Seismology

Seismology is stupidthe Seismology 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 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.

Seismic waves

Main article: Seismic wave

Seismic waves are elastic waves that propagate in solid or fluid materials. Because seismic waves commonly propagate efficiently and interact with internal structure, they provide the highest resolution noninvasive methods for studying Earth's interior. Solids support two fundamental types of seismic waves P-waves, S-waves (both body waves). A seismic wave field may also contain interface waves, such as surface waves. The two basic kinds of surface waves (Rayleigh and Love) which travel along a solid-air interface. Interface waves can be theoretically explained in terms of interacting P- and/or S-waves.

Propagation of seismic wave in the ground and the effect of presence of land mine.

Pressure waves or Primary waves (P-waves), are longitudinal waves that travel at maximum velocity within solids and are therefore the first waves to appear on a seismogram.

S-waves, also called shear or secondary waves, are transverse waves that travel more slowly than P-waves and thus appear later than P-waves on a seismogram. Particle motion is perpendicular to the direction of wave propagation. Shear waves do not exist in fluids with essentially no shear strength, such as air or water.

Surface waves travel more slowly than P-waves and S-waves, but because they are guided by the surface of the Earth (and their energy is thus trapped near the Earth's surface) they can be much larger in amplitude than body waves, and can be the largest signals seen in earthquake seismograms. They are particularly strongly excited when the seismic source is close to the surface of the Earth, such as the case of a shallow earthquake or explosion.

For large enough earthquakes, one can observe the normal modes of the Earth. These modes are excited as discrete frequencies and can be observed for days after the generating event. The first observations 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 Great Chilean earthquake and the 1964 Great Alaskan earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.

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. Pressure waves (P-waves) pass through the core. Transverse or shear waves (S-waves) that shake side-to-side require rigid material so they do not pass through the outer core. Thus, the liquid core causes a "shadow" on the side of the planet opposite of the earthquake where no direct S-waves are observed. The reduction in P-wave velocity of the outer core also causes a substantial delay for P waves penetrating the core from the (seismically faster velocity) mantle.

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, faults, anticlines and other geologic traps in petroleum-bearing rocks, geological faults, rock types, and long-buried giant meteor craters. For example, the Chicxulub impactor, which is believed to have killed the dinosaurs, was localized to Central America by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration.

Using seismic tomography with earthquake waves, the mantle of the Earth has been completely mapped to a resolution of several hundred kilometers. This process has enabled scientists to identify convection cells, mantle plumes and other large-scale features of the inner Earth such as Ultra Low Velocity Zones near the core-mantle boundary.

Seismographs are instruments that sense and record the motion of the Earth. Networks of seismographs today continuously monitor the seismic environment of the planet, allowing for the monitoring and analysis of global earthquakes and tsunami warnings, as well as recording a variety of seismic signals arising from non-earthquake sources ranging from explosions (nuclear and chemical), to pressure variations on the ocean floor induced by ocean waves (the global microseism), to cryospheric events associated with large icebergs and glaciers. Above-ocean meteor strikes as large as ten kilotons of TNT, (equivalent to about 4.2 × 10¹³ J of effective explosive force) have been recorded by seismographs. A major motivation for the global instrumentation of the Earth with seismographs has been for the monitoring of nuclear testing.

One of the first attempts at the scientific study of earthquakes followed the 1755 Lisbon earthquake. Other especially notable earthquakes that spurred major developments in the science of seismology include the 1857 Basilicata earthquake, 1906 San Francisco earthquake, the 1964 Alaska earthquake and the 2004 Sumatra-Andaman earthquake. An extensive list of famous earthquakes can be found on the List of earthquakes page.

Earthquake prediction

Main article: Earthquake prediction

Forecasting a probable timing, location, magnitude and other important features of a forthcoming seismic event is called earthquake prediction. 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 significant warning of impending seismic events. More general forecasts, however, are routinely used to establish 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.

Various attempts have been made by seismologists and others to create effective systems for precise earthquake predictions, including the VAN method. Such methods have yet to be generally accepted in the seismology community.

Notable seismologists

See also: Category:Seismologists

• Aki, Keiiti
• Anderson, John G.
• Beroza, Gregory
• Bolt, Bruce
• Claerbout, Jon
• Dziewonski, Adam Marian
• Ewing, Maurice
• Galitzine, Boris Borisovich
• Gamburtsev, Grigory A.
• Gutenberg, Beno
• Hanks, Thomas, C.
• Hough, Susan
• Hutton, Kate
• Jeffreys, Harold
• Jones, Lucy
• Jordan, Thomas
• Kanamori, Hiroo
• Keilis-Borok, Vladimir
• Knopoff, Leon
• Lehmann, Inge
• Mallet, Robert
• Mercalli, Giuseppe
• Milne, John
• Mohorovičić, Andrija
• Oldham, Richard Dixon
• Sebastião de Melo, Marquis of Pombal
• Press, Frank
• Richards, Paul G.
• Richter, Charles Francis
• Sekiya, Seikei
• Sieh, Kerry
• Paul G. Silver
• Tucker, Brian
• Vidale, John
• Wen, Lianxing
• Winthrop, John
• Zhang Heng

See also

• Ambient Vibrations
• Asteroseismology
• Catastrophe modeling
• Cryoseism
• Doublet earthquake
• Earthquake engineering
• Engineering geology
• Geophysics
• GNS Science (formerly the Institute of Geological and Nuclear Sciences) (in New Zealand)
• Helioseismology
• The IRIS Consortium
• Isoseismal map
• Plate tectonics
• Reflection seismology
• Geophysical migration
• Seismic loading
• Seismic performance analysis
• Seismic source
• Seismite
• Seismo-electromagnetics
• Seismometer
• Seismotectonics
• Volcanology

References

• "Oxford Dictionary of Earth Sciences," edited by Ailsa Allaby and Michael Allaby, Second Edition, Reissued with corrections and new covers, Oxford University Press, 2003.
• Lay, t., ed. 2009. Seismological Grand Challenges in Understanding Earth’s Dynamic Systems. Report to the National Science Foundation, IRIS consortium, 76 pp.