Geoscience Reference
In-Depth Information
Chapter 8
The deep interior of the Earth
8.1 The internal structure of the Earth
8.1.1 Seismic velocities for the whole Earth
The variations of seismic velocities in the Earth are determined from the travel-
time curves for seismic waves (Fig. 4.16). There are two approaches to this
determination: one is an inverse method and the other is forward. In the inverse
problem used in the early determinations, such as that of Jeffreys (see Appendix 3
for details), the velocities are obtained directly from the travel times. In the
forward method, a velocity-depth model is assumed and travel times calculated
and compared with observations. The model is then adjusted until agreement at
the desired level is attained. The surface-wave-dispersion and normal-mode data
(Sections 4.1.3 and 4.1.4) are also used in velocity determination. Standard radial
reference velocity models PREM (Fig. 8.1) and
iasp91
are listed in Appendices
8 and 9. Other radial-velocity models have been constructed to have the best fit to
P- and S-wave arrival times (e.g.,
SP6
and
ak135
). The zones in which particular
refinements of the structure are still taking place are the low-velocity zone in the
upper mantle, the core-mantle boundary region and the transition zone between
the inner core and the outer core - everywhere else the gross P- and S-wave
velocities are known to within better than
0.01 km s
−
1
.
±
Crust
The seismic structure of the continental crust is variable, but it has an average
thickness of 35 km and an average P-wave velocity of about 6.5 km s
−
1
(see
Section 10.1.2). The oceanic crust is thinner, 7-8 km thick, with an average
P-wave velocity of more than 6 km s
−
1
(see Section 9.2.1).
Mantle
The discontinuity between the crust and mantle, which is compositional (Section
8.1.4), is called the Mohorovicic discontinuity. The normal P-wave velocity at
the top of the mantle is 8.1 km s
−
1
.
The uppermost mantle is very heterogeneous, its structure being depen-
dent upon plate processes and history. There does not seem to be a universal
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