Seismic scale

A seismic scale is used to describe the strength or "size" of an earthquake. There are two types of scales: intensity scales that describe the intensity or severity of ground shaking (quaking) at a given location, and magnitude scales that measure the strength of the seismic event itself, usually based on an instrumental record. The intensity and nature of ground shaking depends on the local geology; at a given locality the intensity of shaking depends on the magnitude and distance of the seismic event.

Earthquake magnitude and ground-shaking intensity

The Earth's crust is stressed by tectonic forces. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking.

Magnitude is an estimate of the relative "size" or strength of an earthquake, and thus its potential for causing ground-shaking. It is "approximately related to the released seismic energy."[1]

Isoseismal map for the 1968 Illinois earthquake

Intensity refers to the strength or force of shaking at a given location, and can be related to the peak ground velocity. Prior to the development of strong-motion accelerometers that can measure peak ground speed directly, the intensity of the earth-shaking was estimated on the basis of the observed effects (damage), as categorized on various intensity scales. With an isoseismal map of the observed intensities (see illustration) an earthquake's magnitude can be estimated from both the maximum intensity observed (usually but not always near the epicenter), and from the extent of the area where the earthquake was felt.[2]

The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake,[3] one of the most important being soil conditions. For instance, thick layers of soft soil (such as fill) can amplify seismic waves, often at a considerable distance from the source, while sedimentary basins will often resonate, increasing the duration of shaking. This is why, in the 1989 Loma Prieta earthquake, the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter.[4] Geological structures were also significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland. A similar effect channeled seismic waves between the other major faults in the area.[5]

Seismic intensity scales

The first simple classification of earthquake intensity was devised by Domenico Pignataro in the 1780s.[6] However, the first recognisable intensity scale in the modern sense of the word was drawn up by P.N.G. Egen in 1828; it was ahead of its time. The first widely adopted intensity scale, the Rossi–Forel scale, was introduced in the late 19th century. Since then numerous intensity scales have been developed and are used in different parts of the world.

Country/Region Seismic intensity scale used
 China Liedu scale (GB/T 17742-1999)
 Europe European Macroseismic Scale (EMS-98)[7]
 Hong Kong Modified Mercalli scale (MM)[8]
 India Medvedev–Sponheuer–Karnik scale
 Israel Medvedev–Sponheuer–Karnik scale (MSK-64)
 Japan Shindo scale
 Kazakhstan Medvedev–Sponheuer–Karnik scale (MSK-64)
 Philippines PHIVOLCS Earthquake Intensity Scale (PEIS)
 Russia Medvedev–Sponheuer–Karnik scale (MSK-64)
 Taiwan Shindo scale
 United States Modified Mercalli scale (MM)[9]

Unlike magnitude scales, intensity scales do not have a mathematical basis; instead they are an arbitrary ranking based on observed effects. Most seismic intensity scales have twelve degrees of intensity and are roughly equivalent to one another in values but vary in the degree of sophistication employed in their formulation.

Magnitude scales

Typical seismogram. The compressive P-waves (following the red lines) – essentially sound passing through rock — are the fastest seismic waves, and arrive first, typically in about 10 seconds for an earthquake around 50 km away. The sideways-shaking S-waves (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves; the delay is a direct indication of the distance to the quake. S-waves may take an hour to reach a point 1000 km away. Both of these are body-waves, that pass directly through the earth's crust. Following the S-waves are various kinds of surface-waves — Love waves and Rayleigh waves — that travel only at the earth's surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes — less than roughly 60 km deep — the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake, and cause the most severe damage.

An earthquake radiates energy in the form of different kinds of seismic waves, whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through.[10] Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram, and then measuring one or more characteristics of a wave, such as its timing, orientation, amplitude, frequency, or duration.[11] Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram.

