Seismology

Seismology ( /saɪzˈmɒlədʒi/; from Ancient Greek σεισμός (seismós) meaning "earthquake" and -λογία (-logía) meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes such as explosions. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.

2004 Indonesia Tsunami Complete
Animation of 2004 Indonesia Tsunami

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 of Miletus (c. 585 BCE), Anaximenes of Miletus (c. 550 BCE), Aristotle (c. 340 BCE) and Zhang Heng (132 CE).

In 132 CE, Zhang Heng of China's Han dynasty designed the first known seismoscope.[1][2][3]

In the 17th century, Athanasius Kircher argued that earthquakes were caused by the movement of fire within a system of channels inside the Earth. Martin Lister (1638 to 1712) and Nicolas Lemery (1645 to 1715) proposed that earthquakes were caused by chemical explosions within the earth.[4]

The 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 (1757) and John Michell (1761). Michell determined that earthquakes originate within the Earth and were waves of movement caused by "shifting masses of rock miles below the surface".[5]

From 1857, Robert Mallet laid the foundation of instrumental seismology and carried out seismological experiments using explosives. He is also responsible for coining the word "seismology".[6]

In 1897, Emil Wiechert's theoretical calculations led him to conclude that the Earth's interior consists of a mantle of silicates, surrounding a core of iron.[7]

In 1906 Richard Dixon Oldham identified the separate arrival of P-waves, S-waves and surface waves on seismograms and found the first clear evidence that the Earth has a central core.[8]

In 1910, after studying the April 1906 San Francisco earthquake, Harry Fielding Reid put forward the "elastic rebound theory" 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 behaviour of elastic materials and in mathematics.[9]

In 1926, Harold Jeffreys was the first to claim, based on his study of earthquake waves, that below the mantle, the core of the Earth is liquid.[10]

In 1937, Inge Lehmann determined that within the earth's liquid outer core there is a solid inner core.[11]

By the 1960s, earth science had developed to the point where a comprehensive theory of the causation of seismic events had come together in the now well-established theory of plate tectonics.

Types of seismic wave

Seismogram
Seismogram records showing the three components of ground motion. The red line marks the first arrival of P-waves; the green line, the later arrival of S-waves.

Seismic waves are elastic waves 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 or secondary waves (S-waves). P-waves are longitudinal waves that involve compression 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 are transverse waves 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 perpendicular motion, so S-waves only travel in solids.[12]

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 waves, which have both compressional and shear motions, and Love waves, 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. The primary surface waves are often 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.[12]

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 with discrete frequencies and periods of an hour or shorter. Motion caused by a large earthquake can be observed for up to a month after the event.[12] 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 and the 1964 Alaska earthquake. 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 1906 San Francisco earthquake, the 1964 Alaska earthquake, the 2004 Sumatra-Andaman earthquake, and the 2011 Great East Japan earthquake.

Controlled seismic sources

Seismic waves produced by explosions or vibrating controlled sources are one of the primary methods of underground exploration in geophysics (in addition to many different 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 rocks, faults, rock types, and long-buried giant meteor craters. For example, the Chicxulub Crater, which was caused by an impact that has been 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.[13]

Detection of seismic waves

Seismometer-iceland
Installation for a temporary seismic station, north Iceland highland.

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 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[14] or anthropogenic activities, to incessant signals generated at the ocean floor and coasts induced by ocean waves (the global microseism), to cryospheric events associated with large icebergs and glaciers. Above-ocean meteor strikes with energies as high as 4.2 × 1013 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 the earth's interior

Earthquake wave paths
Seismic velocities and boundaries in the interior of the Earth sampled by seismic waves

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 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.[15]

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 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.[16]

Engineering seismology

Engineering seismology is the study and application of seismology for engineering purposes.[17] 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.[18] There are two principal components of engineering seismology. Firstly, studying earthquake history (e.g. historical[18] and instrumental catalogs[19] of seismicity) and tectonics[20] 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[21], which are then often used to develop ground motion prediction equations[22] (or ground-motion models)[1].

