Seismology (from the Greek seismos (σεισμός), meaning "earthquake," and -logia (-λογία), meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth. In addition, this field includes studies of the effects of earthquakes, such as tsunamis, and diverse seismic sources, such as volcanic, tectonic, oceanic, and atmospheric processes and artificial explosions. Waves produced by such sources are called seismic waves. Recorded by instruments known as seismographs, seismic waves provide an effective way to image the sources and various structures deep within the Earth.
A related field that uses geological techniques to infer information regarding past earthquakes is known as paleoseismology. A recording of Earth's motions as a function of time is called a seismogram.
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 1906 San Francisco earthquake, the 1964 Alaska earthquake, and the 2004 Sumatra-Andaman earthquake.
Types of seismic waves
There are two main types of seismic waves: body waves and surface waves.
Body waves travel through the Earth's interior. They have been further subdivided into primary waves (P waves) and secondary waves (S waves).
- P waves: These waves, also known as pressure waves, are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. In solids, these waves generally travel almost twice as fast as S waves and are therefore the first waves to appear on a seismogram. When generated by an earthquake, they are less destructive than the S waves and surface waves that follow them.
- S waves: These waves, also known as shear waves, are transverse waves, which means that the ground is displaced perpendicularly to the direction of propagation. S waves travel more slowly than P waves and thus appear later than P waves on a seismogram. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S waves are several times larger in amplitude than P waves from earthquake sources.
Surface waves are analogous to water waves and travel just beneath the Earth's surface. They travel more slowly than body waves, but because they are guided by the Earth's surface (and their energy is 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 Earth's surface, as in the case of a shallow earthquake. They can be the most destructive type of seismic wave.
There are two types of surface waves: Rayleigh waves and Love waves. Theoretically, they can be explained in terms of interacting P and S waves.
- Rayleigh waves: Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. They were predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves—roughly 70 percent of the velocity of S waves.
- Love waves: Love waves are surface waves that cause horizontal shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90 percent of the S wave velocity.
Usefulness of seismic waves
Seismic waves produced by explosions or vibrating controlled sources are the primary method of underground exploration. 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 interior 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.
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 twentieth 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.
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 joule (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.
Seismologists routinely use their studies to make general forecasts about potential seismic hazards. Such forecasts estimate the probability of an earthquake of a particular magnitude affecting a particular location within a certain time span. However, most seismologists do not think it possible to develop a system that would provide timely warnings for individual earthquakes.
Various attempts have been made by seismologists and others to create effective systems for precise earthquake predictions, such as the VAN method. Such methods have yet to be generally accepted by the seismology community.
- Aki, Keiiti, and Paul G. Richards. 2002. Quantitative Seismology, 2nd ed. Sausalito, CA: University Science Books. ISBN 0935702962
- Shearer, Peter M. 1999. Introduction to Seismology. Cambridge: Cambridge University Press. ISBN 0521669537
- Sheriff, Robert E., and L.P. Geldart. 1995. Exploration Seismology, 2nd ed. Cambridge: Cambridge University Press. ISBN 0521468264
- Stein, Seth, and Michael Wysession. 2003. An Introduction to Seismology, Earthquakes, and Earth Structure. Malden, MA: Blackwell Pub. ISBN 0865420785
- What is Seismology? What Are Seismic Waves? UPSeis. Retrieved October 16, 2008.
- Introduction to Seismology USGS. Retrieved October 16, 2008.
- IRIS: Incorporated Research Institutions for Seismology Retrieved October 16, 2008.
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