Earthquakes are very, very dangerous. They are caused by slippage in the Earth's crust. The Earth's crust is made up of several large pieces called plates. The plates move slowly and at areas where the plates meet (called faults) tension can build and when it is suddenly released an earthquake occurs. One of the most famous faults in the United States is the San Andreas Fault (pictured from the air at right), which is more than 650 miles long and extends as much as 10 miles deep.

While earthquakes alone can cause extensive damage and death, they can also trigger additional disasters. The great San Francisco earthquake of 1906 sparked many fires which may have caused more death and destruction that the actual earthquake. When earthquakes occur under the ocean they can generate large, powerful waves called tsunami which can travel across the ocean at speeds up to nearly 600 mph. Earthquakes can also cause landslides that can be more deadly than the quake itself.