Eastern Quakes

共享题干题【2012年真题】（一） Eastern Quakes Can Trigger Big Shake
   In the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes. The largest, a magnitude(量)5.6 quake, shook thousands of fans in a college football stadium, caused cracks in a few buildings and scared many people who had never felt a quake before. Oklahoma is not an area of the country famous for its quakes. If you watch the news on TV, you see reports about all sorts of natural disasters. But the most dangerous type of natural disaster, and also the most unpredictable, is the earthquake.
    Researchers at the US Geological Survey(USGS)estimate that several million earthquakes occur globally each year. That may sound scary, but people don't feel many of them because they happen in remote and unpopulated regions. Many quakes happen under the ocean, and others have a very small magnitude.
    Scientists know about small, remote quakes only because of very sensitive electronic devices called seismometers(地震仪). These devices detect and measure the size of ground vibrations(震颤)produced by earthquakes. Altogether, USGS researchers use seismometers to identify and locate about 20000 earthquakes each year. 
   Although earthquakes can happen anywhere in the world, really big quakes occur only in certain areas. The largest ones register a magnitude 8 or higher and happen, on average, only once each year. Such big ones typically occur along the edges of Earth's tectonic plates(构造板块).
   Thctonic plates are huge pieces of Earth's crust(外壳), sometimes many kilometers thick. Often, edges of these plates temporarily lock together. When plates push and scrape(擦)past each other earthquakes occur. On average, tectonic plates move very slowly--bout the same speed as your fingernails grow. 
   But sometimes earthquakes rumble(轰轰作响)through portions of the landscape far from a plate's edges. Although less expected, these "mid-plate" small earthquakes can be stantial damage. Some of the biggest known examples hit the eastern half of the United States two centuries ago. Today, scientists are still puzzling over why the quakes occurred and when similar ones might occur.