Body-waves consist of P-waves that are the first to arrive (see seismogram), or S-waves, or reflections of either. Body-waves travel through rock directly.[1]
The original "body-wave magnitude" – mB or mB (uppercase "B") – was developed by Gutenberg(1945b, 1945c) and Gutenberg & Richter (1956)[2] to overcome the distance and magnitude limitations of the ML scale inherent in the use of surface waves. mB is based on the P- and S-waves, measured over a longer period, and does not saturate until around M 8. However, it is not sensitive to events smaller than about M 5.5.[3] Use of mB as originally defined has been largely abandoned,[4] now replaced by the standardized mBBB scale.[5]
mb scale
The mb or mb scale (lowercase "m" and "b") is similar to mB, but uses only P-waves measured in the first few seconds on a specific model of short-period seismograph.[6] It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions.[7]
Measurement of mb has changed several times.[8] As originally defined by Gutenberg (1945c) mb was based on the maximum amplitude of waves in the first 10 seconds or more. However, the length of the period influences the magnitude obtained. Early USGS/NEIC practice was to measure mb on the first second (just the first few P-waves[9]), but since 1978 they measure the first twenty seconds.[10] The modern practice is to measure short-period mb scale at less than three seconds, while the broadband mBBB scale is measured at periods of up to 30 seconds.[11]
mbLg scale
Differences in the crust underlying North America east of the Rocky Mountains makes that area more sensitive to earthquakes. Shown here: the 1895 New Madrid earthquake, M ~6, was felt through most of the central U.S., while the 1994 Northridge quake, though almost ten times stronger at M 6.7, was felt only in southern California. From USGS Fact Sheet 017–03.
The regional mbLg scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and mN – was developed by Nuttli (1973) for a problem the original ML scale could not handle: all of North America east of the Rocky Mountains. The ML scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to the continent. East of the Rockies the continent is a craton, a thick and largely stable mass of continental crust that is largely granite, a harder rock with different seismic characteristics. In this area the ML scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California.
Nuttli resolved this by measuring the amplitude of short-period (~1 sec.) Lg waves,[12] a complex form of the Love wave which, although a surface wave, he found provided a result more closely related to the mb scale than the Ms scale.[13] Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and MbLg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions.[14]
↑ Bormann, Wendt & Di Giacomo 2013, §3.2.4.4. The "g" subscript refers to the granitic layer through which Lg waves propagate. Chen & Pomeroy 1980, p.4. See also J. R. Kayal, "Seismic Waves and Earthquake Location", here, page 5.
Engdahl, E. R.; Villaseñor, A. (2002), "Chapter 41: Global Seismicity: 1900–1999"(PDF), in Lee, W.H.K.; Kanamori, H.; Jennings, P.C.; Kisslinger, C. (eds.), International Handbook of Earthquake and Engineering Seismology, vol.Part A, Academic Press, pp.665–690, ISBN0-12-440652-1 .
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.
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, Bibcode:1996BuSSA..86..832K, doi:10.1785/BSSA0860030832, S2CID130279928 .
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, doi:10.1061/(asce)0733-9399(2004)130:9(1032) .
Nuttli, O. W. (10 February 1973), "Seismic wave attenuation and magnitude relations for eastern North America", Journal of Geophysical Research, 78 (5): 876–885, Bibcode:1973JGR....78..876N, doi:10.1029/JB078i005p00876 .
Nuttli, O. W. (April 1983), "Average seismic source-parameter relations for mid-plate earthquakes", Bulletin of the Seismological Society of America, 73 (2): 519–535.
An earthquake – also called a quake, tremor, or temblor – is the shaking of the surface of Earth resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those that are so weak that they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume. The word tremor is also used for non-earthquake seismic rumbling.
The Modified Mercalli intensity scale measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake.
Charles Francis Richter was an American seismologist and physicist.
Beno Gutenberg was a German-American seismologist who made several important contributions to the science. He was a colleague and mentor of Charles Francis Richter at the California Institute of Technology and Richter's collaborator in developing the Richter magnitude scale for measuring an earthquake's magnitude.
The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. It was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (ML ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often says "Richter scale" when referring to the moment magnitude scale.
Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.
Earthquake forecasting is a branch of the science of seismology concerned with the probabilistic assessment of general earthquake seismic hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades. While forecasting is usually considered to be a type of prediction, earthquake forecasting is often differentiated from earthquake prediction, whose goal is the specification of the time, location, and magnitude of future earthquakes with sufficient precision that a warning can be issued. Both forecasting and prediction of earthquakes are distinguished from earthquake warning systems, which, upon detection of an earthquake, provide a real-time warning to regions that might be affected.
The International Seismological Centre (ISC) is a non-governmental, nonprofit organisation charged with the final collection, definitive analysis and publication of global seismicity. The ISC was formed in 1964 as an international organisation independent of national governments that would carry on the work of the International Seismological Summary in collecting and analyzing seismic data from around the world, and particularly to handle increased flow of data from the World-Wide Standard Seismograph Network (WWSSN), also established that year. The ISC considers its prime task to be the collection and re-analysis of all available earthquake seismic date in order to produce definitive data on earthquakes. The ISC's catalog is considered "the most complete and authoritative final depository of global earthquake parameter data."
