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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 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 rattle the globe each year. That may sound scary, but people dont 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 , people crowded the streets while engineers inspected buildings. Credit: Wikimedia/Alex Tabak 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. | 
