The speed and data storage limitations of most current computer systems make the cost of realistic three dimensional seismic simulations prohibitive. One exception is the implementation of the earthquake simulation code TRES on the ILLIAC IV. The ILLIAC IV is a parallel processor, consisting of 64 processing elements, each executing the same instruction on different data. The TRES program simulates the propagation of elastic waves emanating from an earthquake using a centered, finite difference scheme over a large three dimensional grid. The algorithms of the program have been modified for the unique computational environment of the ILLIAC IV parallel processor.