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Richter's local magnitude scale was originally intended to be used only in a particular study area in California, and on seismograms recorded on a particular instrument, the Wood-Anderson torsion seismometer.
Richter magnitude is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. The diminution of amplitude due to distance between the earthquake epicenter and the seismometer is corrected for by subtracting the logarithm of the expected amplitude of a magnitude 0 event at that distance. This correction for distance is intended to make the local magnitude an absolute measure of earthquake size. Richter originally reported values to the nearest quarter of a unit but later, decimal numbers were used.
Richter's motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the relatively fewer larger earthquakes observed in California at the time. His inspiration for the technique was the stellar magnitude scale used in astronomy to describe the brightness of stars and other celestial objects.
Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 micrometre on a seismogram recorded using a Wood-Anderson torsion seismometer located 100 km from the earthquake epicenter. This choice was made to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit. Sensitive modern seismographs now routinely record quakes with negative magnitudes.
Because of the limitations of the Wood-Anderson torsion seismometer used to develop the scale, the original ML cannot be calculated for events larger than about 6.8. Many investigators have proposed extensions to the local magnitude scale, the most popular being the surface wave magnitude MS and the body wave magnitude Mb.
The major problem with Richter magnitude is that it is not easily related to physical characteristics of the earthquake source. Furthermore, there is a saturation effect at near 8.3-8.5, owing to the scaling law of earthquake spectra , that causes traditional magnitude methods (such as MS) to yield the same magnitude estimate for events that are clearly of different size. By the beginning of the 21st century, most seismologists considered the traditional magnitude scales to be largely obsolete, being replaced by a more physically meaningful measurement called the seismic moment which is more directly relatable to the physical parameters, such as the dimension of the earthquake rupture , and the energy released from the earthquake. In 1979Events January-February January 1 Sino-American relations: United States and the People's Republic of China establish diplomatic relations January 4 State of Ohio agrees to pay $675,000 to families of dead and injured in Kent State University shootings. seismologist Hiroo KanamoriHiroo Kanamori (born October 17, 1936) is an earthquake seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them. His most visible public contribution has been the moment ma, also of the California Institute of Technology, proposed the Moment Magnitude Scale (MW), which provides a way of expressing seismic moments in a form that can be approximately related to traditional seismic magnitude measurements.
Magnitude must not be confused with intensityIn physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of. Intensity scales, such as the Rossi-Forel and Modified Mercalli Intensity ScaleThe Mercalli Intensity Scale is a semi-quantitative scale used to classify the intensity of an earthquake by examining its effects on people and structures at the Earth's surface. It was conceived by Italian volcanologist Giuseppe Mercalli in 1902, and wa, are used to describe relative earthquake effects. Intensity is sensitive to a host of local site conditions and is not an absolute measurements of earthquake size.
Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world.
The following describes the typical effects of earthquakes of various magnitudes near the epicenter. This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the magnitude, but also on the distance to the epicenter, and geological conditions (certain terrains can amplify seismic signals).
| Descriptor | Richter Magnitudes | Earthquake Effects | Average Annually |
|---|---|---|---|
| Micro | Less than 2.0 | Microearthquakes, not felt. | About 8,000 per day |
| Very minor | 2.0-2.9 | Generally not felt, but recorded. | About 1,000 per day |
| Minor | 3.0-3.9 | Often felt, but rarely causes damage. | 49,000 (estimated) |
| Light | 4.0-4.9 | Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. | 6,200 (estimated) |
| Moderate | 5.0-5.9 | Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. | 800 |
| Strong | 6.0-6.9 | Can be destructive in areas up to about 100 miles across in populated areas. | 120 |
| Major | 7.0-7.9 | Can cause serious damage over larger areas. | 18 |
| Great | 8.0 or greater | Can cause serious damage in areas several hundred miles across. | 1 |
(Adapted from U.S. Geological Survey documents.)
Great earthquakes occur once a year, on average. The largest recorded earthquake was Great Chilean EarthquakeThe Great Chilean Earthquake of May 22, 1960, was an earthquake measuring 9. 5 on the Richter scale (9. It affected southern Chile. Its epicenter was located in Valdivia, Chile, approximately 700 kilometers (435 miles) south of Santiago. The earthquake ca of May 22May 22 is the 142nd day of the year in the Gregorian calendar (143rd in leap years). There are 223 days remaining. Events 1176 Murder attempt by the Hashshashin (Assassins) on Saladin near Aleppo. 1455 Wars of the Roses: First Battle of St Albans Richard,, 1960Events January-February January 1 Independence of Cameroon January 9 Aswan High Dam construction begins in Egypt January 11 Chad declares its independence. January 14 Ralph Chubb, the gay poet and printer, dies at Fair Oak Cottage in Hampshire. January 23 which had a magnitude (MW) of 9.5 ( Chile 1960).
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