Anatomy of an Earthquake
“WE ARE SO ACCUSTOMED TO LIVING ON SOLID EARTH THAT WHEN THAT BEGINS TO SHAKE THE MIND IS OVERWHELMED.”—“THE VIOLENT EARTH.”
“EARTHQUAKES are among the most destructive and powerful forces in nature,” notes The World Book Encyclopedia. That statement is no exaggeration, for the energy released by a severe quake may be 10,000 times greater than that generated by the first atom bomb! Adding to the terror is the fact that earthquakes can happen in any climate, during any season, and at any time of the day. And although scientists may have some idea where powerful temblors are likely to occur, they cannot specify when.
Earthquakes occur as a result of masses of rock changing position below the earth’s surface. This type of activity goes on continuously. Often, the shock waves that result are not powerful enough to be felt at the earth’s surface, but they can be detected and recorded by a seismograph. * At other times, enough rock breaks and enough shift takes place to shake the surface violently.
But why is there constant movement in the earth’s crust? “An explanation is to be found in plate tectonics, a concept which has revolutionized thinking in the Earth’s sciences,” says the National Earthquake Information Center (NEIC). “We now know that there are seven major crustal plates, subdivided into a number of smaller plates,” NEIC adds, “all in constant motion relative to one another, at rates varying from 10 to 130 millimeters [three eighths of an inch to five inches] per year.” Most earthquakes, NEIC says, are confined to narrow belts that define the boundaries of the plates. This is where 90 percent of major earthquakes are likely to occur.
Magnitude and Intensity
The severity of an earthquake may be measured by its magnitude or its intensity. Charles Richter developed a scale in the 1930’s for measuring the magnitude of earthquakes. As the number of seismograph stations grew, new scales based on Richter’s idea were developed. What is called the moment magnitude scale, for example, measures the energy released at the source of the quake.
Of course, these scales do not always reveal the degree of damage inflicted by an earthquake. Consider an earthquake in northern Bolivia in June 1994, with a magnitude of 8.2, which reportedly killed only five people. Yet, the 1976 quake in Tangshan, China—with the smaller 8.0 magnitude—resulted in hundreds of thousands of deaths!
In contrast with magnitude, intensity registration shows the effects that an earthquake has upon people, structures, and the environment. This is a more descriptive measure of the severity of an earthquake in human terms. After all, tremors in themselves do not usually harm people. Instead, collapsing walls, rupturing gas or power lines, falling objects, and the like cause the most injuries and fatalities.
One goal of seismologists is to be able to provide early warnings of earthquake activity. A digital program called the Advanced Seismic Research and Monitoring System is being developed. According to a CNN report, this system—coupled with speedier access and more high-powered software applications—will help officials to “be able to almost instantly pinpoint areas where the most violent shaking from an earthquake has occurred.” This, in turn, will make it easier for authorities to send help into affected areas.
Obviously, being prepared for an earthquake can reduce injuries, minimize property damage and—most important—save lives. Yet, earthquakes continue to occur. So the question arises: How have people been helped to cope with the aftermath?
^ par. 4 A seismograph is a device that measures and records the motion of the ground during an earthquake. The first one was developed in 1890. Today, more than 4,000 seismograph stations are operating worldwide.
[Chart on page 5]
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How Many Earthquakes?
Descriptor Magnitude Average Annually
Great 8 and higher 1
Major 7-7.9 18
Strong 6-6.9 120
Moderate 5-5.9 800
Light 4-4.9 6,200*
Minor 3-3.9 49,000*
Very Minor <3.0 Magnitude 2-3:
about 1,000 per day
about 8,000 per day
Source: National Earthquake Information Center By permission of USGS/National Earthquake Information Center, USA
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Seismogram on pages 4 and 5: Figure courtesy of the Berkeley Seismological Laboratory