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ScienceWeek
GEOPHYSICS: ON SEISMIC NOISE
The following points are made by Richard L. Weaver (Science 2005 307:1568):
1) It is commonly supposed that noise obscures, but does not contain, useful information. Intuition suggests that multiple scattering of waves garbles them into illegibility. Yet insights arising out of a branch of physics called "mesoscopic physics" are challenging this assumption. Theory shows that regardless of scattering, linear waves preserve a residual coherence. This coherence leads to behaviors that confound intuition, such as Anderson localization in which a multiply scattered wave field is confined to a finite volume and unable to diffuse.
2) Such residual coherences can also be useful in seismology. Shapiro et al [1] have analyzed seismic noise to obtain new information on the structure of Earth's crust. By correlating the data from a month of ambient noise, due in part to wave-wave interactions in the ocean [2], detected by 62 long-period seismograph stations in southern California, they determined the seismic response that they would have obtained from Earth's crust if they had applied forces at each of their stations. In particular, they measured the times that it took for seismic surface waves to propagate between every pair of stations. They then used tomographic techniques to create a map of seismic wave velocity with an unprecedented horizontal resolution of 75 to 100 km. The map is consistent with presumed geologic structures to a depth of 20 km. As new high-density seismograph networks come online, such results can be extended throughout the United States.
3) Correlation of seismic noise is a new and intriguing tool with numerous possible applications. Examples include oil exploration without explosives or thumper trucks, seismic wave profiling and deep Earth tomography from arbitrary positions without waiting for an earthquake, and the extraordinary pleasure of using and interpreting a wealth of data that were previously considered worthless.
4) The term "mesoscopic" is taken from low-temperature electronics, where electrons remain quantum mechanically coherent over the almost macroscopic intervals needed for electronic transport in modern small devices. Constructive and destructive interferences of the electron wave lead to a wealth of fascinating phenomena. For example, mesoscopic fluctuations of electronic conductance affect the electronic properties of the devices. The behaviors are not confined to quantum mechanical systems, but are a consequence of linearity and of the constancy in time of the structures. Related phenomena have been observed for acoustic, seismic, and optical waves (3).
5) A transient seismic source such as an earthquake often causes two different sets of seismic waves: a main wave that propagates directly from the source, and a long-duration noisy "coda" consisting of waves (or rays) that have been scattered or reflected at least once. The variations in the intensity of the seismic coda with time have long been known to be characteristic of a region, but independent of the earthquake (4). Recent work [5] found that at least for a region in Mexico, the seismic coda has an additional property: Its energy is distributed in a characteristic way (equipartitioned) among the various types of seismic waves. Such partitioning is a consequence of multiple scattering. The observation thus indicates that coda waves have been scattered several times.
References (abridged):
1. N. M. Shapiro, M. Campillo, L. Stehly, M. H. Ritzwoller, Science 307, 1615 (2005)
2. S. Kedar, F. Webb, Science 307, [682] (2005)
3. S. E. Skipetrov, B. A. van Tiggelen, Eds., Wave Scattering in Complex Media: From Theory to Applications (Kluwer, Dordrecht, Netherlands, 2003)
4. M. Fehler, H. Sato, Pure Appl. Geophys. 160, 541 (2003)
5. R. Hennino et al., Phys. Rev. Lett. 86, 3447 (2001)
Science http://www.sciencemag.org
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Related Material:
ON SEISMIC TOMOGRAPHY AND MANTLE DYNAMICS
In general, the term "tomography" refers to a representation in cross-section in which neighboring 2-dimensional cross-sections are combined to provide a 3-dimensional model. The use of computer-aided tomography (CAT) in medical diagnosis is well-known as a non-invasive method of examining internal organs for abnormal regions. X-rays or ultrasonic waves are absorbed unequally be different materials, and computer-aided tomography consists of studying the attenuation of x-rays or ultrasonic waves that pass through the body in distinctly controlled planar sections. The technique of "seismic tomography" uses the same principles, with the difference that the travel-times of the signals, rather than their attenuation, are observed. Thus, the technique of seismic tomography may be described as the 3-dimensional modeling of the velocity distribution of seismic waves in the Earth. In general, the technique requires powerful computational facilities and sophisticated programming.
