Seismology
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Seismology (; from
Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). Ancient Greek was the language of an ...
σεισμός (''seismós'') meaning "
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object known t ...

earthquake
" and -λογία (''-logía'') meaning "study of") is the scientific study of
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object known t ...

earthquake
s and the propagation of
elastic wave Linear elasticity is a mathematical model of how solid objects deform and become internally stressed due to prescribed loading conditions. It is a simplification of the more general nonlinear theory of elasticity and a branch of continuum mechani ...
s through the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
or through other planet-like bodies. The field also includes studies of earthquake environmental effects such as
tsunamis A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquake An earthquake (also known as a quake, tre ...
as well as diverse
seismic source A seismic source is a device that generates controlled seismic energy used to perform both reflection seismology, reflection and seismic refraction, refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more s ...
s such as volcanic, tectonic, glacial,
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the Deposition (geology), deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaci ...
, oceanic, atmospheric, and artificial processes such as explosions. A related field that uses
geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they change over time. Geology can ...

geology
to infer information regarding past earthquakes is
paleoseismology caused by an earthquake January 26, 1700 Paleoseismology looks at geologic sediments and rock (geology), rocks, for signs of ancient earthquakes. It is used to supplement seismology, seismic monitoring, for the calculation of seismic hazard. ...
. A recording of Earth motion as a function of time is called a
seismogram A seismogram is a graph output by a seismograph A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a r ...

seismogram
. A seismologist is a scientist who does research in seismology.


History

Scholarly interest in earthquakes can be traced back to antiquity. Early speculations on the natural causes of earthquakes were included in the writings of
Thales Thales of Miletus ( ; el, Θαλῆς Thales of Miletus ( ; el, Θαλῆς (ὁ Μιλήσιος), ''Thalēs''; ) was a Greek mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (fr ...

Thales
of Miletus (c. 585 BCE),
Anaximenes of Miletus Anaximenes of Miletus (; grc-gre, Ἀναξιμένης ὁ Μιλήσιος, translit=Anaximenēs ho Milēsios; ) was an Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiqu ...
(c. 550 BCE),
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental quest ...

Aristotle
(c. 340 BCE), and
Zhang Heng Zhang Heng (; AD 78–139), formerly romanized Romanization or romanisation, in linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for ...

Zhang Heng
(132 CE). In 132 CE, Zhang Heng of China's
Han dynasty#REDIRECT Han dynasty The Han dynasty () was the second Dynasties in Chinese history, imperial dynasty of China (202 BC – 220 AD), established by the rebel leader Liu Bang and ruled by the House of Liu. Preceded by the short-lived Qin dynas ...

Han dynasty
designed the first known
seismoscope A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph. The output of ...
. In the 17th century,
Athanasius Kircher Athanasius Kircher (2 May 1602 – 28 November 1680) was a German Society of Jesus, Jesuit scholar and polymath who published around 40 major works, most notably in the fields of comparative religion, geology, and medicine. Kircher has b ...

Athanasius Kircher
argued that earthquakes were caused by the movement of fire within a system of channels inside the Earth.
Martin Lister Martin Lister FRS (12 April 1639 – 2 February 1712) was an English naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning more towards obs ...
(1638 to 1712) and
Nicolas Lemery Nicolas Lémery (or Lemery as his name appeared in his international publications) (17 November 1645 – 19 June 1715), French chemist, was born at Rouen Rouen (, ; or ) is a city on the River Seine in northern France. It is the capital of ...
(1645 to 1715) proposed that earthquakes were caused by chemical explosions within the earth. The
Lisbon earthquake of 1755
Lisbon earthquake of 1755
, coinciding with the general flowering of science in Europe, set in motion intensified scientific attempts to understand the behaviour and causation of earthquakes. The earliest responses include work by
John Bevis John Bevis (10 November 1695 in Salisbury, Wiltshire – 6 November 1771) was an English (people), English Physician, doctor, electrical researcher and astronomer. He is best known for discovering the Crab Nebula in 1731. Bevis also observed a ...
(1757) and
John Michell John Michell (; 25 December 1724 – 21 April 1793) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medie ...
(1761). Michell determined that earthquakes originate within the Earth and were waves of movement caused by "shifting masses of rock miles below the surface." From 1857,
Robert Mallet Robert Mallet, Fellow of the Royal Society, FRS, Royal Irish Academy#Membership, MRIA (3 June 1810 – 5 November 1881), Irish people, Irish geophysicist, civil engineer, and inventor who distinguished himself in research on earthquakes and is s ...
laid the foundation of instrumental seismology and carried out seismological experiments using explosives. He is also responsible for coining the word "seismology." In 1897,
Emil Wiechert Emil Johann Wiechert (26 December 1861 – 19 March 1928) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of ...

