Long-period surface waves from large earthquakes can provide reliable estimates of the source depth, rupture duration, seismic moment, and fault orientation. This requires accurate surface wave phase and amplitude spectra and precise propagation corrections. Standard procedures for estimating spectra of traveling waves in source analyses are inaccurate for periods exceeding 300 s, so the longer periods are usually analyzed by free oscillation analysis if at all. We apply a multiple filter analysis to extract complex spectra for periods up to 500 s suitable for traveling wave spectral inversions. This will allow improved constraints on the rupture process of large earthquakes to be obtained by inversion methods using a single formalism. ¿ American Geophysical Union 1989