We present results of a comparative source study on one of the largest aftershocks of the Borah Peak, Idaho, earthquake series. The mechanism of this event had been previously studied using first motions and teleseismic p-wave data, and we report here our results from analysis of regional Pnl and surface waves. The Pnl waves gave an estimate of the source duration near 5 sec, in agreement with the teleseismic p-waves. Both the source duration and the effect of surface-wave dispersion through laterally heterogeneous crustal structures must be accounted for in the moment-tensor inversions of intermediate-period surface waves. All data sources, both long period and short, gave consistent source mechanisms, and the long periods also yield three independent and consistent source mechanisms, and the long periods also yield three independent and consistent estimates of the seismic moment. The results demonstrate the promise of formal source-parameter retrieval techniques applied to regional data for the study of earthquakes with magnitudes below the magnitude threshold of teleseismic source determinations. ¿ American Geophysical Union 1988