Emil Wiechert
's theoretical calculations led him to conclude that the
Earth's interior The internal structure of Earth, structure of the solid Earth, or simply structure of Earth refers to concentric spherical layers subdividing the Solid earth, i.e., excluding Earth's atmosphere File:Atmosphere gas proportions.svg, Compositi ...
consists of a mantle of silicates, surrounding a core of iron. In 1906
Richard Dixon Oldham Richard Dixon Oldham FRS (; 31 July 1858 – 15 July 1936) was a British geologist who made the first clear identification of the separate arrivals of P-wave A P wave (primary wave or pressure wave) is one of the two main types of elasti ...
identified the separate arrival of
P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves Seismic waves are waves of energy In physics Physics (from grc, φυσική (ἐπιστήμη), physik ...
s, S-waves and surface waves on seismograms and found the first clear evidence that the Earth has a central core. In 1909,
Andrija Mohorovičić Andrija Mohorovičić (23 January 1857 – 18 December 1936) was a Croatian geophysicist. He is best known for the eponymous Mohorovičić discontinuity and is considered as one of the founders of modern seismology Seismology (; from Ancient G ...
, one of the founders of modern seismology, discovered and defined the
Mohorovičić discontinuity The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of ...
. Usually referred to as the "Moho discontinuity" or the "Moho," it is the boundary between the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
's crust and the mantle. It is defined by the distinct change in velocity of seismological waves as they pass through changing densities of rock. In 1910, after studying the April 1906 San Francisco earthquake,
Harry Fielding Reid Harry Fielding Reid (May 18, 1859 – June 18, 1944) was an American geophysicist. He was notable for his contributions to seismology Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancie ...
put forward the "
elastic rebound theory __NOTOC__ 350px, In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed ...
" which remains the foundation for modern tectonic studies. The development of this theory depended on the considerable progress of earlier independent streams of work on the behavior of elastic materials and in mathematics. In 1926,
Harold Jeffreys 160px, Plaque to Sir Harold Jeffreys, Newcastle University Sir Harold Jeffreys, FRS (22 April 1891 – 18 March 1989) was a British mathematician, statistician, geophysicist, and astronomer An astronomer is a scientist in the field of astro ...
was the first to claim, based on his study of earthquake waves, that below the mantle, the core of the Earth is liquid. In 1937,
Inge Lehmann Inge Lehmann (13 May 1888 – 21 February 1993) was a Danish seismologist and geophysicist. In 1936, she discovered that the Earth has a solid inner core inside a molten outer core. Before that, seismologists believed Earth's core to be a ...
determined that within Earth's liquid
outer core Earth's outer core is a fluid layer about thick and composed of mostly iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transi ...
there is a solid
inner core Earth's inner core is the innermost structure of Earth, geologic layer of planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth radius, Earth's radius or 70% of the Moon's radius. Th ...
. By the 1960s, Earth science had developed to the point where a comprehensive theory of the causation of seismic events and geodetic motions had come together in the now well-established theory of
plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
.


