Preliminary Reference Earth Model

The 1980 paper written by Adam M. Dziewonski and Don L. Anderson is one of the most cited scientific work ever. The study presented a new Earth model, PREM, that satisfies the guidelines established by the Standard Earth Model (SEM) Committee.

The concept of the model

The assumptions used in the study as the trial starting value includes the oceanic crust covers two-thirds of the Earth’s surface and the average depth to the Moho is 19 km for the average Earth (11 km under the oceans 35 km under continents).

Also it is recognized the following principal regions within the Earth:

1. Ocean layer
2. Upper and lower crust
3. Region above the low velocity zone (LID), considered to be the main part of the seismic lithosphere. When we finally dropped the assumption of isotropy the distinction between LID and LVZ became less pronounced.
4. Low velocity zone (LVZ).
5. Region between low velocity zone and 400 km discontinuity
6. Transition zone spanning the region between the 400 and 670 km discontinuities
7. Lower mantle. In our work we found it necessary to subdivide this region into three parts connected by second-order discontinuities
8. Outer core
9. Inner core

The gross Earth data set: Three principal subsets of data:

1. Astronomic-geodetic data Radius of the Earth: 6371 km. Mass: 5.974x10^24 kg. I/MR^2: 0.3308. These values were used as constraints.
2. Free oscillation and long-period surface-wave data There is an impressive set of measurements of eigenfrequencies for over 1000 normal modes. The precision of measurements varies appreciably: from up to 4x 10^-3 for some of the toroidal overtones to the 4x 10^-6 for the fundamental radial mode. The set of normal mode data consists of about 900 modes.
3. Body waves

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Inversion and the final model

Discussion