WPP (Wave Propagation Program) is the predecessor of SW4. It discretizes the seismic wave equations using a second order accurate numerical method that satisfies the principle of summation by parts. SW4 (Seismic Waves, 4th order) implements substantial capabilities for 3-D seismic modeling, with a free surface condition on the top boundary, absorbing super-grid conditions on the far-field boundaries [PS-13], and an arbitrary number of point force and/or point moment tensor source terms. Each source time function can have one of many predefined analytical time dependencies, or interpolate a user defined discrete time series. Complex rupture mechanisms can be described with the SRF file format. SW4 supports a fully 3-D heterogeneous material model that can be specified in several formats. It uses a curvilinear mesh near the free surface to honor the free surface boundary condition on a realistic topography. The curvilinear mesh is automatically generated from the description of the topography. To make SW4 more computationally efficient, the seismic wave equations are discretized on a Cartesian mesh below the curvilinear grid. The Cartesian mesh, which extends to the bottom of the computational domain, is also generated automatically.