Seismic wave
Part of a series on |
Earthquakes |
---|
A seismic wave is a
The propagation velocity of a seismic wave 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 Earth's outer core.[2]
Earthquakes create distinct types of waves with different velocities. When recorded by a seismic observatory, their different travel times help scientists locate the quake's hypocenter. In geophysics; the refraction or reflection of seismic waves is used for research into Earth's internal structure. Scientists sometimes generate and measure vibrations to investigate shallow, subsurface structure.
Types
Among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earth's surface.[3]: 48–50 [4]: 56–57
Other modes of wave propagation exist than those described in this article; though of comparatively minor importance for earth-borne waves, they are important in the case of asteroseismology.
- Body waves travel through the interior of the Earth.
- Surface waves travel across the surface. Surface waves decay more slowly with distance than body waves which travel in three dimensions.
- Particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage.
Body waves
Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of
Primary waves
Primary waves (P-waves) are compressional waves that are
Secondary waves
Secondary waves (S-waves) are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Shear waves can not travel through any liquid medium,[6] so the absence of S-waves in earth's outer core suggests a liquid state.
Surface waves
Seismic surface waves travel along the Earth's surface. They can be classified as a form of mechanical surface wave. Surface waves diminish in amplitude as they get farther from the surface and propagate more slowly than seismic body waves (P and S). Surface waves from very large earthquakes can have globally observable amplitude of several centimeters.[7]
Rayleigh waves
Rayleigh waves, also called ground roll, are surface waves that propagate with motions that are similar to those of waves on the surface of water (note, however, that the associated seismic particle motion at shallow depths is typically retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt,
Love waves
Love waves are horizontally
Stoneley waves
A Stoneley wave is a type of boundary wave (or interface wave) that propagates along a solid-fluid boundary or, under specific conditions, also along a solid-solid boundary. Amplitudes of Stoneley waves have their maximum values at the boundary between the two contacting media and decay exponentially towards away from the contact. These waves can also be generated along the walls of a fluid-filled borehole, being an important source of coherent noise in vertical seismic profiles (VSP) and making up the low frequency component of the source in sonic logging.[11] The equation for Stoneley waves was first given by Dr. Robert Stoneley (1894–1976), Emeritus Professor of Seismology, Cambridge.[12][13]
Normal modes
Free oscillations of the Earth are standing waves, the result of interference between two surface waves traveling in opposite directions. Interference of Rayleigh waves results in spheroidal oscillation S while interference of Love waves gives toroidal oscillation T. The modes of oscillations are specified by three numbers, e.g., nSlm, where l is the angular order number (or spherical harmonic degree, see Spherical harmonics for more details). The number m is the azimuthal order number. It may take on 2l+1 values from −l to +l. The number n is the radial order number. It means the wave with n zero crossings in radius. For spherically symmetric Earth the period for given n and l does not depend on m.
Some examples of spheroidal oscillations are the "breathing" mode 0S0, which involves an expansion and contraction of the whole Earth, and has a period of about 20 minutes; and the "rugby" mode 0S2, which involves expansions along two alternating directions, and has a period of about 54 minutes. The mode 0S1 does not exist because it would require a change in the center of gravity, which would require an external force.[3]
Of the fundamental toroidal modes, 0T1 represents changes in Earth's rotation rate; although this occurs, it is much too slow to be useful in seismology. The mode 0T2 describes a twisting of the northern and southern hemispheres relative to each other; it has a period of about 44 minutes.[3]
The first observations of free oscillations of the Earth were done during the great 1960 earthquake in Chile. Presently the periods of thousands of modes have been observed. These data are used for constraining large scale structures of the Earth's interior.
