Theoretical Seismology studies the propagation and generation of seismic waves in the earth. Because of heterogeneity of seismic wave velocities in the interior of our planet, seismic wave propagation is very complex: seismic waves are refracted, reflected, and converted by numerous internal discontinuities, and they are scattered and attenuated by inhomogeneities. They are also trapped as surface waves in the shallow, low-velocity layers of the earth. The kinematic and dynamic properties of seismic waves are used by seismologists to study the interior of the earth and to determine the rupture process that generates earthquakes.