The various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter, and are adjusted so the mid-range approximately correlates with the original "Richter" scale.[12]

Richter magnitude scale

The first scale for measuring earthquake magnitudes, developed in 1935 by Charles F. Richter and popularly known as the "Richter" scale, is actually the Local magnitude scale, label ML or ML.[13] Richter established two features now common to all magnitude scales. First, the scale is logarithmic, so that each unit represents a ten-fold increase in the amplitude of the seismic waves.[14] As the energy of a wave is 101.5 times its amplitude, each unit of magnitude represents a nearly 32-fold increase in the energy (strength) of an earthquake.[15]

Second, Richter arbitrarily defined the zero point of the scale to be where an earthquake at a distance of 100 km makes a maximum horizontal displacement of 0.001 millimeters (1 µm, or 0.00004 in.) on a seismogram recorded with a Wood-Anderson torsion seismograph.[16] Subsequent magnitude scales are calibratedted to be approximately in accord with the original "Richter" (local) scale around magnitude 6.[17]

All "Local" (ML) magnitudes are based on the maximum amplitude of the ground shaking, without distinguishing the different seismic waves. They underestimate the strength of distant earthquakes (over ~600 km) because of attenuation of the S-waves, of deep earthquakes because the surface waves are smaller, and of strong earthquakes (over M ~7) because they do not take into account the length of the shaking (the coda). The original "Richter" scale, developed in the geological context of Southern California and Nevada, was later found to be inaccurate for earthquakes in the central and eastern parts of the continent.[18] All these problems prompted development of other scales.

Most seismological authorities, such as the United States Geological Survey, report earthquake magnitudes as moment magnitude (below), which the press describes as "Richter magnitude".[19]

Other "Local" magnitude scales

Richter's original "local" scale has been adapted for other localities. These may be labelled "ML", or with a lowercase "l", either Ml, or Ml.[20] Whether the values are comparable depends on whether the local conditions have been adequately determined and the formula suitably adjusted.[21]

Japanese Meteorological Agency magnitude scale

In Japan, for shallow (depth < 60 km) earthquakes within 600 km, the Japanese Meteorological Agency calculates[22] a magnitude labeled MJMA, MJMA, or MJ. (These should not be confused with moment magnitudes JMA calculates, which are labeled Mw(JMA) or M(JMA).) The magnitudes are based (as typical with local scales) on the maximum amplitude of the ground motion; they agree "rather well"[23] with the seismic moment magnitude Mw in the range of 4.5 to 7.5,[24] but underestimate larger magnitudes.

Surface-wave magnitude scale

The surface-wave magnitude scale, variously denoted as Ms, MS, and Ms, is based on a procedure developed by Beno Gutenberg in 1942[25] for measuring shallow earthquakes stronger or more distant than Richter's original scale could handle. Notably, it measured the amplitude of surface waves (mostly Love waves and Rayleigh waves, which generally produce the largest amplitudes) for a period of "about 20 seconds"[26] The Ms scale approximately agrees with ML at ~6, then diverges by as much as half a magnitude.[27]

A modification – the "Moscow-Prague formula" – was proposed in 1962, and recommended by the IASPEI in 1967; this is the basis of the standardized Ms20 scale (Ms_20, Ms(20)).[28] A "broad-band" variant (Ms_BB, Ms(BB)) measures the largest velocity amplitude in the Rayleigh-wave train for periods up to 60 seconds.[29] The MS7 scale used in China is a variant of Ms calibrated for use with the Chinese-made "type 763" long-period siesmograph.[30]

Moment magnitude scale

Because of the limitations of the magnitude scales, a new, more uniformly applicable extension of them, known as moment magnitude (Mw) scale for representing the size of earthquakes, was introduced by Thomas C. Hanks and Hiroo Kanamori in 1977. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of moment in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying slip displacement—as well as the amount of energy released. So while seismic moment, too, is calculated from seismograms, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for observed earthquakes range over more than 15 orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes.

Duration and Coda magnitude scales

Md designates various scales that estimate magnitude from the duration or length of some part of the seismic wave-train. This is especially useful for measuring local or regional earthquakes, both powerful earthquakes that might drive the seismometer off-scale (a problem with the analog instruments formerly used) and preventing measurement of the maximum wave amplitude, and weak earthquakes, whose maximum amplitude is not accurately measured. Even for distant earthquakes, measuring the duration of the shaking (as well as the amplitude) provides a better measure of the earthquake's total energy. Measurement of duration is incorporated in some modern scales, such as Mwpd and mBc.[31]

Mc scales usually measure the duration or amplitude of a part of the seismic wave, the coda. For short distances (less than ~100 km) these can provide a quick estimate of magnitude before the quake's exact location is known.[32]

Macroseismic magnitude scales

Magnitude scales generally are based on instrumental measurement of some aspect of the seismic wave as recorded on a seismogram. Where such records do not exist, magnitudes can be estimated from reports of the macroseismic events such as described by intensity scales.[33]

One approach for doing this (developed by Beno Gutenberg and Charles Richter in 1942[34]) relates the maximum intensity observed (presumably this is over the epicenter), denoted I0 (capital I, subscripted zero), to the magnitude. It has been recommended that magnitudes calculated on this basis be labeled Mw(I0),[35] but are sometimes labeled with a more generic Mms.

