A least squares waveform inversion technique to determine the source parameters of moderate size earthquakes is presented. The technique involves an iterative scheme which makes use of an error function determined by the cross correlation of a long-period seismogram and a synthetic. Both the data and the synthetics are normalized to make the solution insensitive to absolute amplitudes. Making use of the three fundamental faults, the error functions can be written as a series of cross correlations multiplied by constants corresponding to source orientation. Once the cross correlations are computer, the source orientation is determined iteratively, and only the constants have to be recalculated. The technique has been applied to long-period regional seismograms where the beginning portion of the vertical and radial components containing Pn and PL (referred to as Pnl) is inverted. Three western United States earthquakes which have been well studied by other authors were used as test samples. The inversion yields fault plane solutions in good agreement with those in the literature. Moments determined in the inversion are also in excellent agreement with those determined teleseismically.