Preliminary Reference Earth Model

PREM ( Preliminary Reference Earth Model Abbreviation for ) is a reference model for seismic velocities, density, pressure and other physical parameters in the Earth's interior. It is beside the IASP91 reference model one of the most commonly used earth models.

Basics

The propagation of seismic waves through the earth system is dependent on the distribution of seismic velocities in the Earth's interior. This again depends on the elastic parameters that vary with the pressure and density. From the lifetime of different seismic wave trains that run from one earthquake source to different distant stations, conclusions can be drawn on the global change of the velocities with depth.

Different wave phases run it on different ray paths through the earth body and therefore provide information from different depths. For a variety of individual measured values ​​of the gradual progression of the speed curve can be determined.

PREM

The Preliminary Reference Earth Model was developed in 1981 by Adam M. Dziewoński and Don L. Anderson. It is a 1D velocity model for the Earth. The radius data refer to the mean earth radius of 6371 km.

For a more precise determination of the change in density with depth in addition to seismic waves and oscillations of the earth were considered. Local speed changes by small-scale heterogeneities have been eliminated by averaging over a large number of individual measurements. An advantage over other reference models is also the inclusion of anisotropy in the upper 200 km.

The figure shows the velocity curves (light and dark green) compared according to PREM directly with those of the IASP91 model ( gray and black). Striking differences are, for example, in the depths of seismic discontinuities. So the Mohorovičić discontinuity ( crust-mantle boundary) is located 10 km shallower than in IASP91 by PREM, while the thickness of the mantle transition zone (MTZ ) is greater by about 20 km to PREM. How IASP91 also takes into account only the PREM continental crust. For the much thinner oceanic crust both are not valid.

A significant difference to IASP91 is also the very pronounced jump in velocity within the upper mantle at 220 km ( Lehmann discontinuity ), which is represented by recent earth models rather than change in the gradient instead of a real jump in speed.