Precise matter event locations of 344 aftershocks of the May 1980 Mammoth Lake earthquakes indicate a complex pattern of faulting within the hypocentral region. On the east three of the ML 6+shocks lie along a a N-S line but are associated with an echelon, vertical clusters of aftershocks striking N15¿E , in good agreement with swarms of fractures in the epicentral area and in general agreement with short- and long-period fault plane solutions for the Mammoth Lakes sequence. To the northwest, one of the large events was located at the end of the WNW striking vertical zone of aftershocks along the southern boundary of Long Valley caldera. On the bottom, the hypocentral zone appears to be bounded by a plane striking NNW and dipping east, parallel to the Sierran frontal fault system. The results of this study do not support the idea that the large shocks were associated with the rapid formation of NNW striking vertical dikes.