In order to assess the existence and impact of mild nonlinear contributions to the attenuation of seismic signals from underground explosions, free-field motion data from underground 5.3-kt nuclear test Salmon have been examined. These data, which were taken in a salt dome at ranges from 1665 to 660 m, show moderate strains (10-3 to 10-4) which may provide nonlinear attenuation. The attenuation over an order of magnitude in peak amplitude can be described approximately by an attenuation function Q of a bit less than 10; however, the calculated waveform using this constant Q is noticeably wider than the data. A linear but frequency dependent Q which decreases with decreasing frequency gives a reasonable fit to much of the waveform changes as well as the peak amplitude decay with range. The higher-speed precursor which precedes the main pulse in the data cannot be described by this linear Q. With a spherical finite difference calculation driven by the 166-m Salmon pulse, it is found that a rapid shear modulus decrease at a 10-4 strain threshold can reproduce the observed precursor and other features of the pulses at greater ranges when a linear broad absorption band with Q≈10 is also added. The attenuation of the Salmon pulse thus can be attributed in part to a nonlinear effect of material failure as well as to a conventional linear mechanism. There are reasons to believe that the attenuation is not linear, but the Salmon data alone cannot verify this view. ¿American Geophysical Union 1990