Long-period regional distance (2¿<Δ12¿) body waves are strongly influenced by the waveguide nature of the crust. If the seismic source is known, it is possible to parameterize the waveforms in terms of gross structural properties of crustal thickness and Pn velocity. Since the regional distance waveforms are a product of interference of multiple reflected and mode converted phases, change in crustal thickness of Pn velocity changes the relative timing of the various phases. Approximate Green's function for a suite of crustal structures can be constructed by stretching or squeezing the Green's function of an average crustal model, which is the basis of an interative, least squares inversion procedure for crustal structure. The crustal structure of the western U.S. is inverted as a test of the technique.