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  3. How Earthquake Works [A]An earthquake is one of the most terrifying phenomena that nature can whip up. We generally think of the

How Earthquake Works [A]An earthquake is one of the most terrifying phenomena that nature can whip up. We generally think of the

admin2016-01-05  23
问题                         How Earthquake Works
[A]An earthquake is one of the most terrifying phenomena that nature can whip up. We generally think of the ground we stand on as "rock-solid" and completely stable. An earthquake can shatter that perception instantly, and often with extreme violence.
Earthquake facts
[B]An earthquake is a vibration that travels through the earth’s crust. Technically, a large truck that rumbles down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to think of earthquakes as events that affect a fairly large area, such as an entire city. All kinds of things can cause earthquakes: volcanic eruptions, meteor(流星)impacts, underground explosions(an underground nuclear test, for example), collapsing structures(such as a collapsing mine). But the majority of naturally-occurring earthquakes are caused by movements of the earth’s plates. We only hear about earthquakes in the news every once in a while, but they are actually an everyday occurrence on our planet.
[C]The vast majority of these 3 million quakes are extremely weak. The law of probability also causes a good number of stronger quakes to happen in uninhabited places where no one feels them. It is the big quakes that occur in highly populated areas that get our attention. Earthquakes have caused a great deal of property damage over the years, and they have claimed many lives. In the last hundred years alone, there have been more than 1. 5 million earthquake-related fatalities. Usually, it’s not the shaking ground itself that claims lives—it’s the associated destruction of man-made structures and the instigation(发起)of other natural disasters, such as tsunamis, avalanches and landslides.
Plate tectonics(板块构造)
[D]The biggest scientific breakthrough in the history of seismology—the study of earthquakes—came in the middle of the 20 th century, with the development of the theory of plate tectonics. Scientists proposed the idea of plate tectonics to explain a number of peculiar phenomenon on earth, such as the apparent movement of continents over time, the clustering of volcanic activity in certain areas and the presence of huge ridges at the bottom of the ocean.
[E]The basic theory is that the surface layer of the earth—the lithosphere—is comprised of many plates that slide over the lubricating asthenosphere layer. At the boundaries between these huge plates of soil and rock, three different things can happen: [F]* Plates can move apart—If two plates are moving apart from each other, hot, melted rock flows up from the layers of covering below the lithosphere. This magma comes out on the surface(mostly at the bottom of the ocean), where it is called lava. As the lava cools, it hardens to form new lithosphere material, fdling in the gap. This is called a divergent(分叉的)plate boundary.
[G]* Plates can push together—If the two plates are moving toward each other, one plate typically pushes under the other one. This subducting plate sinks into the lower mantle layers, where it melts. At some boundaries where two plates meet, neither plate is in a position to subduct(潜没)under the other, so they both push against each other to form mountains. The lines where plates push toward each other are called convergent plate boundaries.
[H]* Plates slide against each other—At other boundaries, plates simply slide by each other—one moves north and one moves south, for example. While the plates don’t drift directly into each other at these transform boundaries, they are pushed tightly together. A great deal of tension builds at the boundary.
[I]Where these plates meet, you’ll find faults—breaks in the earth’s crust where the blocks of rock on each side are moving in different directions. Earthquakes are much more common along fault lines than they are anywhere else on the planet.
Richter scale
[J]The Richter Scale is used to rate the magnitude of an earthquake—the amount of energy it released. This is calculated using information gathered by a seismograph. Upon the Richter Scale, whole-number jumps indicate a tenfold increase. In this case, the increase is in wave amplitude. That is, the wave amplitude(幅度)in a level 6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude increases 100 times between a level 7 earthquake and a level 9 earthquake. The amount of energy released increases 31.7 times between whole number values.
[K]The largest earthquake on record registered a 9. 5 on the currently used Richter Scale, though there have certainly been stronger quakes in Earth’s history. The majority of earthquakes register less than 3 on the Richter Scale. These tremors, which aren’t usually felt by humans, are called microquakes. Generally, you won’t see much damage from earthquakes that rate below 4 on the Richter Scale. Major earthquakes generally register at 7 or above. Richter ratings only give you a rough idea of the actual impact of an earthquake. As we’ve seen, an earthquake’s destructive power varies depending on the composition of the ground in an area and the design and placement of manmade structures.
[L]Richter Scale ratings are determined soon after an earthquake, once scientists can compare the data from different seismograph stations. Mercalli ratings, on the other hand, can’t be determined until investigators have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating.
Predicting earthquakes
[M]We understand earthquakes a lot better than we did even 50 years ago, but we still can’t do much about them. They are caused by fundamental, powerful geological processes that are far beyond our control. These processes are also fairly unpredictable, so it’ s not possible at this time to tell people exactly when an earthquake is going to occur. The first detected seismic waves will tell us that more powerful vibrations are on their way, but this only gives us a few minutes warning, at most.
[N]Scientists can say where major earthquakes are likely to occur, based on the movement of the plates in the earth and the location of fault zones. They can also make general guesses of when they might occur in a certain area, by looking at the history of earthquakes in the region and detecting where pressure is building along fault lines. These predictions are extremely vague, however— typically on the order of decades. Scientists have had more success predicting aftershocks, additional quakes following an initial earthquake. These predictions are based on extensive research of aftershock patterns. Seismologists can make a good guess of how an earthquake originating along one fault will cause additional earthquakes in connected faults.
[O]So what can we do about earthquakes? The major advances over the past 50 years have been in preparedness—particularly in the field of construction engineering. In 1973, the Uniform Building Code, an international set of standards for building construction, added specifications to fortify buildings against the force of seismic waves. This includes strengthening support material as well as designing buildings so they are flexible enough to absorb vibrations without falling or deteriorating. It’s very important to design structures that can take this sort of punch, particularly in earthquake-prone areas.
[P]In the future, improvements in prediction and preparedness should further minimize the loss of life and property associated with earthquakes. But it will be a long time, if ever, before we’ll be ready for every substantial earthquake that might occur. Just like severe weather and disease, earthquakes are an unavoidable force generated by the powerful natural processes that shape our planet. All we can do is to increase our understanding of the phenomenon and develop better ways to deal with it.
Scientists are more successful in predicting aftershocks than an initial earthquake.
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