Source parameters of moderate to large size (Mw>4.5) earthquakes in the western United States from 1992 to 1994 are determined by point source moment tensor inversion of complete long-period (T>35--50 s) ground motions recorded at regional distances (1¿--12¿). Stable long-period signals are obtained by low-pass filtering records from the very broadband seismometers recently deployed in several networks in the western United States. These signals are dominated by fundamental mode Rayleigh and Love waves, which have very simple waveforms due to the limited dispersion on the short paths to regional stations. Since long-period motions are relatively insensitive to the attenuation model and crustal structure used in the inversion, they provide robust constraints on the seismic moment and faulting geometry as long as adequate azimuthal coverage is available. Comparisons of solutions for 21 events with results of other regional and teleseismic wave inversions are made to assess the model dependence and uncertainties of our regional centroid moment tensor (RCMT) solutions. RCMT inversion has limited source depth resolution for shallow crustal events, but the focal mechanism and seismic moment determinations prove quite stable over a range of source depths in the crust, as well as over a range of crustal propagation models. Simultaneous waveform inversion of shorter-period body wave signals can improve the source depth resolution. By applying path corrections for heterogeneous crustal structure, shorter-period surface wave energy can also be inverted, allowing the methodology to be extended to lower-magnitude regional events as well. The RCMT procedure requires minimal signal processing, only a sparse broadband network, and a simple laterally homogeneous propagation model; thus it can readily be automated and applied in near real time to events in the magnitude range from 4.5 to 7.5 distributed over an area as large as the western United States. ¿ American Geophysical Union 1995