The Richter Scale

By Nancy Steinbach

This is the VOA Special English SCIENCE REPORT.

A powerful earthquake shook western India late last month. Thousands of people were killed. The quake mainly affected the Indian state of Gujarat, but was felt in Pakistan, Bangladesh and Nepal.

Earthquakes are caused by movements of the surface of the Earth. Scientists measure their power on the Richter Scale. The recent earthquake in India measured seven-point-seven.

The Richter Scale was developed in Nineteen-Thirty-Five by Charles Richter of the California Institute of Technology. It measures the amount of energy released by an earthquake.

Energy flows from the center of an earthquake in waves. Scientists measure the waves with a device called a seismograph. A special pen is connected to the device. It moves across a piece of paper whenever the Earth moves.

Scientists use seismographs to measure movements in areas of the Earth that have suffered earthquakes before. They find the center of a new earthquake by studying seismographs operating near it. They use the devices to learn the strength of the earthquake at its center. They rate it on the Richter Scale.

An earthquake with a rating of two or less is not usually felt by people and is recorded only on local seismographs. A rating of more than six means the earthquake is strong enough to destroy buildings.

Each step on the Richter Scale represents a big increase in power. For example, an earthquake rated five is ten times stronger than an earthquake rated four. A quake rated six is ten times stronger than one rated five. . . and so on.

The Richter Scale has no upper limit. Scientists sometimes discuss the size of earthquakes as moderate, large, major or great. A moderate earthquake measures five on the Richter Scale. A large one measures six. A major earthquake measures seven. And a great earthquake measures eight or higher. The largest known earthquakes have been measured between eight-point-eight and eight-point-nine.

The United States National Earthquake Information Center reports about earthquakes all over the world. It says there are between twelve-thousand and fourteen-thousand earthquakes each year. Scientists say they expect about eighteen major earthquakes and one great earthquake in any one year.

This VOA Special English SCIENCE REPORT was written by Nancy Steinbach.