Another approach is to make an isoseismal map showing the area over which a given level of intensity was felt. The size of the "felt area" can also be related to the magnitude (based on the work of Frankel 1994 and Johnston 1996). While the recommended label for magnitudes derived in this way is M0(An),[36] the more commonly seen label is Mfa. A variant, MLa, adapted to California and Hawaii, derives the Local magnitude (ML) from the size of the area affected by a given intensity.[37]

Peak Ground Velocity (PGV) and Peak Ground Acceleration (PGA) are measures of the force that causes destructive ground shaking.[38] In Japan, a network of strong-motion accelerometers provides PGA data that permits site-specific correlation with different magnitude earthquakes. This correlation can be inverted to estimate the ground shaking at that site due to an earthquake of a given magnitude at a given distance. From this a map showing areas of likely damage can be prepared within minutes of an actual earthquake.[39]

Tsunami magnitude scale

Earthquakes that generate tsunamis generally rupture relatively slowly, delivering more energy at longer periods (lower frequencies) than generally used for measuring magnitudes. Any skew in the spectral distribution can result in larger, or smaller, tsunamis than expected for a nominal magnitude.[40] The tsunami magnitude scale, Mt, is based on a correlation by Katsuyuki Abe of earthquake seismic moment (M0) with the amplitude of tsunami waves as measured by tidal gauges.[41] Originally intended for estimating the magnitude of historic earthquakes where seismic data is lacking but tidal data exist, the correlation can be reversed to predict tidal height from earthquake magnitude.[42] (Not to be confused with the height of a tidal wave, or run-up, which is an intensity effect controlled by local topography.) Under low-noise conditions, tsunami waves as little as 5 cm can be predicted, corresponding to an earthquake of M ~6.5.[43]

