Broadband seismic waveform data recorded in the distance range of 1 to 11 km from two underground explosions detonated at Pahute Mesa of the Nevada Test Site have been used to estimate the source time functions of the second-order moment tensors of these events. Data were available from 8 three-component stations for the explosion Harzer and from 11 stations for the explosion Chancellor. Stable moment tensor estimates with a good signal-to-noise ratio were obtained in the frequency range 0.2 to 5.0 Hz. The moment tensors are dominated by the diagonal elements, but a significant asymmetry is present, with vertical force couples having a different time dependence than the horizontal force couples. When the isotropic parts of the moment tensors are interpreted as reduced displacement potentials the result are in reasonable agreement with theoretical and empirical models, although the parameters which are estimated with these models show systematic differences from published scaling relationships. These differences may be related to the fact that the explosions were detonated in unsaturated materials or to the effects of spall in surficial layers above the source.