Richter scale: Difference between revisions

Prior to the development of the magnitude scale, the only measure of an earthquake's strength or "size" was a subjective assessment of the intensity of shaking observed near the [[epicenter]] of the earthquake, categorized by various [[seismic intensity scales]] such as the [[Rossi-Forel scale]]. ("Size" is used in the sense of the quantity of energy released, not the size of the area affected by shaking, though higher-energy earthquakes do tend to affect a wider area, depending on the local geology.) In 1883, [[John Milne]] surmised that the shaking of large earthquakes might generate waves detectable around the globe, and in 1899 E. Von Rehbur Paschvitz observed in Germany seismic waves attributable to an earthquake in [[Tokyo]].<ref>{{Harvnb|Bolt|1993|p=47}}.</ref> In the 1920s, [[Harry O. Wood]] and [[John August Anderson|John A. Anderson]] developed the ''Wood–Anderson [[seismometer|Seismograph]]''seismograph, one of the first practical instruments for recording seismic waves.<ref>{{Harvnb|Hough|2007|p=}};</ref> Wood then built, under the auspices of the [[California Institute of Technology]] and the [[Carnegie Institution for Science|Carnegie Institute]], a network of seismographs stretching across [[Southern California]].<ref>{{Harvnb|Hough|2007|p=57}}.</ref> He also recruited the young and unknown Charles Richter to measure the seismograms and locate the earthquakes generating the seismic waves.<ref>{{Harvnb|Hough|2007|pp=57, 116}}.</ref>

In 1931, [[Kiyoo Wadati]] showed how he had measured, for several strong earthquakes in Japan, the amplitude of the shaking observed at various distances from the epicenter. He then plotted the logarithm of the amplitude against the distance and found a series of curves that showed a rough correlation with the estimated magnitudes of the earthquakes.<ref>{{Harvnb|Richter|1935|p=2}}.</ref> Richter resolved some difficulties with this method<ref>{{Harvnb|Richter|1935|pp=1–5}}.</ref> and then, using data collected by his colleague [[Beno Gutenberg]], he produced similar curves, confirming that they could be used to compare the relative magnitudes of different earthquakes.<ref>{{Harvnb|Richter|1935|pp=2–3}}.</ref>

ToAdditional developments were required produce a practical method of assigning an absolute measure of magnitude required additional developments. First, to span the wide range of possible values, Richter adopted Gutenberg's suggestion of a [[logarithm]]ic scale, where each step represents a tenfold increase of magnitude, similar to the magnitude scale used by astronomers [[Apparent magnitude|for star brightness]].<ref>[pending]</ref> Second, he wanted a magnitude of zero to be around the limit of human perceptibility.<ref>{{Harvnb|Richter|1935|p=14}}: {{Harvnb|Gutenberg|Richter|1936|p=183}}.</ref> Third, he specified the Wood–Anderson seismograph as the standard instrument for producing seismograms. Magnitude was then defined as "the logarithm of the maximum trace amplitude, expressed in [[microns]]", measured at a distance of {{Convert|100|km|mi|abbr=on}}. The scale was calibrated by defining a magnitude 0 shock as one that produces (at a distance of {{Convert|100|km|mi|abbr=on}}) a maximum amplitude of 1&nbsp;micron (1&nbsp;µm, or 0.001&nbsp;millimeters) on a seismogram recorded by a {{ill|Wood-Anderson torsion seismometer|pt|Sismógrafo de torção de Wood-Anderson|lt=Wood–Anderson torsion seismograph}}.<ref>{{Harvnb|Richter|1935|p=5}}. See also {{Harvnb|Hutton|Boore|1987|p=1}}; {{Harvnb|Chung|Bernreuter|1980|p=10}}.</ref> Finally, Richter calculated a table of distance corrections,<ref>{{Harvnb|Richter|1935|p=6}}, Table I.</ref> in that for distances less than 200 kilometers<ref>{{Harvnb|Richter|1935|p=32}}.</ref> the attenuation is strongly affected by the structure and properties of the regional geology.<ref>{{Harvnb|Chung|Bernreuter|1980|p=5}}.</ref>