Stress Propagation of seismic waves is caused by oscillating motions of the solid materials within the Earth. In turn, these oscillating motions are produced by external forces. Transmission of these forces is usually described in terms of stress and strain. In geophysics, stresses are often expressed in bars.
Stresses are tensors because it is a vector divided by another vector.
A graphical representation of stresses is given in the diagrams below. There are two different types of stresses. When a force is acting normal to a surface, it is called a normal stress. When a force is acting parallel to a surface, it is called a shear stress. Stresses can also be expressed in matrix forms as indicated below.