The surface wave magnitude scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This magnitude scale is related to the local magnitude scale proposed by Charles Francis Richter in 1935, with modifications from both Richter and Beno Gutenberg throughout the 1940s and 1950s. It is currently used in People's Republic of China as a national standard for categorising earthquakes.
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, , and one on body waves, . 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 , using an equation similar to that used for .
The Richter scale, also called the Richter magnitude scale, Richter's magnitude scale, and the Gutenberg–Richter scale, is a measure of the strength of earthquakes, developed by Charles Francis Richter and presented in his landmark 1935 paper, where he called it the "magnitude scale". This was later revised and renamed the local magnitude scale, denoted as ML or ML .
In the early morning hours of August 16, 1931, a powerful earthquake occurred in West Texas with a maximum Mercalli intensity of VIII (Severe). Estimates of its magnitude range between 5.8–6.4 mb, making it the most powerful earthquake ever recorded in Texas history. Its epicenter was near the town of Valentine, Texas; there, the earthquake caused damage to many homes and buildings. The earthquake may have been caused by movement along oblique-slip faulting in West Texas, the most seismically-active region in the state. Shaking from the earthquake was perceptible within a 400 mi (640 km) radius of the epicenter, affecting four U.S. states and northern Mexico. Several foreshocks and aftershocks accompanied the primary temblor, with the aftershocks continuing until at least November 3, 1931. The main earthquake caused no fatalities, though several people sustained minor injuries; the damage in Valentine amounted to $50,000–$75,000.
The 1982 North Yemen earthquake hit near the city of Dhamar, North Yemen on December 13. Measuring 6.2 on the moment magnitude scale, with a maximum perceived intensity of VIII (Severe) on the Mercalli intensity scale, as many as 2,800 people were killed and another 1,500 injured. The shock occurred within several hundred kilometers of a plate boundary in a geologically complex region that includes active volcanoes and seafloor spreading ridges. Yemen has a history of destructive earthquakes, though this was the first instrumentally recorded event to be detected on global seismograph networks.
In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within 0.4 magnitude – distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock and decrease in magnitude and frequency according to known laws.
Harry Oscar Wood (1879–1958) was an American seismologist who made several significant contributions in the field of seismology in the early twentieth-century. Following the 1906 earthquake in San Francisco, California, Wood expanded his background of geology and mineralogy and his career took a change of direction into the field of seismology. In the 1920s he co-developed the torsion seismometer, a device tuned to detect short-period seismic waves that are associated with local earthquakes. In 1931 Wood, along with another seismologist, redeveloped and updated the Mercalli intensity scale, a seismic intensity scale that is still in use as a primary means of rating an earthquake's effects.
The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period, and of the threat to the built environment. This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses. A variety of hazard metrics can be calculated with UCERF3; a typical metric is the likelihood of a magnitude M 6.7 earthquake in the 30 years since 2014.
Energy class – also called energy class K or K-class , and denoted by K – is a measure of the force or magnitude of local and regional earthquakes used in countries of the former Soviet Union, and Cuba and Mongolia. K is nominally the logarithm of seismic energy radiated by an earthquake, as expressed in the formula K = log ES. Values of K in the range of 12 to 15 correspond approximately to the range of 4.5 to 6 in other magnitude scales; a magnitude Mw 6.0 quake will register between 13 and 14.5 on various K-class scales. The energy class system was developed by seismologists of the Soviet Tadzhikskaya Complex [Interdisciplinary] Seismological Expedition established in the remote Garm (Tajikistan) region of Central Asia in 1954 after several devastating earthquakes in that area.
Seismic intensity scales categorize the intensity or severity of ground shaking (quaking) at a given location, such as resulting from an earthquake. They are distinguished from seismic magnitude scales, which measure the magnitude or overall strength of an earthquake, which may, or perhaps may not, cause perceptible shaking.
The 1912 Maymyo earthquake or Burma earthquake struck Burma on the morning of May 23, with an epicentre near Taunggyi and Pyin Oo Lwin in Shan State. The earthquake was initially calculated at 8.0 on the surface wave magnitude scale (Ms ) by Beno Gutenberg and Charles Francis Richter, and described by them as being one of the most remarkable seismic events in the early 1900s. Recent re-evaluation of the earthquake, however, have revised the magnitude to 7.6–7.9. It was preceded by two foreshocks on May 18 and 21 with respective intensities V and VII on the Rossi–Forel scale, while the mainshock was assigned IX. Shaking was felt throughout most of Burma, parts of Siam and Yunnan; an area covering approximately 375,000 square miles. It was one of the largest earthquakes in the country.
The Japan Meteorological Agency magnitude scale is a seismic magnitude scale set by the Japan Meteorological Agency.
The Wood–Anderson seismometer is a torsion seismometer developed in the United States by Harry O. Wood and John August Anderson in the 1920s to record local earthquakes in southern California. It photographically records the horizontal motion. The seismometer uses a pendulum of 0.8g, its period is 0.8 seconds, its magnification is 2,800 times, and its damping constant is 0.8. Charles Francis Richter developed the Richter magnitude scale using the Wood–Anderson seismometer.
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