EFFECTS OF 3D SURFACE TOPOGRAPHY ON REGIONAL AND TELESEISMIC SIGNALS FROM UNDERGROUND EXPLOSIONS

Recent underground nuclear tests conducted by North Korea have reaffirmed the importance of near-source rough surface topography for the generation of complex seismic signals observed at regional and teleseismic distances. Complex reflections from the irregular 3D surface scatter into both horizontally and vertically propagating wave fields, producing directionally dependent focusing and defocusing of P and S wave fields, conversion of P-to-Rg high frequency energy, and generating complex signal codas, complicating robust event discrimination and precise estimation of burial depth (and hence yield). Quantifying these effects requires numerical methods capable of accurately accounting for the 3D interactions with a rough surface. We are applying two distinct finite difference methods with which we have extensive experience to quantitatively determine the effects of near-source rough topography on seismic waves from shallow underground nuclear explosions, using high-resolution topography models for the North Korea test site and exploring the effects of precise emplacement location relative to the topography. Preliminary work indicates that the very complexity of the wave field that complicates waveform interpretation holds the key to event quantification if the topographic effects are accurately modeled. High-resolution near-source models are being coupled to regional and teleseismic propagators using two methods; representation theorem and embedded slowness array calculations, and comparisons will be made with availabl