High-frequency hybrid modelling of near-source topographic effects at teleseismic distances: The Degelen mountain case study

We apply a hybrid method that couples global Instaseis databases (van Driel et al., 2015) with a local finite-difference code WPP (Nilsson et al., 2007) to study the 1960s-1980s nuclear explosions located at the USSR Degelen mountain test site. Observed teleseismic P waves (up to 2 Hz) display strong near-source signatures, yet the relative importance of contributing factors – such as explosion depth and yield, scattering from near-source topography and geological heterogeneities, as well as non-linear effects – are not well understood. An analysis of teleseismic waveforms suggest that these features are dependent on the source location within the Degelen mountain range, while depths and yields do not show a consistent effect. We therefore propose that the change in signal characteristics on teleseismic waveforms is related to the mountainous topography in the source region and we turn to deterministic hybrid modelling to test the effect of Degelen topography at teleseismic distances. Despite simplistic modelling assumptions, we achieve an excellent fit with the observed waveforms. Amplitudes are in good agreement and many observed features are reproduced by synthetic seismograms at 2 Hz, highlighting the importance of near-source 3-D effects on long-range wave propagation. Hybrid modelling of more realistic high-frequency scenarios could ultimately lead to waveform-based constraints on explosion locations, for example via grid-search methods or more advanced learning algorithms, or even improve nuclear discrimination methods.