Source parameters for 50 earthquakes occurring between 1915 and 1988 have been used to examine the faulting characteristics of magnitude 5.5 to 7.8 earthquakes within the western U. S. cordillera, an area extending from the eastern Sierra Nevada to the Great Plains and from northwestern Montana to Trans-Pecos Texas. Source parameters used in this study include seismic information from first-motion analyses, body and surface waveform modeling, geodetic studies, and geological studies of surface faulting. The principal results of this analysis include: (1) all earthquakes occurred on faults dipping 38° or more, and no evidence for listric of low angle planar faulting was found, (2) all M ≧ 7.0 earthquakes occurred at depths ≧ 12 km and were composed of multiple subevents, (3) most earthquakes (>70 per cent) had unilateral ruptures, and (4) no individual subevent had a rupture length >21 km.

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