tremendous destruction. When shock waves from an earthquake strike some types of soil, they result in a condition called liquefaction. Liquefaction occurs when these soils lose their ability to bear weight and behave much like quicksand. Buildings which have been constructed on these soils quickly topple and may even be swallowed up. This phenomena was observed in the big earthquake which struck San Francisco in 1906.
Subsea tremors may result in a tsunami. A tsunami is another name for a tidal wave but because tides have nothing to do with the creation of these waves, the term tsunami is used instead. These waves move with tremendous speed and power and can devastate coastal communities where the sea bottom "lifts" the water as it rolls up on the land.
Measuring the Intensity of Earthquakes
The most well-known method of measuring the intensity of an earthquake is the Richter scale. The Richter scale is named after an American seismologist named Charles Francis Richter, and measures the amount of energy released at the focus of a quake. It uses a logarithmic scale that runs from "1" to "9." Because this scale is logarithmic, each number is actually an increase of ten times than the number which precedes it. Thus, a 7.0 earthquake is ten times more powerful than a 6.0 and 100 times more powerful than a 5.0. To allow a greater degree of precision, a decimal equivalent was provided. At one time it, was believed that an earthquake with a magnitude of 8.5 was the most powerful possible but new seismic measuring techniques have revealed that it is possible to reach 9.5.
Another method of measuring the intensity of earthquakes was developed at the turn of the century by an Italian seismologist by the name of Giuseppe Mercalli. The Mercalli scale measures shaking with gradations from I to XII. Since the effects of the earthquake diminish with the distance it is from the focus of the quake (epicenter), the Mercalli rating depends upon where the measurement is taken. The lowest rating (I) is one which is felt by very few people and the highest (XII) is one which causes total destruction. An earthquake with a magnitude of 3 to 4 on the Richter scale is roughly equivalent to II to III on the Mercalli scale and a magnitude 8 to 9 on the Richter scale is roughly equivalent to a XI to XII on the Mercalli scale.
Predicting Earthquakes
Earthquake prediction has met with mixed results, however some success has been reported in recent years. Several countries, most notably the United States, China, Japan and Russia, are actively supporting research in this area. In 1975, just two days before a magnitude 7.3 earthquake destroyed the city of Haicheng, the Chinese government evacuated 90,000 residents. The evidence used to support the prediction was a series of foreshocks occurring in the area five years earlier. Other possible clues being investigated are water levels in wells, changes in the earth's magnetic field and the behavior of animals. So far though, no reliable method exists for predicting when and where an earthquake will occur nor how strong it is likely to be.
Earthquakes in Southern California
Southern California has been plagued by numerous earthquakes which have not only caused death and destruction, but have resulted in the most stringent building codes anywhere in the world. Buildings and other structures erected under these new laws have resulted in safer and more durable construction than ever before. Because of the frequency of earthquake tremors, the people living in and around the greater Los Angeles area are among the most knowledgeable of this subject in the United States. Earthquake mitigation and preparedness is an ongoing process and each tremor brings new information on how to deal with the next one. Children are taught earthquake drills in elementary school. Police and Fire Departments practice building rescues. Municipalities conduct table top disaster management exercises. Freeway overpasses, bridges and dams are reinforced. The California Institute of Technology in Pasadena is regarded as one of the foremost earthquake research centers in the world.
One of the most active earthquake fault systems anywhere on the globe runs through Southern California. The San Andreas Fault emerges from the Pacific Ocean and runs through more than 600 miles through California. The fault marks the boundary where the North American and the Pacific tectonic plates meet. This boundary is easily visible from the air and is the most well known fault system in the United States.
At 0431 hours, on the morning of January 17, 1994, a magnitude 6.7 earthquake struck near the City of Northridge in the San Fernando Valley of Los Angeles. By 0500, the Los Angeles County Emergency Operations Center was staffed and operating and thirty minutes later, initial damage assessments were compiled. The earthquake, which came to be known as the "Northridge earthquake," collapsed freeway overpasses, damaged roadways, shut off power, started fires, closed and evacuated hospitals and resulted in at least 72 deaths and an estimated $30 billion in damages. The relief effort was orchestrated by the County's Emergency Operations Center and required the efforts of all local police and fire departments, many public utility companies, the California National Guard, the American Red Cross and a host of other agencies. Tent cities were erected, food and water dispersed, fires put out and numerous buildings searched and evacuated. Although this earthquake earned the title of "the most expensive disaster in the history of the United States," the preparation and training of all concerned proved to be highly effective and greatly contributed to limiting the amount of death and injury.
As the next century looms near, Southern California prepares for the "big one." Despite the tremendous impact of the Northridge earthquake, no one believes that it is the last nor even the largest which will strike the greater Los Angeles area. A new 36,000 square foot Emergency Operations Center has been constructed in East Los Angeles. A series of laws and procedures entitled the "Standardized Emergency Management System" has continued to make the Mutual Aid System even more effective. The Los Angeles Sheriff's Department has taken a leading role in establishing courses in crisis action planning, crisis decision making, command and control, operations, logistics and intelligence. Liaisons and working groups with other agencies and disciplines has provided additional insight and contributed to a high state of readiness as they prepare for the next major earthquake.
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