Tools

Seismological instruments can generate large amounts of data. Systems for processing such data include:

Notable seismologists

See also

Notes

  1. ^ Needham, Joseph (1959). Science and Civilization in China, Volume 3: Mathematics and the Sciences of the Heavens and the Earth. Cambridge: Cambridge University Press. pp. 626–635.
  2. ^ Dewey, James; Byerly, Perry (February 1969). "The early history of seismometry (to 1900)". Bulletin of the Seismological Society of America. 59 (1): 183–227.
  3. ^ Agnew, Duncan Carr (2002). "History of seismology". International Handbook of Earthquake and Engineering Seismology. 81A: 3–11.
  4. ^ Udías, Agustín; Arroyo, Alfonso López (2008). "The Lisbon earthquake of 1755 in Spanish contemporary authors". In Mendes-Victor, Luiz A.; Oliveira, Carlos Sousa; Azevedo, João; Ribeiro, Antonio (eds.). The 1755 Lisbon earthquake: revisited. Springer. p. 14. ISBN 9781402086090.
  5. ^ Member of the Royal Academy of Berlin (2012). The History and Philosophy of Earthquakes Accompanied by John Michell's 'conjectures Concerning the Cause, and Observations upon the Ph'nomena of Earthquakes'. Cambridge Univ Pr. ISBN 9781108059909.
  6. ^ Society, The Royal (2005-01-22). "Robert Mallet and the 'Great Neapolitan earthquake' of 1857". Notes and Records. 59 (1): 45–64. doi:10.1098/rsnr.2004.0076. ISSN 0035-9149.
  7. ^ Barckhausen, Udo; Rudloff, Alexander (14 February 2012). "Earthquake on a stamp: Emil Wiechert honored". Eos, Transactions American Geophysical Union. 93 (7): 67. Bibcode:2012EOSTr..93...67B. doi:10.1029/2012eo070002.
  8. ^ "Oldham, Richard Dixon". Complete Dictionary of Scientific Biography. 10. Charles Scribner's Sons. 2008. p. 203.
  9. ^ "Reid's Elastic Rebound Theory". 1906 Earthquake. United States Geological Survey. Retrieved 6 April 2018.
  10. ^ Jeffreys, Harold (1926-06-01). "On the Amplitudes of Bodily Seismic Waues". Geophysical Journal International. 1: 334–348. Bibcode:1926GeoJ....1..334J. doi:10.1111/j.1365-246X.1926.tb05381.x. ISSN 1365-246X.
  11. ^ Hjortenberg, Eric (December 2009). "Inge Lehmann's work materials and seismological epistolary archive". Annals of Geophysics. 52 (6). doi:10.4401/ag-4625.
  12. ^ a b c Gubbins 1990
  13. ^ Schulte et al. 2010
  14. ^ Naderyan, Vahid; Hickey, Craig J.; Raspet, Richard (2016). "Wind-induced ground motion". Journal of Geophysical Research: Solid Earth. 121 (2): 917–930. doi:10.1002/2015JB012478.
  15. ^ Wen & Helmberger 1998
  16. ^ Hall 2011
  17. ^ Plimer, Richard C. SelleyL. Robin M. CocksIan R., ed. (2005-01-01). "Editors". Encyclopaedia of Geology. Oxford: Elsevier. pp. 499–515. doi:10.1016/b0-12-369396-9/90020-0. ISBN 978-0-12-369396-9.
  18. ^ a b Ambraseys, N. N. (1988-12-01). "Engineering seismology: Part I". Earthquake Engineering & Structural Dynamics. 17 (1): 1–50. doi:10.1002/eqe.4290170101. ISSN 1096-9845.
  19. ^ Wiemer, Stefan (2001-05-01). "A Software Package to Analyze Seismicity: ZMAP". Seismological Research Letters. 72 (3): 373–382. doi:10.1785/gssrl.72.3.373. ISSN 0895-0695.
  20. ^ Bird, Peter; Liu, Zhen (2007-01-01). "Seismic Hazard Inferred from Tectonics: California". Seismological Research Letters. 78 (1): 37–48. doi:10.1785/gssrl.78.1.37. ISSN 0895-0695.
  21. ^ Douglas, John; Aochi, Hideo (2008-10-10). "A Survey of Techniques for Predicting Earthquake Ground Motions for Engineering Purposes" (PDF). Surveys in Geophysics. 29 (3): 187–220. Bibcode:2008SGeo...29..187D. doi:10.1007/s10712-008-9046-y. ISSN 0169-3298.
  22. ^ Douglas, John; Edwards, Benjamin (2016-09-01). "Recent and future developments in earthquake ground motion estimation". Earth-Science Reviews. 160: 203–219. doi:10.1016/j.earscirev.2016.07.005.
  23. ^ Lee, W. H. K.; S. W. Stewart (1989). "Large-Scale Processing and Analysis of Digital Waveform Data from the USGS Central California Microearthquake Network". Observatory seismology: an anniversary symposium on the occasion of the centennial of the University of California at Berkeley seismographic stations. University of California Press. p. 86. Retrieved 2011-10-12. The CUSP (Caltech-USGS Seismic Processing) System consists of on-line real-time earthquake waveform data acquisition routines, coupled with an off-line set of data reduction, timing, and archiving processes. It is a complete system for processing local earthquake data ...
  24. ^ Akkar, Sinan; Polat, Gülkan; van Eck, Torild, eds. (2010). Earthquake Data in Engineering Seismology: Predictive Models, Data Management and Networks. Geotechnical, Geological and Earthquake Engineering. 14. Springer. p. 194. ISBN 978-94-007-0151-9. Retrieved 2011-10-19.