See also

Notes

  1. ↑ Bormann, Wendt & Di Giacomo 2013, p. 37. The relationship between magnitude and the energy released is complicated. Se §3.1.2.5 and §3.3.3 in Bormann et al. 2013 for details.
  2. ↑ Bormann, Wendt & Di Giacomo 2013, §3.1.2.1.
  3. ↑ Bolt 1993, p. 164 et seq..
  4. ↑ Bolt 1993, pp. 170–171.
  5. ↑ Bolt 1993, p. 170.
  6. ↑ David Alexander (1993). Natural Disasters (First ed.). Springer Science+Business Media. p. 28. ISBN 978-0-412-04741-1.
  7. ↑ "The European Macroseismic Scale EMS-98". Centre Européen de Géodynamique et de Séismologie (ECGS). Retrieved 2013-07-26.
  8. ↑ "Magnitude and Intensity of an Earthquake". Hong Kong Observatory. Retrieved 2008-09-15.
  9. ↑ "The Severity of an Earthquake". U.S. Geological Survey. Retrieved 2012-01-15.
  10. ↑ See Bolt 1993, Chapters 2 and 3, for a very readable explanation of these waves and their interpretation. J. R. Kayal's excellent description of seismic waves can be found here.
  11. ↑ See Havskov & Ottemöller 2009, §1.4, pp. 20–21, for a short explanation, or MNSOP-2 EX 3.1 2012 for a technical description.
  12. ↑ Chung & Bernreuter 1980, p. 1.
  13. ↑ Kanamori 1983, p. 187.
  14. ↑ Richter 1935, p. 7.
  15. ↑ Spence, Sipkin & Choy 1989, p. 61.
  16. ↑ Richter 1935, pp. 5; Chung & Bernreuter 1980, p. 10. Subsequently redefined by Hutton & Boore 1987 as 10 mm of motion by an ML  3 quake at 17 km.
  17. ↑ Chung & Bernreuter 1980, p. 1; Kanamori 1983, p. 187, figure 2.
  18. ↑ Chung & Bernreuter 1980, p. ix.
  19. ↑ The USGS policy for reporting magnitudes to the press was posted at USGS policy, but has been removed. A copy can be found at http://dapgeol.tripod.com/usgsearthquakemagnitudepolicy.htm.
  20. ↑ Bormann, Wendt & Di Giacomo 2013, §3.2.4, p. 59.
  21. ↑ See Datasheet 3.1 in NMSOP-2 for a partial compilation and references.
  22. ↑ Katsumata 1996; Bormann, Wendt & Di Giacomo 2013, §3.2.4.7, p. 78; Doi 2010.
  23. ↑ Bormann & Saul 2009, p. 2478.
  24. ↑ See also figure 3.70 in NMSOP-2.
  25. ↑ Gutenberg 1945a; based on work by Gutenberg & Richter 1936.
  26. ↑ Gutenberg 1945a.
  27. ↑ Kanamori 1983, p. 187.
  28. ↑ Bormann, Wendt & Di Giacomo 2013, pp. 81–84.
  29. ↑ MNSOP-2 DS 3.1 2012, p. 8.
  30. ↑ Bormann et al. 2007, p. 118.
  31. ↑ Bormann, Wendt & Di Giacomo 2013, §3.2.4.5.
  32. ↑ Bormann, Wendt & Di Giacomo 2013, §3.2.4.5, pp. 71–72.
  33. ↑ Musson & Cecić 2012, p. 2.
  34. ↑ Gutenberg & Richter 1942.
  35. ↑ Grünthal 2011, p. 240.
  36. ↑ Grünthal 2011, p. 240.
  37. ↑ Stover & Coffman 1993, p. 3.
  38. ↑ Makris & Black 2004, p. 1032.
  39. ↑ Doi 2010.
  40. ↑ Bormann, Wendt & Di Giacomo 2013, §3.2.6.7, p. 124.
  41. ↑ Abe 1979; Abe 1989, p. 28. More precisely, Mt is based on far-field tsunami wave amplitudes in order to avoid some complications that happen near the source. Abe 1979, p. 1566.
  42. ↑ Blackford 1984, p. 29.
  43. ↑ Abe 1989, p. 28.

Sources

  • Abe, K. (April 1979), "Size of great earthquakes of 1837 – 1874 inferred from tsunami data", Journal of Geophysical Research, 84 (B4): 1561–1568, doi:10.1029/JB084iB04p01561 .
  • Abe, K. (September 1989), "Quantification of tsunamigenic earthquakes by the Mt scale", Tectonophysics, 166 (1-3): 27–34, doi:10.1016/0040-1951(89)90202-3 .
  • Bolt, B. A. (1993), Earthquakes and geological discovery, Scientific American Library, ISBN 0-7167-5040-6 .
  • Bormann, P.; Saul, J. (2009), "Earthquake Magnitude" (PDF), Encyclopedia of Complexity and Applied Systems Science, 3, pp. 2473–2496 .
  • Frankel, A. (1994), "Implications of felt area-magnitude relations for earthquake scaling and the average frequency of perceptible ground motion", Bulletin of the Seismological Society of America, 84 (2): 462–465 .
  • Grünthal, G. (2011), "Earthquakes, Intensity", in Gupta, H., Encyclopedia of Solid Earth Geophysics, pp. 237–242, ISBN 978-90-481-8701-0 .
  • Gutenberg, B.; Richter, C. F. (1936), "On seismic waves (third paper)", Gerlands Beiträge zur Geophysik, 47: 73–131 .
  • Gutenberg, B.; Richter, C. F. (1942), "Earthquake magnitude, intensity, energy, and acceleration", Bulletin of the Seismological Society of America: 163–191, ISSN 0037-1106 .
  • Johnston, A. (1996), "Seismic moment assessment of earthquakes in stable continental regions — II. Historical seismicity", Geophysical Journal International, 125 (3): 639–678 .
  • Katsumata, A. (June 1996), "Comparison of magnitudes estimated by the Japan Meteorological Agency with moment magnitudes for intermediate and deep earthquakes.", Bulletin of the Seismological Society of America, 86 (3): 832–842 .
  • Makris, N.; Black, C. J. (September 2004), "Evaluation of Peak Ground Velocity as a "Good" Intensity Measure for Near-Source Ground Motions", Journal of Engineering Mechanics, 130 (9): 1032–1044 .
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