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  3. How Earthquakes Work An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of th

How Earthquakes Work An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of th

admin2009-04-23  30
问题                                         How Earthquakes Work
    An earthquake is one of the most terrifying phenomena that nature can dish up. We generally think of the ground we stand on as "rock-solid" and completely stable. But an earthquake can shatter that perception instantly. Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused earthquakes. Even today there is still a certain amount of mystery surrounding them, but scientists have a much clearer understanding. There has been enormous progress in the past century: Scientists have identified the forces that cause earthquakes, and developed technology that can tell us an earthquake’s magnitude and origin. The next hurdle is to find a way of predicting earthquakes.
Shaking Ground
    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, as we’ll see in the next section.
    We only hear about earthquakes in the news every once in a while, but they are actually an everyday occurrence on our planet. According to the United States Geological Survey, more than three million earthquakes occur every year. That’s about 8000 a day, or one every 11 seconds! The vast majority of these 3 million quakes are extremely weak.
Sliding Plates
    The biggest scientific breakthrough in the history of seismology(地震学)—the study of earthquakes came in the middle of the 20th century, with the development of the theory of plate tectonics(板块构造). The basic theory is that the surface layer of the earth—the lithosphere—is comprised of many plates that slide over the lubricating mantle(地幔) layer. At the boundaries between these huge plates of soil and rock, three different things can happen:
    -Plates can move apart—If two plates are moving apart from each other, hot, molten rock flows up from the layers of mantle below the lithosphere.
    -Plates can push together—If the two plates are moving toward each other, one plate typically pushes under the other one. At some boundaries where two plates meet, neither plate is in a position to subduct under the other, so they both push against each other to form mountains.
    -Plates slide against each other—At other boundaries, plates simply slide by each other—they are pushed tightly together. A great deal of tension builds at the boundary.
    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.
Faults
    Scientists identify four types of faults, characterized by the position of the fault plane, the break in the rock and the movement of the two rock blocks:
    -In a normal fault (see animation below), the fault plane is nearly vertical. These faults occur where the crust is being pulled apart, due to the pull of a divergent plate boundary.
    -The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up and the footwall pushes down. This sort of fault forms where a plate is being compressed.
    -A thrust fault moves the same way as a reverse fault, but the fault line is nearly horizontal. This is the sort of fault that occurs in a converging plate boundary.
    -In a strike-slip fault, the blocks of rock move in opposite horizontal directions. These faults form when the crust pieces are sliding against each other, as in a transform plate boundary.
    In all of these types of faults, the different blocks of rock push very tightly together, creating a good deal of friction as they move. If this friction level is high enough to keep the blocks from sliding against each other, the forces in the plates will continue to push the rock, increasing the pressure applied at the fault. If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks will suddenly snap forward.
Making Waves
    When a sudden break or shift occurs in the earth’s crust, the energy radiates out as seismic waves(地震波),just as the energy from a disturbance in a body of water radiates out in wave form. Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth. Surface waves sometimes called long waves, or simply L waves—are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations.
    Surface waves are something like the waves in a body of water—they move the surface of the earth up and down. This generally causes the worst damage because the wave motion rocks the foundations of manmade structures. L waves are the slowest moving of all waves, so the most intense shaking usually comes at the end of an earthquake.
Rating Magnitude and Intensity
    Whenever a major earthquake is in the news, you’ll probably hear about its Richter Scale rating. You might also hear about its Mercalli Scale rating, though this isn’t discussed as often. These two ratings describe the power of the earthquake from two different perspectives.
    The Richter Scale is used to rate the magnitude of an earthquake—the amount of energy it released. The Richter Scale is logarithmic, meaning that whole-number jumps indicate a tenfold increase. That is, the wave amplitude(振幅) 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.
    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. The extent of damage is rated on the Mercalli Scale.
     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.
Dealing with Earthquakes
    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.
    Another component of preparedness is educating the public. The United States Geological Survey(USGS) and other government agencies have produced several brochures explaining the processes involved in an earthquake and giving instructions on how to prepare your house for a possible earthquake, as well as what to do when a quake hits.
    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. All we can do is to increase our understanding of the phenomenon and develop better ways to deal with it.
选项 A、Y
B、N
C、NG
答案B
解析 由题干中的nowadays定位到首段第5句Even today...，再由made...progress定位到首段倒数第2句， There has been enormous progress in the past century...题干中的have not made huge progress与文中There has been enormous progress语义相反，尽管第5句中 there is still a certain amount of mystery与they have not fully understood属同义转述，但文中该句强调的是科学家已经对地震有了很清楚的了解。注意but转折处易设题。
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