P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the
Notation
The naming of seismic waves is usually based on the wave type and its path; due to the theoretically infinite possibilities of travel paths and the different areas of application, a wide variety of nomenclatures have emerged historically, the standardization of which - for example in the IASPEI Standard Seismic Phase List - is still an ongoing process.[14] The path that a wave takes between the focus and the observation point is often drawn as a ray diagram. Each path is denoted by a set of letters that describe the trajectory and phase through the Earth. In general, an upper case denotes a transmitted wave and a lower case denotes a reflected wave. The two exceptions to this seem to be "g" and "n".[14][15]
c | the wave reflects off the outer core |
d | a wave that has been reflected off a discontinuity at depth d |
g | a wave that only travels through the crust |
i | a wave that reflects off the inner core |
I | a P-wave in the inner core |
h | a reflection off a discontinuity in the inner core |
J | an S wave in the inner core |
K | a P-wave in the outer core |
L | a Love wave sometimes called LT-Wave (Both caps, while an Lt is different) |
n | a wave that travels along the boundary between the crust and mantle |
P | a P wave in the mantle |
p | a P wave ascending to the surface from the focus |
R | a Rayleigh wave |
S | an S wave in the mantle |
s | an S wave ascending to the surface from the focus |
w | the wave reflects off the bottom of the ocean |
No letter is used when the wave reflects off of the surfaces |
For example:
- ScP is a wave that begins traveling towards the center of the Earth as an S wave. Upon reaching the outer core the wave reflects as a P wave.
- sPKIKP is a wave path that begins traveling towards the surface as an S-wave. At the surface, it reflects as a P-wave. The P-wave then travels through the outer core, the inner core, the outer core, and the mantle.
Usefulness of P and S waves in locating an event
In the case of local or nearby earthquakes, the difference in the
A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in
At teleseismic distances, the first arriving P waves have necessarily travelled deep into the mantle, and perhaps have even refracted into the outer core of the planet, before travelling back up to the Earth's surface where the seismographic stations are located. The waves travel more quickly than if they had traveled in a straight line from the earthquake. This is due to the appreciably increased
The travel time must be calculated very accurately in order to compute a precise hypocenter. Since P waves move at many kilometers per second, being off on travel-time calculation by even a half second can mean an error of many kilometers in terms of distance. In practice, P arrivals from many stations are used and the errors cancel out, so the computed epicenter is likely to be quite accurate, on the order of 10–50 km or so around the world. Dense arrays of nearby sensors such as those that exist in California can provide accuracy of roughly a kilometer, and much greater accuracy is possible when timing is measured directly by cross-correlation of seismogram waveforms.
See also
References
- (PDF) from the original on 24 August 2016.
- ^ Shearer 2009, Introduction
- ^ a b c Shearer 2009, Chapter 8 (Also see errata Archived 2013-11-11 at the Wayback Machine)
- ISBN 978-14443-1131-0.
- ^ Poisson, S. D. (1831). "Mémoire sur la propagation du mouvement dans les milieux élastiques" [Memoir on the propagation of motion in elastic media]. Mémoires de l'Académie des Sciences de l'Institut de France (in French). 10: 549–605.
- ^ "Seismic Waves". Burke Museum of Natural History and Culture. Retrieved March 24, 2019.
- ISBN 978-0-08-086012-1.
- ^ Rayleigh, Lord (1885). "On waves propagated along the plane surface of an elastic solid". Proceedings of the London Mathematical Society. 17: 4–11.
- ISBN 0-521-46826-4.
- ^ Love, A.E.H. (1911). Some problems of geodynamics; …. London, England: Cambridge University Press. pp. 144–178.
- ^ "Schlumberger Oilfield Glossary. Stoneley wave". Archived from the original on 2012-02-07. Retrieved 2012-03-07.
- .
- ^ Robert Stoneley, 1929 – 2008.. Obituary of his son with reference to discovery of Stoneley waves.
- ^ ISSN 0895-0695.
- ISBN 9780080489223.
Sources
- Shearer, Peter M. (2009). Introduction to Seismology. Cambridge University Press. ISBN 978-0-521-88210-1.