A major instrumentation program undertaken in the Rio Blanco experiment involved the burial of 20 accelerometers and 16 velocity gauges at varying depths from the ground surface and distances from the emplacement well. The objective was to measure the extent of spall (tensile failure of the earth materials). A preliminary analysis of the accelerometer data indicates that the Rio Blanco spall zone can be described as a broad shallow dish with a most probable depth of <350 ft and a radius of <24 000 ft. Spalled material experiences a period of free fall (-1-g acceleration) that can be identified in both acceleration and velocity traces. Acceleration records show conclusively that spall occurred above 149 ft at the emplacement well, but not below 604 ft. Instruments placed at depths of 350 and 450 ft appear not to have recorded spall characteristics, but these records must be analyzed in greater detail for a definitive statement. The results are in general agreement with predetonation predictions.