Simulation of topographic effects on seismic waves from shallow explosions near the North Korean nuclear test site with emphasis on shear wave generation

Abstract

We performed high‐resolution (8 Hz) three‐dimensional simulations of ground motions from shallow explosions in the presence of rough surface topography near the North Korean nuclear test site to study elastic propagation effects with emphasis on theoretical aspects of shear wave generation. Interaction with rough topography causes significant P‐to‐Rg scattering along the surface with amplification of high‐frequency (2–8 Hz) shear waves relative to the flat Earth case. Shear waves of different polarizations are coupled by topographic scattering. Rg precursors composed of P‐to‐Rg conversions traveling as surface waves have the spectral amplitudes comparable to the P wave, while the Rg phase has the low‐frequency (0.5–3 Hz) spectral shape of the Rg from the flat case plus the high‐frequency (3–8 Hz) P wave spectra. Motions at near‐vertical takeoff angles corresponding to teleseismic propagation are increased or decreased indicating that waves are focused or defocused by topographic features above the source. Topographic roughness has a dramatic effect as short‐wavelength features (<2–5 km) are included. Higher frequencies are amplified by topography, including frequencies corresponding to wavelengths shorter than the shortest topographic scale length. Overall topography enhances energy propagating along the surface near the source, amplifies surface waves, and tends to balance SV‐ and SH‐polarized motions, all of which impact shear wave observations used for nuclear explosion monitoring. Further simulation studies could elucidate how the wavefield emerging from a topographically rough area ultimately propagates to regional and/or teleseismic distances.