Inversion of complete long-period ground motions is used for rapid determination of source mechanisms of Mw≥5 earthquakes in the western United States. The first few minutes of ground motions on local and regional distance waveforms are inverted for point-source centroid moment tensors. Waveforms recorded at regional distances (300--1500 km) can be adequately modeled at periods longer than 50 s using the laterally homogeneous PREM model [Dziewonski and Anderson, 1981>; at closer distances (<300 km) waves with periods as short as 35 s can be analyzed using PREM. Best double-couple focal mechanisms for 18 recent large western U.S. earthquakes compare very well with those obtained by other methods. Our procedure, interfaced with automated data retrieval and source location methods, is a viable tool for reliable determination of seismic moment and fault mechanism within 30 minutes after an earthquake. ¿ American Geophysical Union 1993