Fast Facts on Earthquakes

Seismologists, the scientists who study earthquakes, estimate that several million earthquakes occur each year. Only a handful cause major damage, as the one that struck Central Asia on Oct. 8 did. Here are some facts on why earthquakes occur, how they are measured and the types of damage they can cause.

The Earth is covered by a series of tectonic plates that move slowly in different directions.

If the plates start to smash together, pressure can slowly build between two plates, known as the fault line. When the pressure is overwhelming, the rock on one side of the fault suddenly slips under the other -- an earthquake.

Some earthquakes produce a visible cracking of the planet's surface, but quakes that occur deeper in the Earth's surface may not be visible. The fault lines rumble for only a few minutes, sometimes seconds.

During an earthquake, especially larger ones, people near the location of the quake will first feel a swaying or small jerking motion, then a pause, followed by a more intense rolling or jerking motion.

Foreshocks or aftershocks may occur before and after the earthquake. They are generally much smaller in magnitude and occur more often after deep earthquakes.

Earthquakes cause vibrations, and a seismograph machine records the motion of the ground during a quake. Scientists cannot predict a quake, but they can detect smaller regional vibrations along known fault lines that may indicate a bigger quake is coming.

Earthquakes are measured according to magnitude, which is based on several factors, including the strength of surface vibrations felt near the quake and other movements detected deep in the Earth. With each point increase in magnitude, the quake's strength is 10 times larger than the previous point.

A 4.0 magnitude will be felt by many people in a quake's area, but the extent of the damage will likely be broken windows and dishes. A 6.0 is more destructive and may collapse ordinary buildings and overturn furniture. An 8.0 or higher can destroy almost all standing structures and damage bridges, dams and roads.

Since 1900, the largest measured earthquake on record was a magnitude 9.5 in Chile on May 22, 1960. The second-largest was a magnitude 9.2 quake in Alaska on March 28, 1964, the biggest ever in the United States.

Earthquakes near urban areas cause more human destruction. When an earthquake's seismic wave vibrates beneath buildings, the foundation shakes, and the motion moves upward. These upward motions can weaken or collapse man-made structures, such as homes, dams and power plants.

Sources: United States Geological Survey, the Multidisciplinary Center for Earthquake Engineering Research, Caribbean Disaster Emergency Response Agency