The deformable porous media possess four distinct seismic wave processes, namely, fast- and slow-waves each associated with compression and shear deformations. The fast compressional (shear) wave is essentially motion of a porous medium as a whole such that constituent phases undergo volume (shape) change in the same manner. The linear momentum fluxes are associated with these motions and they describe transport of translational kinetic energy. The slow compressional (shear) wave is basically motions of constituent phases undergoing volume (shape) in equal but opposite manner such that the medium as a whole is at rest. This new mode of deformation amounts to existence of an intrinsic angular momentum (spin). The slow waves are basically spin fluxes and they describe transport of rotational kinetic energy.
Pratap N. Sahay,
"Nature of waves in deformable porous media", Proc. SPIE 2571, Mathematical Methods in Geophysical Imaging III, (1 September 1995); doi: 10.1117/12.218502; https://doi.org/10.1117/12.218502