The solid earth extends from the earth's surface down to its centre. It usually does not include oceans and atmosphere. The mechanical properties of the solid earth are described by the continuum mechanics approach. This means that the earth is split into small volume elements where for each element the material properties are prescribed. Material properties like density and elastic rigidity may vary with depth, which results in a spherically symmetric structure. General physical principles are applied to describe the interaction between the elements. Integration over the volume gives the response of the entire solid earth to an applied external forcing. Here the lithosphere and mantle are of main interest.