Vertical deformation which has been produced by underground nuclear explosions and which includes both the primary effects of the explosion and the secondary effects of triggered faulting has been studied by means of data from high-accuracy level-line surveys. The results from several large tests indicate that for a megatonsize explosion the primary effects are expected to produce permanent vertical displacements of less than 2 to 3 cm at distances greater than 2 to 3 km from GZ (ground zero). Larger vertical displacements at more than several kilometers from GZ are commonly associated with movement on pre-existing faults. In areas that are uncomplicated by multiple structural features, the vertical displacements associated with fault movement vary, in a regular manner, to a considerable distance from the fault. The general pattern of fault-related displacements is similar to that predicted by dislocation theories of faulting but is modified by an overall tendency to subsidence. Analysis of vertical displacements in the vicinity of a fault activated by one explosion indicates a depth of faulting of about 1,500 m. Most fractures observed at the surface may be relatively shallow features only indirectly related to the processes which produce the aftershock sequences that follow large explosions.

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