There are many models that attempt to describe the dynamics of seismology that include plasticity, temperature effect, dissipation and viscosity. While these details are important, useful results of dynamic deformation can be obtained from simple models. The essential features are the internal geometry of porosity and cracked medium. They involve the specific surface or the average distance between one crack to another. This entails the internal sizes of micro and mesostructure that are not considered by classical mechanics. Only one new parameter is introduced in the simple model, the specific surface while the other parameters are the Lame constants and density. (C) 2002 Elsevier Science Ltd. All rights reserved.