Emhmml

Figure 2.23 (a) Distribution of isobars of strength in compression (after Kirby, 1980). (b) Distribution of isobars of strength in extension (after Kirby, 1980).

E and m are as previously defined, and h is the thickness of the plate, in the engineering sense, or, here, refers to the thickness of the strong layer.

When dealing with engineering problems, it is reasonable to assume that a specific metal will have a constant value of E, but one cannot reasonably make this assumption for the upper oceanic lithosphere. It is therefore pertinent to refer to data relating to the elastic moduli of the mantle as listed in Preliminary Reference Earth Model (PREM) or another seismically inferred data set of mantle elasticity.

Seismology gives values for the bulk modulus (K), the shear modulus (G) and Poisson's ratio (which can be readily transposed into Poisson's number m). If one assumes that the mantle is homogeneous and isotropic, one can calculate the value of E from the known values of G and m, using the relationship:

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