On October 17, 1989, a giant earthquake shook Northern California. Bridges, roadways, and buildings collapsed throughout the area. Scores of people were killed, and thousands more were injured or left homeless.

You might be surprised to know it wasn't the only quake to rattle the earth that day. Each day, more than 1,000 quakes occur in the world. People don't notice most quakes, though. Many occur far underground or beneath the ocean. Others are too small for anyone to feel.

Why do quakes occur? The earth's surface is actually made of large pieces, or plates. Sometimes, the plates grind against each other. Pressure builds underneath them. Finally, the pressure escapes through cracks, or faults, between the plates. It causes the surface of the earth to shake.

To describe the strength of quakes, scientists use a scale of numbers called the Richter scale. The Richter scale grows by powers of 10. An increase of 1 point means the strength of a quake is 10 time greater than the level before it. Here's how it works:

An earthquake registering 2.0 on the Richter scale is 10 times stronger than a quake registering 1.0. A quake registering 3.0 is 10 X 10 or 100 times stronger than a quake registering 1.0 A 4.0 is 10 X 10 X 10 or 1,000 times greater than 1.0 and so on.



Scholastic Math, December 1989.