Info

200 400 600 800

Depth, km

Figure 3.3. Structure of the upper mantle from the spherical Earth model PREM (Dziewonski and Anderson, 1981).

p, 1011 Pa

Figure 3.4. Dependences of the Earth's adiabatic bulk modulus Ka and shear modulus or rigidity / on pressure in the spherical Earth model PREM (Dziewonski and Anderson, 1981).

p, 1011 Pa

Figure 3.4. Dependences of the Earth's adiabatic bulk modulus Ka and shear modulus or rigidity / on pressure in the spherical Earth model PREM (Dziewonski and Anderson, 1981).

and rigidity with pressure through the mantle are shown in Figure 3.4. PREM is constructed assuming three first-order discontinuities in the upper mantle, at depths of 220 km, 400 km, and 650km. Like its predecessor PEM (Parametric Earth Model, Dziewonski et al., 1975), the profiles consist of polynomials within each layer. PREM assumes isotropy everywhere except between the Moho and 220 km depth, where transverse anisotropy is permitted. PREM also contains anelasticity, through the quality factor Q (inverse attenuation), and frequency-dependent elastic moduli. A major improvement over earlier models, such as Bullen's, is the presence of fine structure. Despite the fact that fine structure is the least certain component, it is the main feature that distinguishes one spherical model from another. Small differences in fine structure can lead to different interpretations regarding composition and its variation with depth in the mantle, which in turn may lead to grossly different inferences about the style of convection. Modern spherical Earth models (e.g., IASPEI1991, see Kennett, 1991) have verified the major subdivisions listed in Table 3.1. These divisions provide the basis for discussions of the Earth's composition.

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