This chapter focuses on the software for calculating earthquake ground motions from finite faults in vertically varying media. The COMPSYN (Complete Synthetic) software uses the numerical techniques of Spudich and Archuleta to evaluate the representation theorem integrals on a fault surface. The COMPSYN package has two primary strengths. First, the Green's functions include the complete response of the Earth structure, so that all P and S waves, surface waves, leaky modes, near-field terms, and static displacements are included in the calculated seismograms. Second, the codes are computationally fairly fast (typical computation times are minutes) compared to fully three-dimensional codes, so that the user can simulate ground motions from many hypothetical rupture models in minimal time. The ISOSYN (Isochrone-integration Synthetic) applications use the isochrone-integration technique of Bernard and Madariaga (1984) and Spudich and Frazer (1984) as their basic calculational algorithm. These applications are appropriate for calculating high-frequency ground motions near large earthquake ruptures. The ISOSYN package has two primary strengths. First, it is computationally very fast (typical computation times of seconds), so that in principle the user can simulate ground motions from thousands of hypothetical rupture models in minimal time. Second, the graphical applications display many physical quantities on the fault that help the users understand and interpret the source effects, such as directivity and radiation patterns, that affect the calculated seismograms.
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