Perhaps you remember being in an earthquake--the ground rumbles, hanging lamps begin to sway back and forth, shelves begin to rattle or spill their contents, the floor and walls shake. Even if you do not remember seeing or feeling an earthquake, you have probably lived through thousands of tiny earthquakes during your lifetime. The earth is constantly creating earthquakes. Earthquakes are among the most devastating natural events that occur on Earth and are a reminder that our planet is a dynamic, changing body.

An earthquake is the shaking of the earth caused by pieces of the crust of the earth that suddenly shift. The crust, the thin outer layer, is mostly cold and brittle rock compared to the rock deeper inside. The most common cause of earthquakes is faulting. A fault is a break in the earth's crust along which movement occurs. The study of this movement is known as plate tectonics.

Most earthquakes occur in narrow belts along the boundaries of crustal plates, particularly where the plates push together or slide past each other. At times, the plates are locked together, unable to release the accumulating energy. When this energy grows strong enough, the plates break free. When two pieces that are next to each other get pushed in different directions, they will stick together for a long time (many years), but eventually the forces pushing on them will force them to break apart and move. This sudden shift in the rock shakes all of the ground around it.

Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. The New Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred within the North American plate.

Earthquakes in History

The scientific study of earthquakes is comparatively new. Until the 18th century, few factual descriptions of earthquakes were recorded, and the natural cause of earthquakes was not very well understood. Those who did look for natural causes often reached conclusions that seem ridiculous today; one popular theory was that earthquakes were caused by air rushing out of caverns deep in the Earth's interior.

The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years. Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occured in the mid-16th century.

The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is estimated to be about 8, occurred on the morning of December 16, 1811. Another great earthquake occurred on January 23, 1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continuous between these great earthquakes and continued for months afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense effects were in a sparsely populated region, the destruction of human life and property was slight. If just one of these enormous earthquakes occurred in the same area today, millions of people and buildings and other structures worth billions of dollars would be affected.

The San Francisco earthquake of 1906 was one of the most destructive in the recorded history of North America. The earthquake and the fire that followed killed nearly 700 people and left the city in ruins. The Alaska earthquake of March 27, 1964, was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of almost 500,000 square miles. The ground motion near the epicenter was so violent that the tops of some trees were snapped off. One hundred and fourteen people (some as far away as California) died as a result of this earthquake, but loss of life and property would have been far greater had Alaska been more densely populated.

Predicting Earthquakes

Since the 1970's, seismologists have been testing different ways of predicting earthquakes. The goal is to be able to predict the location, strength, and time of occurrence of a particular earthquake. Scientists believe that an earthquake will occur when stress in the earth at a given place is larger than the rock's strength. Unfortunately, both stress and strength are very difficult to measure, especially deep within the Earth. Scientists are looking for easier ways to measure changes in stress and strength, such as a sudden lowering of ground water levels, tilts and bulges in the Earth's surface, changes in the speed of P and S waves, increases of rare gases in well water, changes in Earth's magnetic field. Unfortunately, none of these have been shown to be consistent predictors or earthquakes in general.

Attempts at long-term prediction of the very largest earthquakes along the world's plate boundaries have been somewhat more successful. The goal is to identify areas of plate boundaries that are likely to experience great earthquakes in the next decade or so. The speed of movement of the plates and a pattern of earthquakes are used to locate areas where stress is high enough to possibly cause an earthquake to occur. A successful long-term prediction of the 1989 Loma Prieta, California earthquake (M=6.9) was made several years before it occurred.