Preliminary reference Earth model PREM

Figure 2 Preliminary reference Earth model (PREM) (source Dziewonski and Anderson, 1981).

Probably the most comprehensive box model for the whole Earth is the GERM model (Geochemical Earth Reference Model, Staudigel et al. (1998), http //www.earthref.org/), Fig. 1.1. The purpose of this model is to provide a geochemical reference model for the Earth, similar to the Preliminary Reference Earth Model (PREM) used in geophysics (see Chapter 3, Section 3.1). In detail the model... 

The densities of the different layers within the Earth can be also be inferred by combining information from P- and S-body waves with data from surface wave oscillation periods. This then permits a density-depth profile to be constructed for the Earth, the mantle section of which is shown in Fig. 3.1. These data have been progressively improved and refined into a reference model for the Earth showing the average depth-velocity-density structure of the Earth. These data were first presented as a Preliminary Reference Earth Model (PREM -Dziewonski Anderson, 1981), the mantle part of which was subsequently refined by Montagner and Anderson (1989). 

These tests show that the average thickness of the seismic lithosphere (crust plus upper-mantle lid) can be as much as c. 160 km and the minimum S-wave velocity beneath the lid can be as high as c. 4.45kms and still produce synthetic waveforms that match the observed waveforms. A thicker lid or higher S-wave velocities at depth below the lid are not consistent with the regional seismic waveforms. It is the high S-wave velocity lid that is unique to the upper mantle of the shield below that, the S-wave velocity is not significantly different from PREM (Preliminary Reference Earth Model) (Fig. 2). 

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