TRANSIENT AND RESIDUAL STRAINS FROM LARGE UNDERGROUND EXPLOSIONS

Tectonic strain readjustments associated with large underground explosions have been observed at the Nevada Test Site. The BENHAM event of December 19, 1968 produced a peak quasi-static radial strain of 1.2 X 10-7 at a distance of 29 kin. This strain transient was followed by an exponential return to the initial state with a time constant of 13 minutes, and is interpreted as the direct elastic response of the medium to a time varying pressure in the BENHAM cavity. An upper bound on the tectonic strain release was determined to be 0.7 X 10-s. Using these measure-ments it is estimated that the permanent and quasi-static strains associated with this explosion could significantly effect local earthquake occurrences out to distances of about 15 kin. The size distribution of aftershocks of this explosion resembles that seen in model experiments of brittle fracture, in which the distribution is controlled by the dimensions of inhomogeneities in the medium