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

Abstract

[1] 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.Citation: Rodgers, A. J., N. A. Petersson, and B. Sjogreen (2010), Simulation of topographic effects on seismic waves from shallow explosions near the North Korean nuclear test site with emphasis on shear wave generation,

The role of topography and lateral velocity heterogeneities on near-source scattering and ground-motion variability

“…We attribute this phenomena to intense topography-induced mode conversion, described in other studies (e.g. Clouser & Langston 1995;Komatitsch & Vilotte 1998;Rodgers et al 2010), resulting in long-period surface waves forming most of the late-coda, as in Fig. 4.…”

“…Out-of-plane effects are particularly important in presence of rough topography, and phase conversion can be widely observed (e.g. Hestholm et al 1999;Rodgers et al 2010).…”

Direct simulation Monte Carlo method with a focal mechanism algorithm

“…Although event 6 is the largest event, and the focal mechanism is well defined, the M sum of DSMC-2 is larger than that of DMSC-1. This might be related to the topographic effect, whose contribution of amplitude variation is well known in rough terrain (Geli et al, 1988;Rodgers et al, 2010). The location of event 6 shows rough topography with a high mountainous area (Figure 6), which possibly caused amplitude errors in the waveform inversion using the simplified one-dimensional model.…”

Serpentine: Finite Difference Methods for Wave Propagation in Second Order Formulation

“…This code has shown excellent scaling on up to 32,768 processors and has enabled seismic wave calculations with up to 26 Billion grid points. The WPP calculations have resulted in several publications in the field of computational seismology, e.g., [28,2,1,29].…”