ABSTRACT
This chapter examines the physical properties of the materials that make up the Earth. Seismology is the science that analyzes the causes of earthquakes and the propagation of waves within Earth and on its surface. In seismology, the source of the earthquake is called the focus or hypocenter. Although seismic waves are very different from electromagnetic waves in general, and light waves in particular, they share some properties. Similar to light in optics, a point source of mechanical shock establishes spherical wave surfaces around itself. A wave surface is the geometric locus of points that have been affected by deformation and at the same time are vibrating in phase. A seismic ray is reflected when it intersects the interface of two media that have different velocities. The study of seismic responses at different points on the Earth has revealed the geometry and composition of layers within the Earth.