Types of seismic wave

Seismic waves are
elastic waves Linear elasticity is a mathematical model of how solid objects deform and become internally stressed due to prescribed loading conditions. It is a simplification of the more general nonlinear theory of elasticity and a branch of continuum mechani ...
that propagate in solid or fluid materials. They can be divided into ''body waves'' that travel through the interior of the materials; ''surface waves'' that travel along surfaces or interfaces between materials; and ''normal modes'', a form of standing wave.


Body waves

There are two types of body waves, pressure waves or primary waves (P-waves) and
shear Shear may refer to: Textile production * Animal shearing, the collection of wool from various species **Sheep shearing *The removal of Nap (textile), nap during wool cloth production Science and technology Engineering *Shear strength (soil), the ...
or secondary waves (
S-wave __NOTOC__ In seismology Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divi ...
s). P-waves are
longitudinal wave Longitudinal waves are waves in which the vibration of the medium is parallel to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical wave, Mechanical longitudinal ...

longitudinal wave
s that involve
compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
and expansion in the direction that the wave is moving and are always the first waves to appear on a seismogram as they are the fastest moving waves through solids.
S-waves __NOTOC__ In seismology Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided ...
are
transverse wave
transverse wave
s that move perpendicular to the direction of propagation. S-waves are slower than P-waves. Therefore, they appear later than P-waves on a seismogram. Fluids cannot support transverse elastic waves because of their low shear strength, so S-waves only travel in solids.


Surface waves

Surface waves are the result of P- and S-waves interacting with the surface of the Earth. These waves are dispersive, meaning that different frequencies have different velocities. The two main surface wave types are
Rayleigh wave Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructi ...

Rayleigh wave
s, which have both compressional and shear motions, and
Love wave 300px, How Love waves work In elastodynamics, Love waves, named after Augustus Edward Hough Love, are horizontally polarized surface waves. The Love wave is a result of the interference of many shear waves ( S-waves) guided by an elastic laye ...

Love wave
s, which are purely shear. Rayleigh waves result from the interaction of P-waves and vertically polarized S-waves with the surface and can exist in any solid medium. Love waves are formed by horizontally polarized S-waves interacting with the surface, and can only exist if there is a change in the elastic properties with depth in a solid medium, which is always the case in seismological applications. Surface waves travel more slowly than P-waves and S-waves because they are the result of these waves traveling along indirect paths to interact with Earth's surface. Because they travel along the surface of the Earth, their energy decays less rapidly than body waves (1/distance2 vs. 1/distance3), and thus the shaking caused by surface waves is generally stronger than that of body waves, and the primary surface waves are often thus the largest signals on earthquake seismograms. Surface waves are strongly excited when their source is close to the surface, as in a shallow earthquake or a near-surface explosion, and are much weaker for deep earthquake sources.


Normal modes

Both body and surface waves are traveling waves; however, large earthquakes can also make the entire Earth "ring" like a resonant bell. This ringing is a mixture of
normal modes A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidal A sine wave or sinusoid is a curve, mathematical curve that describes a smooth periodic oscillation. A sine wave is a continuous wave. ...
with discrete frequencies and periods of approximately an hour or shorter. Normal mode motion caused by a very large earthquake can be observed for up to a month after the event. The first observations of normal modes were made in the 1960s as the advent of higher fidelity instruments coincided with two of the largest earthquakes of the 20th century the
1960 Valdivia earthquake The 1960 Valdivia earthquake ( es, link=no, Terremoto de Valdivia) or the Great Chilean earthquake (''Gran terremoto de Chile'') on 22 May 1960 was the most powerful earthquake ever recorded. Various studies have placed it at 9.4–9.6 on the mo ...

1960 Valdivia earthquake
and the
1964 Alaska earthquake The 1964 Alaskan earthquake, also known as the Great Alaskan earthquake and Good Friday earthquake, occurred at 5:36 PM AKST on Good Friday Good Friday is a Christian holiday commemorating the crucifixion of Jesus and his death at Calvary. It ...
. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.


