Ground motion velocity and acceleration data recorded within the spall region on over 30 Pahute Mesa explosions have been analyzed for characterization of the spall source. These data provide observations of peak spall velocities and the spatial extent of spall from which estimates of the volume and mass of spalled material were obtained, along with the yield scaling. Spall velocities at ground zero are higher, and the apparent attenuation of velocities is lower for explosions below the water table compared to shots above the water table in tuff or rhyolite. As a result, the lateral and depth extents of spall are larger for shots below the water table, and the estimated spall mass is three times larger than the estimate for explosions above the water table. The lateral and depth extents were found to scale as the quarter root of the yield (W) as opposed to the familiar cube-root scaling, and assuming a cylindrically symmetric volume, the mass scales as W0.77±.05. At 150 kt, the estimated mass for a shot below the water table is 15 times greater than the first estimates made by Viecelli. While this estimate suggests a much larger spall source than originally believed, not all of the mass is necessarily involved in generating seismic waves recorded in the far field, and the coupling efficiency of spall remains an outstanding, unresolved problem.