Seismic tomography

The seismic tomography is a tool Géophysique using speeds of the seismic waves to study the variations of the temperatures inside the terrestrial sphere. Its essential use is the realization of the cartography of heterogeneities of the terrestrial, very useful coat to establish bonds between tectonics lithospheric and the mantellic convection.

Principle

A natural or caused seism emits seismic waves in all the directions since its hearth or hypocentre. These waves are various natures and different speeds; waves P, waves S etc The physical properties of the crossed medium (moduli of elasticity of Lamé μ and λ, and density ρ) condition the speed of a wave and the way which she traverses. The speed of a wave P in a point is given by Vp = ((λ + 2μ)/ρ) 1/2 and that of a wave S by: Vs = (μ/ρ) 1/2.

With the depth, the pressure increases as well as the density of the rocks and the coefficients elastic. The latter grow more quickly than the density, thus the speed of the waves increases in-depth. The seismic waves follow a curved trajectory. A wave arrives at a seismic station in a certain time, and it is possible to determine a named physical size “run time” of a wave in the coat. This time is function of the angular distance between the two stations and depth of the seism.

If the seismic wave crosses a lithological anomaly in the coat, typically a plunging lithosphere, its real run time will be different from theoretical (which considers the homogeneous coat), because of local modification of density and moduli of elasticity. The anomaly in experiments measured speed is overall about the thousandths of run time, which constrained measurements with being precise.

By recutting the data of anomalies speed of a very great number of ways of waves, it is possible to chart heterogeneities and mathematically to create a model of in-depth coat. Concretely, they are local charts “out of cut” of the coat to a given depth.

Nomenclature of the seismic waves used in tomography

The speed of the two types of waves P and S varies according to the density of crossed material. This propagation velocity of the waves is proportional to the density of crossed materials. The less dense the crossed layer is, plus the waves are propagated slowly. Moreover, when a wave P arrives not perpendicularly on a zone of transition (interface coat - core for example), a small portion of its energy is then converted into another form of wave (a fraction of P becomes S then). The interpretation of the seismographic statements is thus difficult because the layouts of many types of waves overlap there which should be disentangled and which one must explain the origin. To find itself there a little better, one indicated all these waves by different letters which one can then progressively combine of their evolution (see table below).

Wave P Wave S coat P S external core K core interns I J

Thus a PP wave is a wave P which, after having undergone a reflection under the external surface of the terrestrial sphere, remained in the coat before reappearing on the surface where it is detected. A wave PKP will be a wave P which arises on the surface after having crossed the external core liquidates (way = coat/core ext./coat). One can thus lengthen name as much as necessary. Let us take a rather complex example: a quasi vertical wave crossing the terrestrial sphere right through after having rebounded on the surface and to be last twice (with the outward journey and the return) by the core and seed will reappear on the affublée surface of the nice nickname, palindrome completely unpronouncable, of PKIKPPKIKP!

(Fig: Nomenclature of the seismic waves)

History of the discoveries due to the seismic tomography

During the XXe century, several essential discoveries were made thanks to the seismic tomography.

In 1909, Andrija Mohorovičić detects under the Croatia the interface crust/coat called from now on by the close friends, and in homage to its discoverer, Moho.
In 1912, Beno Gutenberg (1889 - 1960) replaces the interface coat/core to 2900 km of depth thanks to the study of the waves P.
In 1926, Harold Jeffreys (1891 - 1989) establishes the fluidity of the metal core.
In 1936, Inge Lehmann (1888 - 1993) discovers seed: metal part inside the core. Its solidity will be established during the following decades.

In same time, of 1923 with 1952, other geophysicists (Adams, Williamson, Bullen, Birch…) work on equations allowing to determine the variation of the density with and the increase of in-depth pressure. From now on, the essence of the structure of our sphere is posed. Remain to improve dynamic comprehension of it interns for better including/understanding its evolution, its sudden starts, the variations of the magnetic field, etc

Tomography of the lithosphere

The tomography high-resolution at a shallow depth makes it possible to visualize the progressive increase in density of the oceanic plates, which cool while moving away from the oceanic dorsals. Contrary, the continental shields archaean have seismic speeds abnormally raised, which mean that they are dense and thus cold. This is with their bad thermal conduction and their strong thickness, that the tomography estimates sometimes at 300 km.

The area of Afars has an interesting characteristic: tomography reveals that it is abnormally very hot, which means that the asthenosphere deep and hot goes up under the continental crust, and is most probably likely to cause the opening of an ocean.

Tomography of the higher coat

Observations

The models obtained by regional seismic tomography highlight bands inclined at abnormally high seismic speed plunging in the coat. The seismic data indicate that they are plates lithospheric in subduction, plunging at least until the interface higher coat - lower coat. These structures are classically observed under the Aegean Sea, where the Africa plate plunges under the Crete.

One does not know exactly until where can plunge a plate. The situations observed are varied: in the case of the Peaceful subduction of the area of the Mariannes, the plunging plate returns clearly in the lower coat, whereas on the level of the Japan, it seems to stagnate and be flattened in extreme cases higher coat - lower coat.

Role of the phases of olivine

This phenomenon would be with the transitions from phase of olivine. To 410 km of depth the transition olivine β - spinel occurs, which is exothermic. As the plunging plate is colder than the coat bordering and lowers the isotherms to its proximity, this transition occurs less deeply than under normal conditions, and Archimedes forces it is increased. On the contrary, the transition between spinel and perovskite to 660 km of depth - marking the limit between higher coat and lower coat, is endothermic, and thus occurs with a depth abnormally raised, which results in to reduce the force of Archimedes, to even cancel it, which implies a possible stagnation of the plate to this limit. However, the lack of information on this subject leaves outstanding many questions, for example the role of the transitions from phase of “minor” minerals of the coat in the dynamics of plunging of the plates.

Tomography of the lower coat

The seismic tomography reveals that the lower coat is less heterogeneous than the higher coat, and the anomalies observed do not have a bond with current tectonics lithospheric. Abnormally cold zones are however observed, which would correspond to the zones where oceanic lithosphere was subduite in the past.

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