Earthquakes

One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake. Other notable earthquakes that spurred major advancements in the science of seismology include the
1857 Basilicata earthquake The 1857 Basilicata earthquake (also known as the Great Neapolitan earthquake) occurred on December 16 in the Basilicata it, Lucano (man) it, Lucana (woman) , population_note = , population_blank1_title = , population_blank1 = ...
, the 1906 San Francisco earthquake, the 1964 Alaska earthquake, the 2004
Sumatra-Andaman earthquake The 2004 Indian Ocean earthquake and tsunami (also known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake) occurred at 07:58:53 in local time (UTC+7 UTC+07:00 is an identifier for a UTC offset, tim ...
, and the 2011
Great East Japan earthquake Great may refer to: Descriptions or measurements * Great, a relative measurement in physical space, see Size * Greatness, being divine, majestic, superior, majestic, or transcendent People with the name * "The Great", a historical suffix to people ...
.


Controlled seismic sources

Seismic waves produced by explosions or vibrating controlled sources are one of the primary methods of Exploration geophysics, underground exploration in geophysics (in addition to many different Electromagnetic radiation, electromagnetic methods such as induced polarization and magnetotellurics). Controlled-source seismology has been used to map salt domes, anticlines and other geologic traps in petroleum-bearing rock (geology), rocks, Fault (geology), faults, rock types, and long-buried giant meteor Impact crater, craters. For example, the Chicxulub Crater, which was caused by an impact that has been Cretaceous–Paleogene extinction event, implicated in the extinction of the dinosaurs, was localized to Central America by analyzing ejecta in the Cretaceous–Paleogene boundary, and then physically proven to exist using seismic maps from oil exploration.


Detection of seismic waves

Seismometers are sensors that detect and record the motion of the Earth arising from elastic waves. Seismometers may be deployed at the Earth's surface, in shallow vaults, in boreholes, or Ocean-bottom seismometer, underwater. A complete instrument package that records seismic signals is called a seismograph. Networks of seismographs continuously record ground motions around the world to facilitate the monitoring and analysis of global earthquakes and other sources of seismic activity. Rapid location of earthquakes makes tsunami warnings possible because seismic waves travel considerably faster than tsunami waves. Seismometers also record signals from non-earthquake sources ranging from explosions (nuclear and chemical), to local noise from wind or anthropogenic activities, to incessant signals generated at the ocean floor and coasts induced by ocean waves (the global microseism), to cryosphere, cryospheric events associated with large icebergs and glaciers. Above-ocean meteor strikes with energies as high as 4.2 × 1013 joule, J (equivalent to that released by an explosion of ten kilotons of TNT) have been recorded by seismographs, as have a number of industrial accidents and terrorist bombs and events (a field of study referred to as forensic seismology). A major long-term motivation for the global seismographic monitoring has been for the detection and study of nuclear testing.


Mapping Earth's interior

Because seismic waves commonly propagate efficiently as they interact with the internal structure of the Earth, they provide high-resolution noninvasive methods for studying the planet's interior. One of the earliest important discoveries (suggested by Richard Dixon Oldham in 1906 and definitively shown by Harold Jeffreys in 1926) was that the
outer core Earth's outer core is a fluid layer about thick and composed of mostly iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transi ...
of the earth is liquid. Since S-waves do not pass through liquids, the liquid core causes a "shadow" on the side of the planet opposite the earthquake where no direct S-waves are observed. In addition, P-waves travel much slower through the outer core than the mantle. Processing readings from many seismometers using seismic tomography, seismologists have mapped the mantle of the earth to a resolution of several hundred kilometers. This has enabled scientists to identify convection cells and other large-scale features such as the large low-shear-velocity provinces near the core–mantle boundary.


