共用题干 Eastern Quakes Can Trigger Big Shakes
In the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes. The largest,a magnitude 5 .6 quake,shook thousands of fans in a college football stadium,caused cracks in a few buildings and rattled the nerves of 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一 hurricanes,tornadoes,flooding and wildfires,to name a few. But the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U. S. Geological Survey estimate that several million earthquakes rattie the globe each year. That may sound scary,but people don"t feel many of the tremors because they happen in remote and unpopulated regions.Many quakes happen under the ocean, and others have a very small magnitude,or shaking intensity.
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23, 2011,was felt from central Georgia to southeastern Canada. In many urban areas,including Washington, D. C.,and New York City(Wall Street shown),people crowded the streets while engineers inspected buildings.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seismometers.These devices detect and measure the size of ground vibrations
produced by earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 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.
Tectonic plates are huge pieces of Earth"s crust,sometimes many kilometers thick. These plates cover our planet"s surface like a jigsaw puzzle. Often,jagged edges of these plates temporarily lock together. When plates jostle and scrape past each other,earthquakes occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate"s edges. Although less expected,these”mid-plate” tremors can do substantial damage. Some of the biggest known examples rattled 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. Whenever tectonic plates move,earthquakes happen.