Small-scale effects on global seismic waves

Thesis

Earth structure is multi-scale, yet capturing such a broad range of complex heterogeneities with seismic wave propagation is computationally prohibitive. In this thesis we present an Instaseis-based (van Driel et al., 2015) framework for injection type hybrid methods that addresses this challenge and provides 5,000-100,000 speed-up relative to full 3-D global solvers without compromising on the full physics. The modified Instaseis interface allows to couple the global wave-propagation solver, AxiSEM (Nissen-Meyer et al., 2014), with an arbitrary local 3-D solver of choice, and thus embed any heterogeneous 3-D domain within a spherically symmetric Earth model. The global 1-D medium accounts for source and paths effects, while the local 3-D domain optimally honours complexities in the model to resolve local scattering effects where necessary. The method can reach the highest frequencies observed teleseismically (1-4 Hz) and opens global wave propagation to a new parameter space.

The Instaseis-based method is applied to study the 1960s-1980s nuclear explosions from the USSR test site in the Degelen mountains. Observed waveforms up to 2 Hz bear a strong signature of near-source effects, yet the relative importance of contributing factors – such as explosion depth and yield, scattering off topography and geological heterogeneities, as well as non-linear effects – is not well understood. An analysis of the dataset suggests that waveform features are dependent on the source location in the Degelen mountain range, though simple measures do not indicate a straightforward relationship with topography. We therefore turn to deterministic hybrid modelling to evaluate waveform variations stemming from topographic interactions in the vicinity of the source. Amplitudes are in good agreement and many observed features are reproduced by synthetic seismograms at 2 Hz, showing that topography significantly contributes to waveform complexity at teleseismic distances.

Authors: Pieńkowska-Côte, M

• DOI: 10.5287/ora-z5ajxa5mw
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Degelen; nuclear monitoring; coupled solvers; wave scattering; hybrid method; numerical wave propagation; teleseismic waves
• Subjects: Seismology; Geophysics; High performance computing; Elastic wave propagation