The apparent explosion moment: Inferences of volumetric moment due to source medium damage by underground nuclear explosions

Classical explosion source theory relates isotropic seismic moment to the steady state level of the reduced displacement potential. The theoretical isotropic moment for an incompressible source region M-t is proportional to cavity volume V-c created by pressurization of materials around the point of energy release. Source medium damage due to nonlinear deformations caused by the explosion will also induce volume change V-d and radiate seismic waves as volumetric, double-couple, and compensated linear vector dipole (CLVD) body force systems. A new source model is presented where K is a relative measure of moment M-CLVD with respect to the net moment from volumetric sources V-c and V-d. K values from moment tensor inversions steadily decrease from similar to 2.5 at lower yields to similar to 1.0 for the highest-yield shots on Pahute Mesa. A value of 1.0 implies M-CLVD = 0 and, by inference, small V-d. We hypothesize that the extent to which damage adds (or subtracts) volumetric moment is controlled by material properties and dynamics of stress wave rebound, shock wave interactions with the free surface, gravitational unloading, and slapdown of spalled near-surface layers. This hypothesis is tested by comparing measurements of isotropic moment M-I with estimates of M-t based on V-c scaling relationships and velocity-density models. The results support the hypothesis and the conclusion that M-I represents the apparent explosion moment since it has contributions from direct effects due to cavity formation and indirect effects due to material damage. Implications for yield estimation using M-I are discussed in general and for the North Korean tests.