Rayleigh waves from underground nuclear explosions which are accompanied by high levels of tectonic strain release are observed to be reversed and time delayed relative to Rayleigh waves from ''normal'' underground explosions. The ''time delays'' can be explained as an interference effect between the explosion and tectonic source time functions which magnifies the true phase difference between the source functions. The phase of the explosion source time function is advanced relative to the phase of the tectonic source function, and the magnitude of the time advance is directly related to the amount of overshoot in the explosion source time function. The phase distortion is more complex than a simple time delay, and may appear as a constant phase shift with no linear trend. Apparent time advance are also possible; however because of small differences between the excitation functions of the explosion and tectonic sources at shallow depths, the phase shift will appear more frequently as an apparent time delay. Observations of Rayleigh wave phase shifts from underground explosions at the Soviet East Kazakh test site recorded at SRO stations are in very good agreement with simulated phase shifts derived from synthetic calculations.