Natural events such as volcanic eruptions and meteor impacts can cause earthquakes, but the majority of naturally-occurring earthquakes are triggered by movement of the earth's plates.
The surface of the earth is made up of 20 constantly moving plates. As the plates shift, tension is created, and as its strength increases it can cause the crust to break. When a break occurs, the stress is released as energy that moves through the Earth in the form of waves. These waves are earthquakes.
The National Earthquake Information Center (NEIC) records an average of 20,000 earthquakes every year (about 50 a day) around the world. There are, however, millions of earthquakes estimated to occur every year that are too weak to be recorded.
Almost 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire"; a region that encircles the Pacific Ocean and is home to 452 volcanoes (over 75 percent of the world's active and dormant volcanoes).
Each year the southern California area has about 10,000 earthquakes; the majority of which go unnoticed. If there is a large earthquake, however, the aftershock sequence will produce many more earthquakes of all magnitudes for months.
The largest recorded earthquake in the United States was a magnitude 9.2 that struck Prince William Sound, Alaska on March 28, 1964.
The largest recorded earthquake in the world was a magnitude 9.5 in Chile on May 22, 1960.
When the Chilean earthquake occurred in 1960, seismographs recorded seismic waves that traveled around the world. These seismic waves shook the entire earth for many days.
Normally, it's not the shaking ground itself that claims lives during an earthquake; it's the associated destruction of man-made structures and the instigation of other natural disasters such as tsunamis, avalanches, and landslides.
An undersea earthquake in the Indian Ocean triggered a series of devastating tsunamis on Dec. 26, 2004. The tsunamis struck along the coasts of most landmasses bordering the Indian Ocean, killing more than 225,000 people in 11 countries and inundating coastal communities with waves up to 100 feet high.
Alaska is the most earthquake-prone state and one of the most seismically active regions in the world, experiencing a magnitude 7.0 earthquake almost every year, and a magnitude 8.0 or greater earthquake approximately once every 14 years.