Numerical Simulations of Surface Topography Effects on Shallow Explosion Ground Motions with Applications to S-Wave Generation and North Korean Nuclear Tests

Abstract:

Observations of nuclear explosions ubiquitously reveal shear-waves in the form of high-frequency Sn and Lg at regional distances and long-period surface waves at teleseismic distances. While a number of mechanisms have been shown to generate shear-waves from explosions, including non-linear processes near the source, elastic scattering and tectonic release, it is likely that all mechanisms act to some extent in each emplacement scenario. We performed numerical simulations of seismic ground motions excited by shallow explosions in the presence of rough surface topography to investigate elastic scattering and mode-conversion mechanisms of S-wave generation. Massively parallel simulations were performed at high-resolution to capture the high frequencies (up to 8 Hz) of interest to low-yield nuclear explosion monitoring. Simulations were performed with SW4, an elastic finite difference code that uses a conforming curvilinear mesh to model the near-surface region with a non-planar free surface. Results show that the rough free surface generates significant S-waves by P-to-Rg scattering. We performed simulations with real surface topography from the North Korean (DPRK) nuclear test site and synthetic stochastic topography to investigate how surface topography governs local and regional distance S-waves. We find that S-wave amplitudes along the surface in the presence of topography are increased several-fold on average within 10 km of the source. Topographic scattering leads to SH amplitudes of up to 50% of SV within 10 km, leading to equipartitioning of shear wave energy on the horizontal components. Scattered energy between the direct P and Rg waves has the amplitude spectra of the direct P-wave but travels with the Rg velocity, clearly illustrating the P-to-Rg mechanism for shear-wave generation. The amplitude spectra of Rg waves in the presence of topography are shaped by the low frequency (0.5-2.5 Hz) peaked spectrum of the flat case plus the high frequency (> 2 Hz) P-wave spectrum. We will attempt to relate simulation results to observations of recent DPRK nuclear tests.