The Richter scale: what it is and what it measures
28 Jan 2001 By Clarence Fernandez
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SINGAPORE (Reuters) - Most people have heard of the Richter scale -- the measure of the strength of an earthquake -- but what does it really measure and what does it mean? The U.S. Geological Survey estimated that the January 26 earthquake, feared to have killed 15,000 in India, measured 7.9 on the scale. The quake which killed 2,400 people in Taiwan in September 1999 measured 7.6 -- so does that mean the Indian quake was just slightly stronger than the one in Taiwan? The answer is no. The Indian quake was almost twice as strong as the one in Taiwan, according to the Richter calculation. "The Richter scale is a logarithmic scale," seismic analyst Bill Smith at the Survey's National Earthquake Information Centre in Colorado told Reuters. "So each (whole) unit on the scale represents 10 times the ground shaking of the next lower (whole) number. "A 3/10s difference on the scale represents a factor of about two in the amplitude, or shaking motion, which is how far the ground actually moves." As the strength of an earthquake rises by each whole number up the Richter scale, the quake releases 31 times more energy. Introduced in 1935, the scale is named for American physicist Charles F. Richter of the California Institute of Technology, who evolved it from patterns discovered by studying more than 200 earthquakes a year. The Richter scale is a more objective, quantitative basis of measuring earthquakes than the other widely-used standard, the 12-point Mercalli scale, the U.S. Geological Survey says. The Richter scale does not measure an earthquake's effects, but gives its strength in terms of the energy released, as measured by seismographs. NO UPPER LIMIT The scale starts at one and has no upper limit; each unit is 10 times greater than the one before. Mathematicians call this method of arranging numbers on a scale logarithmic. For instance, the Survey says, a magnitude of 5.3 might be calculated for a moderate earthquake, and a strong earthquake might be classed as magnitude 6.3. Last week's earthquake in India it has classed as major. But because the Richter scale has a logarithmic basis, each whole number increase in magnitude on the scale represents a 10-fold increase in an earthquake's measured amplitude. A quake's strength is ranked on the basis of the maximum amplitude of the signal recorded by a seismograph and how far the instrument is from the earthquake. Today, state-of-the-art seismic systems use telephone lines and satellites to link to a central computer, which churns out preliminary estimates within minutes. Earthquakes with a magnitude of about 2.0 or less are called microearthquakes; people don't usually feel them and they are usually tracked only by local seismographs. Events ranking about 4.5 or greater -- of which there may be several thousand every year -- are strong enough to be recorded by sensitive instruments all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. The San Francisco earthquake of 1906 has since been calculated at 8.3. Another in Mexico in 1985 registered 8.1 on the scale. On average, one earthquake of such size occurs somewhere in the world each year, the U.S. Geological Survey says. The Richter scale has no upper limit, but the largest known shocks have had magnitudes in the 8.8 to 8.9 range. In 1950, a quake measuring 8.6 on the scale struck northeast India, killing hundreds of people. WORST QUAKE IN HISTORY In a quake measuring possibly nine on the scale, more than 830,000 people died when the most devastating earthquake in recorded history hit China's Shaanxi province in 1556. China suffered the worst quake in its modern history on July 28, 1976 when the northeastern city of Tangshan was completely razed, and at least 240,000 people, possibly hundreds of thousands more, were killed. The Richter reading was 7.8. By contrast, the Mercalli scale, is more subjective in assessing the impact of earthquakes. For example, an earthquake registering five on the Mercalli scale is defined as having made furniture shake and churchbells ring, but having triggered little or no damage. But an earthquake that registered 12 on the Mercalli scale would have destroyed all man-made objects, and created new topography by forming new lakes, huge falls of rock and major earth faults. Russia also has a 12-point scale, and Japan a seven-point system.
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