The moment tensor solutions determined from both long-period body and susrface waves for most of the large earthquakes in the 1980 Mammoth Lakes sequence have substantial deviations from a pure double-couple mechanism. Julian (1983) and Aki (1984) argue that these deviations require nonshear faulting seismic sources such as a compensated linear vector dipole (CLVD) which could represent magmatic dike injection. Here we reexamine the moment tensor solutions and investigate the effects of the non-double-couple source on body wave synthetics. An error function which is based on the cross correlation of a synthetic and an observed waveform was used to assess the difference in fit between the complete moment tensor solution and one constrained to be a double couple. For the main shock (May 25, 1980; 1633) the sum of the errors differed by less than 5% for the two models. Similarly, the errors differed by only 3% for the aftershock on May 27, 1980. The conclusion is that the CLVD mechanism is not resolved in the long-period data. In addition, the complete moment tensor solution has an extra parameter; a &khgr;2 test was applied to normalize the number of parameters, and it was found that the complete solution was slightly less significant than the double-couple solution. Finally, the complete solution was compared to a pure CLFV solution; for all the events examined, the total moment tensor is as far removed from a CLVD as it is a double couple. Although this does not rule out the possibility of dike injection, it strongly suggests that faulting can account for the long-period seismic observations in the 1980 swarm.