完形填空 As scientists learn more about the causes of earthquakes, they become bette

完形填空

As scientists learn more about the causes of earthquakes, they become better able to predict them．First of all, they know that quakes are 1 to occur in areas where the tectonic(地壳构造的)plates push 2 each other．They 3 these areas closely, using different instruments．They 4 the vibrations(震动)of the earth with seismographs(地震仪), which 5 on rolls of paper all movements, both large and small．Scientists usually use the Richter scale(里氏震级), 6 after a famous seismologist, for these measurements．When an earthquake measures over 4.5 on the Richter scale, it is 7 enough to cause damage．Scientists have determined that a(an)8 is not likely to have a large 9 when many small quakes have been 10．When the small quakes stop, then pressure 11 and a strong quake is more likely．

There are several other 12 to predict earthquakes．Scientists can measure the tiny 13 in the tilt(倾斜)of the land that happen before earthquakes occur．Increases in the amount of radon(氡), a substance found in deep 14, as well as a high level of water in the wells, can also 15 that an earthquake is likely to occur．In addition, scientists have found that some animals, like dogs, behave 16 before an earthquake：they become 17 and howl(嚎叫)．

Seismologists have 18 predicted several earthquakes, such as the earthquake which occurred in Zhangjiakou, China in 1998．19, not all earthquakes are predictable, and a system for predicting is not yet 20．

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