Surface wave magnitude

Surface wave magnitude

The Surface wave magnitude (MS) is one of the magnitude scales used in seismology to measure the "size" of an earthquake. It is based on measurements in Rayleigh surface waves that primarily travel along the uppermost layers of the earth. It is currently used in People's Republic of China as a national standard (GB 17740-1999) for measuring earthquake impacts.cite web
publisher=General Administration of Quality Supervision, Inspection, and Quarantine of P.R.C.
author=XU Shaokui, LU Yuanzhong, GUO Lucan, CHEN Shanpei, XU Zhonghuai, XIAO Chengye, FENG Yijun (许绍燮、陆远忠、郭履灿、陈培善、许忠淮、肖承邺、冯义钧)
url=http://www.dccdnc.ac.cn/html/zcfg/gfxwj1.jsp
title=Specifications on Seismic Magnitudes (地震震级的规定)
date=1999-04-26
accessdate=2008-09-14
language=Chinese]

Surface wave magnitude was initially developed in 1950's by the same researchers who developed the local magnitude scale ML in order to improve resolution on larger earthquakes:cite paper
publisher=USGS
author=William L. Ellsworth
url=http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm
title=SURFACE-WAVE MAGNITUDE (MS) AND BODY-WAVE MAGNITUDE (mb)
date=1991
accessdate=2008-09-14] Quotation|The successful development of the local-magnitude scale encouraged Gutenberg and Richter to develop magnitude scales based on teleseismic observations of earthquakes. Two scales were developed, one based on surface waves, MS, and one on body waves, mb.

Surface waves with a period near 20 s generally produce the largest amplitudes on a standard long-period seismograph, and so the amplitude of these waves is used to determine MS, using an equation similar to that used for ML.| William L. Ellsworth |The San Andreas Fault System, California (USGS Professional Paper 1515), 1990-1991

Recorded magnitudes of earthquakes during that time, commonly attributed to Richter, could be either MS or ML.

Definition

The formula to calculate surface wave magnitude is: [It is obvious that the entire formula cannot stand dimensional analysis without additional qualifications. References here provide no such qualification.] M = lg(A/T)max + σ(Δ)where A is the maximum particle displacement in surface waves (vector sum of the two horizontal displacements) in μm, T is the corresponding period in s, Δ is the epicentral distance in °, and σ(Δ) = 1.66 lg(Δ) + 3.5According to GB 17740-1999, the two horizontal displacements must be measured at the same time or within 1/8 of a period; if the two displacements have different periods, weighed sum must be used: T = (TNAN + TEAE) / (AN + AE)where AN is the north-south displacement in μm,　AE is the east-west displacement in μm,　TN is the period corresponding to AN in s, and TE is the period corresponding to AE in s.

Other studies

Vladimír Tobyáš and Reinhard Mittag proposed to relate surface wave magnitude to local magnitude scale ML, usingcite paper
publisher=Studia Geophysica et Geodaetica
author=Vladimír Tobyáš and Reinhard Mittag
url=http://www.springerlink.com/content/lv140444x5m01362/
title=Local magnitude, surface wave magnitude and seismic energy
date=1991-02-06
accessdate=2008-09-14] Ms = –3.2 + 1.45 ML.Other formulas include three revised formulae proposed by CHEN Junjie et al: [cite paper
publisher=Journal of Seismological Research (《地震研究》)
author=CHEN Junjie, CHI Tianfeng, WANG Junliang, CHI Zhencai (陈俊杰, 迟天峰, 王军亮, 迟振才)
url=http://scholar.ilib.cn/Abstract.aspx?A=dzyj200201016
title=Study of Surface Wave Magnitude in China (中国面波震级研究)
date=2002-01-01
accessdate=2008-09-14
language=Chinese] Ms = log (Amax/T) + 1.541og△ + 3.53 Ms = log (Amax/T) + 1.731og△ + 3.27and Ms = log (Amax/T) -6.21og△ + 20.6

ee also

*Moment magnitude
*Seismic scale

Notes and References

External links

*cite paper
publisher=USGS
author=Robert E. Wallace, ed.
url=http://www.johnmartin.com/earthquakes/eqsafs/
title=The San Andreas Fault System, California (Professional Paper 1515)
date=1991
accessdate=2008-09-14
* [http://earthquake.usgs.gov/learning/glossary.php?termID=118 Visual Glossary - magnitude] - USGS
* [http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/earthquake_size.html Earthquake Size]


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