The following points are made by T. Tanimoto and T. Ley (Proc. Nat. Acad. Sci. 2000 97:12409):
1) The authors point out that the advent of the theory of plate tectonics approximately 30 years ago established that most near-surface geological phenomena such as earthquakes, volcanoes, and mountain belts can be understood in the context of a unifying model of interacting surface plates. However, our understanding of this system has largely been limited to detailed kinematics of plate motions, leaving the nature of the driving motions in the interior as a puzzle. Questions such as what is the configuration of convection, and how are surface tectonics controlled by internal processes, have long been raised, but a lack of tools and a lack of evidence prevented evaluation of various hypotheses. Thus, most views regarding mantle dynamics remained highly speculative until recently. Seismic tomography, which emerged in the early 1980s, has provided a major probe of the dynamical system of which plates are just the surface veneer.
2) The primary question concerning mantle dynamics is whether mantle convection occurs in mantle-wide convective cells or whether it involves a layered system, with separate flow regimes in the upper mantle (i.e., above 650 kilometers) and lower mantle. One of the most exciting results from work during the last 5 years is the verification of deep penetration of former oceanic lithosphere into the lower mantle. Tomography shows thickened tabular extensions of subducted material to depths as great as 2000 kilometers directly below deep subduction zones where earthquakes occur in oceanic slabs down to approximately 650-kilometer depth. Thus, strictly layered mantle convection can now be ruled out with good confidence.
3) In summary, seismic tomography has resulted in breakthrough advances in the last two decades, revealing fundamental geodynamical processes throughout the Earth's mantle and core. Convective circulation of the entire mantle is taking place, with subducted oceanic lithosphere sinking into the lower mantle, overcoming the resistance to penetration provided by the phase boundary near 650-kilometer depth that separates the upper and lower mantle. The boundary layer at the base of the mantle has been revealed to have complex structure, involving local stratification, extensive structural anisotropy, and massive regions of partial melt. The Earth's high *Rayleigh number convective regime is now recognized to be much more interesting and complex than suggested by textbook cartoons, and continued advances in seismic tomography, geodynamic modeling, and high-pressure-high-temperature mineral physics will be needed to fully quantify the complex dynamics of our planet's interior.
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes by ScienceWeek:
Rayleigh number: The Rayleigh number is a dimensionless parameter used in the theory of fluid dynamics. In general, the Rayleigh number provides a determination of when convection is initiated in a fluid.
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Related Material:
ON SEISMIC ANISOTROPY AND MANTLE DEFORMATION
The following points are made by M.K. Savage (Revs. Geophys. 1999 37:65):
1) Shear wave splitting measurements now allow us to examine deformation in the lithosphere and upper asthenosphere with lateral resolution less than 50 km. In an anisotropic medium, one component of a shear wave travels faster than the orthogonal component. The difference in speed causes the waves to separate; this phenomenon is called "shear wave splitting". The polarization of the fast component and the time delay between the components provide simple measurements to characterize the anisotropy. Strain aligns highly anisotropic olivine crystals in the mantle, which is the most likely cause of splitting measured from records of distant earthquakes. The seismic community is in the fundamental stages of determining the relations between strain and anisotropy, measuring anisotropy around the world, and determining how much is formed by past and present lithospheric deformation and how much is formed by crustal-asthenospheric sources.
2) The mantle appears isotropic between 600 km depth and the D" layer at the top of the core-mantle boundary. Shear wave anisotropy of up to 4% is ubiquitous in the upper 200 km of the crust and mantle. Evidence for stronger and deeper anisotropy is less common. Anisotropy in the transition zone between 400 and 600 km and in the D" layer may be patchy. Transcurrent deformation at plate boundaries appears to be one of the best mechanisms for causing splitting on nearly vertically traveling waves by aligning foliation planes and the fast axes of olivine within the lithosphere parallel to the boundary and in the most favorable orientation for splitting. Similar deformation may also contribute to anisotropy observed at convergent margins.
3) Shear wave splitting data are challenging conventional beliefs about mantle flow. Simple models of asthenosphere diverging at spreading centers and flowing downward beneath subduction zones appear to be only part of the story, with significant components of flow parallel to ridges and trenches. Parallelism between fast polarizations of waves passing through the deep mantle beneath cratons and surficial geological strain indicators has been used to suggest that the mantle at depths of several hundred kilometers beneath the cratons may have been stable since the initial deformation in the Archean. New paths of investigation include testing a wider range of anisotropic symmetry systems and more complicated models by examining variations in splitting as a function of earthquake arrival angle and distance and by numerical modeling of waveforms and of proposed deformation scenarios.
Reviews of Geophysics http://www.agu.org/rog/
ScienceWeek http://scienceweek.com
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