Richter magnitude scale

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The Richter magnitude scale (or more correctly local magnitude M_L scale) assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismometer output. Measurements have no limits and can be either positive or negative.

Contents 1 Development 2 Richter magnitudes 3 Problems with the Richter scale 4 See also 5 References 6 External links

Development

Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg, both of the California Institute of Technology, the 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 originally reported values to the nearest quarter of a unit, but decimal numbers were used later. His motivation for creating the local magnitude scale was to separate the vastly larger number of smaller earthquakes from the few 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 100 km from the earthquake epicenter. This choice was intended to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit, and 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 M_L 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 M_S and the body wave magnitude m_b. These traditional magnitude scales have largely been superseded by the implementation of methods for estimating the seismic moment and its associated moment magnitude scale.

Richter magnitudes

The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 31 times the amount of energy released.

Events with magnitudes of about 4.6 or greater are strong enough to be recorded by any of the seismographs in 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, the depth of the earthquake's focus beneath the epicenter, and geological conditions (certain terrains can amplify seismic signals).

Description	Richter Magnitudes	Earthquake Effects	Frequency of Occurrence
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 per year (est.)
Light	4.0-4.9	Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.	6,200 per year (est.)
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 per year
Strong	6.0-6.9	Can be destructive in areas up to about 100 miles across in populated areas.	120 per year
Major	7.0-7.9	Can cause serious damage over larger areas.	18 per year
Great	8.0-8.9	Can cause serious damage in areas several hundred miles across.	1 per year
Rarely, great	9.0-9.9	Devastating in areas several thousand miles across.	1 per 20 years
Meteoric	10.0+	Never recorded; see below for equivalent seismic energy yield.	Unknown

(Adapted from U.S. Geological Survey documents.)

Great earthquakes occur once a year, on average. The largest recorded earthquake was the Great Chilean Earthquake of May 22, 1960 which had a magnitude (M_W) of 9.5 (Chile 1960).

The following table needs review. It currently lists the approximate energy equivalents in terms of TNT explosive force ^[1] - though note that the energy here is that of the underground energy release (ie a small atomic bomb blast will not simply cause light shaking of indoor items) rather than the overground energy release; the majority of energy transmission of an earthquake is not transmitted to and through the surface, but is instead dissipated into the crust and other subsurface structures.

Richter Approximate Magnitude	Approximate TNT for Seismic Energy Yield	Example
0.5	5.6 kg (12.4 lb)	Hand grenade
1.0	32 kg (70 lb)	Construction site blast
1.5	178 kg (392 lb)	WWII conventional bombs
2.0	1 metric ton	late WWII conventional bombs
2.5	5.6 metric tons	WWII blockbuster bomb
3.0	32 metric tons	Massive Ordnance Air Blast bomb
3.5	178 metric tons	Chernobyl nuclear disaster, 1986
4.0	1 kiloton	Small atomic bomb
5.0	32 kiloton	Nagasaki atomic bomb
5.5	178 kilotons	Little Skull Mtn., NV Quake, 1992
6.0	1 megaton	Double Spring Flat, NV Quake, 1994
6.5	5.6 megatons	Northridge quake, 1994
7.0	50 megatons	Tsar Bomba, largest thermonuclear weapon ever tested (magnitude seen on seismographs reduced because detonated 4 km in the atmosphere.)
7.5	178 megatons	Landers, CA Quake, 1992
8.0	1 gigaton	San Francisco, CA Quake, 1906
9.0	5.6 gigatons	Anchorage, AK Quake, 1964
9.3	32 gigatons	2004 Indian Ocean earthquake
10.0	1 teraton	estimate for a 20 km rocky bolide impacting at 25 km/s

Problems with the Richter scale

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 near 6.3-6.5, owing to the scaling law of earthquake spectra, that causes traditional magnitude methods (such as M_S) 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 1979, seismologists Tom Hanks and Hiroo Kanamori, also of the California Institute of Technology, proposed the moment magnitude scale (M_W), which provides a way of expressing seismic moments in a form that can be approximately related to traditional seismic magnitude measurements.

See also

• Seismic scale
• Japan Meteorological Agency seismic intensity scale

References

1. ^ What is Richter Magnitude?, with mathematic equations

Wikipedia information about Richter magnitude scale This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Richter magnitude scale". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.