Seismology and society


Earthquake prediction

Forecasting a probable timing, location, magnitude and other important features of a forthcoming seismic event is called earthquake prediction. Various attempts have been made by seismologists and others to create effective systems for precise earthquake predictions, including the VAN method. Most seismologists do not believe that a system to provide timely warnings for individual earthquakes has yet been developed, and many believe that such a system would be unlikely to give useful warning of impending seismic events. However, more general forecasts routinely predict seismic hazard. Such forecasts estimate the probability of an earthquake of a particular size affecting a particular location within a particular time-span, and they are routinely used in earthquake engineering. Public controversy over earthquake prediction erupted after Italian authorities indicted six seismologists and one government official for manslaughter in connection with 2009 L'Aquila earthquake, a magnitude 6.3 earthquake in L'Aquila, Italy on April 5, 2009. The indictment has been widely perceived as an indictment for failing to predict the earthquake and has drawn condemnation from the American Association for the Advancement of Science and the American Geophysical Union. The indictment claims that, at a special meeting in L'Aquila the week before the earthquake occurred, scientists and officials were more interested in pacifying the population than providing adequate information about earthquake risk and preparedness.


Engineering seismology

Engineering seismology is the study and application of seismology for engineering purposes. It generally applied to the branch of seismology that deals with the assessment of the seismic hazard of a site or region for the purposes of earthquake engineering. It is, therefore, a link between earth science and civil engineering. There are two principal components of engineering seismology. Firstly, studying earthquake history (e.g. historical and instrumental catalogs of seismicity) and tectonics to assess the earthquakes that could occur in a region and their characteristics and frequency of occurrence. Secondly, studying strong ground motions generated by earthquakes to assess the expected shaking from future earthquakes with similar characteristics. These strong ground motions could either be observations from accelerometers or seismometers or those simulated by computers using various techniques, which are then often used to develop ground motion prediction equations (or ground-motion model


Tools

Seismological instruments can generate large amounts of data. Systems for processing such data include: * CUSP (Caltech-USGS Seismic Processing) * RadExPro seismic software * SeisComP3


Notable seismologists

* Keiiti Aki, Aki, Keiiti * Nicholas Ambraseys, Ambraseys, Nicholas * Don L. Anderson, Anderson, Don L. * Bruce Bolt, Bolt, Bruce * Jon Claerbout, Claerbout, Jon * Adam Dziewonski, Dziewonski, Adam Marian * Maurice Ewing, Ewing, Maurice * Boris Borisovich Galitzine, Galitzine, Boris Borisovich * Grigoriy A. Gamburtsev, Gamburtsev, Grigory A. * Beno Gutenberg, Gutenberg, Beno * Susan Hough, Hough, Susan * Harold Jeffreys, Jeffreys, Harold * Lucy Jones, Jones, Lucy * Hiroo Kanamori, Kanamori, Hiroo * Vladimir Keilis-Borok, Keilis-Borok, Vladimir * Leon Knopoff, Knopoff, Leon * Inge Lehmann, Lehmann, Inge * James B. Macelwane, Macelwane, James * Robert Mallet, Mallet, Robert * Giuseppe Mercalli, Mercalli, Giuseppe * John Milne, Milne, John * Andrija Mohorovičić, Mohorovičić, Andrija * Richard Dixon Oldham, Oldham, Richard Dixon * Fusakichi Omori, Omori, Fusakichi * Sebastião José de Carvalho e Melo, Sebastião de Melo, Marquis of Pombal * Frank Press, Press, Frank * Paul G. Richards, Richards, Paul G. * Charles Francis Richter, Richter, Charles Francis * Seikei Sekiya, Sekiya, Seikei * Kerry Sieh, Sieh, Kerry * Paul Silver, Paul G. Silver * Ross Stein, Stein, Ross * Brian Tucker, Tucker, Brian * John Vidale, Vidale, John * Lianxing Wen, Wen, Lianxing * John Winthrop (educator), Winthrop, John *
Zhang Heng Zhang Heng (; AD 78–139), formerly romanized Romanization or romanisation, in linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for ...

Zhang Heng


See also

* (starquakes) * * * * * * * * * * * * * * * * * *


Notes


References

* * * * * * * * * * * * *


External links


European-Mediterranean Seismological Center
real-time earthquake information website.
Seismological Society of America

Incorporated Research Institutions for Seismology

USGS Earthquake Hazards Program


(UCSB ERI) {{Authority control Seismology, Earthquake engineering,