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 Earths tectonic plates.
Tectonic plates are huge pieces of Earths crust, sometimes many kilometers thick. These plates cover our planets 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 about the same speed as your fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plates 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.
16. Oklahoma is an area often experiencing natural disasters.
A. Right B. Wrong C. Not mentioned
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