References

External links

Accretion disk

An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.

Advanced National Seismic System

The Advanced National Seismic System (ANSS) is a collaboration of the U.S. Geological Survey (USGS) and regional, state, and academic partners that collects and analyzes data on significant earthquakes to provide near real-time (generally within 10 to 30 minutes) information to emergency responders and officials, the news media, and the public. Such information is used to anticipate the likely severity and extent of damage, and to guide decisions on the responses needed.Data is collected by eleven regional seismic networks and the National Seismic Network ("ANSS backbone") of dedicated stations, with additional inputs from overseas seismic networks. Analysis is done at the regional data centers, and at the USGS National Earthquake Information Center (NEIC), with the results posted at the USGS earthquake web page (https://earthquake.usgs.gov/earthquakes/).

The National Strong Motion Project of the ANSS has instrumented 168 structures to record their response to very strong shaking. This data is used in research on earthquake-resistant engineering.

Aftershock

An aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to known laws. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.

Asteroseismology

Asteroseismology or astroseismology is the study of oscillations in stars. Because a star's different oscillation modes are sensitive to different parts of the star, they inform astronomers about the internal structure of the star, which is otherwise not directly possible from overall properties like brightness and surface temperature. Asteroseismology is closely related to helioseismology, the study of stellar oscillations specifically in the Sun. Though both are based on the same underlying physics, more and qualitatively different information is available for the Sun because its surface can be resolved.

Earthquake zones of India

The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes. A World Bank and United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050. The latest version of seismic zoning map of India given in the earthquake resistant design code of India [IS 1893 (Part 1) 2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version, which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.

Epicenter

The epicenter, epicentre or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.

Fault (geology)

In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.

Geophone

A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth.

Helioseismology

Helioseismology, a term coined by Douglas Gough, is the study of the structure and dynamics of the Sun through its oscillations. These are principally caused by sound waves that are continuously driven and damped by convection near the Sun's surface. It is similar to geoseismology, or asteroseismology (also coined by Gough), which are respectively the studies of the Earth or stars through their oscillations. While the Sun's oscillations were first detected in the early 1960s, it was only in the mid-1970s that it was realised that the oscillations propagated throughout the Sun and could allow scientists to study the Sun's deep interior. The modern field is separated into global helioseismology, which studies the Sun's resonant modes, and local helioseismology, which studies all the waves propagating at the Sun's surface.Helioseismology has contributed to a number of scientific breakthroughs. The most notable was to show the predicted neutrino flux from the Sun could not be caused by flaws in stellar models and must instead be a problem of particle physics. The so-called solar neutrino problem was ultimately resolved by neutrino oscillations.

The experimental discovery of neutrino oscillations was recognized by the 2015 Nobel Prize for Physics.

Helioseismology also allowed accurate measurements of the quadrupole (and higher-order) moments of the Sun's gravitational potential, which are consistent with general relativity. The first helioseismic calculations of the Sun's internal rotation profile showed a rough separation into a rigidly-rotating core and differentially-rotating envelope. The boundary layer is now known as the tachocline and is thought to be a key component for the solar dynamo. Although it roughly coincides with the base of the solar convection zone—also inferred through helioseismology—it is conceptually a distinct entity.

Helioseismology benefits most from continuous monitoring of the Sun, which began first with uninterrupted observations from near the South Pole over the southern summer. In addition, observations over multiple solar cycles have allowed helioseismologists to study changes in the Sun's structure over decades. These studies are made possible by global telescope networks like the Global Oscillations Network Group (GONG) and the Birmingham Solar Oscillations Network (BiSON), which have been operating for over 20 years.

Hypocenter

A hypocenter (or hypocentre) (from Ancient Greek: ὑπόκεντρον [hypόkentron] for 'below the center') is the point of origin of an earthquake or a subsurface nuclear explosion. In seismology, it is a synonym of the focus. The term hypocenter is also used as a synonym for ground zero, the surface point directly beneath a nuclear airburst.

Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrología

The National Institute for Seismology, Vulcanology, Meteorology and Hydrology of Guatemala (in Spanish: Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrología (INSIVUMEH)) is a scientific agency of the Guatemalan government. The agency was created to study and monitor atmospheric, geophysical and hydrological phenomena and events, its hazards to Guatemalan society, and to provide recommendations to the government and the private sector in the occurrence of natural disasters. The agency has four major science disciplines, concerning Seismology, Vulcanology, Meteorology and Hydrology.

INSIVUMEH was created in March 1976, shortly after the heavy 1976 Guatemala earthquake and is part of Ministry of Communications, Infrastructure and Housing.

Lunar seismology

Lunar seismology is the study of ground motions of the Moon and the events, typically impacts or moonquakes, that excite them.

Marsquake

A marsquake is a quake which, much like an earthquake, would be a shaking of the surface or interior of the planet Mars as a result of the sudden release of energy in the planet's interior, such as the result of plate tectonics, which most quakes on Earth originate from, or possibly from hotspots such as Olympus Mons or the Tharsis Montes. The detection and analysis of marsquakes could be informative to probing the interior structure of Mars, as well as identifying whether any of Mars's many volcanoes continue to be volcanically active or not.

Quakes have been observed and well-documented on the Moon, and there is evidence of past quakes on Venus, but current seismic activity of Mars has not been definitely detected. Some estimates suggest that marsquakes occur as rarely as once every million years or more. Nevertheless, compelling evidence has been found that Mars has in the past been seismically active, with clear magnetic striping over a large region of southern Mars. Magnetic striping on Earth is often a sign of a region of particularly thin crust splitting and spreading, forming new land in the slowly separating rifts; a prime example of this being the Mid-Atlantic Ridge. However, no clear spreading ridge has been found in this region, suggesting that another, possibly non-seismic explanation may be needed.

The 4,000 km (2,500 mi) long canyon system, Valles Marineris, has been suggested to be the remnant of an ancient Martian strike-slip fault. However, even if it was at some point an active fault, it is unknown whether the fault is still active, or if it has "frozen" into place.

P-wave

A P-wave is one of the two main types of elastic body waves, called seismic waves in seismology. P-waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any affected location or at a seismograph. P-waves may be transmitted through gases, liquids, or solids.

Philippine Institute of Volcanology and Seismology

The Philippine Institute of Volcanology and Seismology (PHIVOLCS Tagalog pronunciation: [ˈfivolks]; Filipino: Surian ng Pilipinas sa Bulkanolohiya at Sismolohiya) is a Philippine national institution dedicated to provide information on the activities of volcanoes, earthquakes, and tsunamis, as well as other specialized information and services primarily for the protection of life and property and in support of economic, productivity, and sustainable development. It is one of the service agencies of the Department of Science and Technology.

PHIVOLCS monitors volcano, earthquake, and tsunami activity, and issues warnings as necessary. It is mandated to mitigate disasters that may arise from such volcanic eruptions, earthquakes, tsunamis, and other related geotectonic phenomena.

Reflection seismology

Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite or Tovex blast, a specialized air gun or a seismic vibrator, commonly known by the trademark name Vibroseis. Reflection seismology is similar to sonar and echolocation. This article is about surface seismic surveys; for vertical seismic profiles, see VSP.

S-wave

In seismology, S-waves, secondary waves, or shear waves (sometimes called an elastic S-wave) are a type of elastic wave, and are one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves.The S-wave is a transverse wave, meaning that, in the simplest situation, the oscillations of the particles of the medium is perpendicular to the direction of wave propagation, and the main restoring force comes from shear stress.

Its name, S for secondary, comes from the fact that it is the second direct arrival on an earthquake seismogram, after the compressional primary wave, or P-wave, because S-waves travel slower in rock. Unlike the P-wave, the S-wave cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S-waves opposite to where they originate. They can still appear in the solid inner core: when a P-wave strikes the boundary of molten and solid cores at an oblique angle, S-waves will form and propagate in the solid medium. When these S-waves hit the boundary again at an oblique angle they will in turn create P-waves that propagate through the liquid medium. This property allows seismologists to determine some physical properties of the inner core.

Seismic wave

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy. Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer.

The propagation velocity of seismic waves depends on density and elasticity of the medium as well as the type of wave. Velocity tends to increase with depth through Earth's crust and mantle, but drops sharply going from the mantle to the outer core.Earthquakes create distinct types of waves with different velocities; when reaching seismic observatories, their different travel times help scientists to locate the source of the hypocenter. In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, and man-made vibrations are often generated to investigate shallow, subsurface structures.

Seismometer

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 such a device—formerly recorded on paper (see picture) or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and characterize earthquakes, and to study the Earth's internal structure.

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