Homogenized moment tensor and the effect of near‐field heterogeneities on nonisotropic radiation in nuclear explosion

Abstract

AbstractWe investigate the effect of small‐scale heterogeneities close to a seismic explosive source, at intermediate periods (20–50 s), with an emphasis on the resulting nonisotropic far‐field radiation. First, using a direct numerical approach, we show that small‐scale elastic heterogeneities located in the near‐field of an explosive source, generate unexpected phases (i.e., long period S waves). We then demonstrate that the nonperiodic homogenization theory applied to 2‐D and 3‐D elastic models, with various pattern of small‐scale heterogeneities near the source, leads to accurate waveforms at a reduced computational cost compared to direct modeling. Further, it gives an interpretation of how nearby small‐scale features interact with the source at low frequencies, through an explicit correction to the seismic moment tensor. In 2‐D simulations, we find a deviatoric contribution to the moment tensor, as high as 21% for near‐source heterogeneities showing a 25% contrast of elastic values (relative to a homogeneous background medium). In 3‐D this nonisotropic contribution reaches 27%. Second, we analyze intermediate‐periods regional seismic waveforms associated with some underground nuclear explosions conducted at the Nevada National Security Site and invert for the full moment tensor, in order to quantify the relative contribution of the isotropic and deviatoric components of the tensor. The average value of the deviatoric part is about 35%. We conclude that the interactions between an explosive source and small‐scale local heterogeneities of moderate amplitude may lead to a deviatoric contribution to the seismic moment, close to what is observed using regional data from nuclear test explosions.