Simulated Effects of Shallow Crustal Heterogeneity, Surface Topography, and Seismic Source Depth on Coda Wave Generation for Magnitude-Based Depth Discrimination
Abstract
The nuclear explosion monitoring community has historically utilized seismic discrimination methods at regional distances to distinguish large-yield explosions from earthquakes. In a recent study, we found that a new discrimination technique based on comparing ML-MC values between earthquakes and explosions performs well at local distances (<100 km). However more analysis of its performance is required to establish its sensitivity to source depth and source type.
Previous work focused on testing the effectiveness of ML-MC in various geologic settings suggests that coda wave excitation from shallow sources increases MC relative to ML. The main contributors to coda wave energy, which directly affect ML-MC, are low velocity surface layers, crustal heterogeneity, and surface topography. In order to evaluate their separate contributions, we perform numerical simulations using 1D layered models with and without small-scale variability and surface topography. These simulations are performed in the frequency range 0-5 Hz using SW4 code. We simulate ground motion from a tectonic earthquake and a ripple-fire explosion recorded by the University of Utah Seismograph Stations. The events are epicentrally co-located and have ML of 1.95 and 1.41, respectively. We use a 1D velocity model for the Wasatch Front in Utah and include a surficial low velocity layer to better reproduce the observed shallow wave trapping. Scattering from crustal heterogeneity is generated by random correlated velocity perturbations in the background 1D velocity model. Our simulations also include surface topography, to quantify its contribution to scattering. Comparisons between synthetic and recorded waveforms are used to determine the parameters that contribute the most to wave scattering and wave phases used in estimating ML-MC. Furthermore, the depth of the earthquake is varied to investigate source depth effects on wave scattering. We find that wave scattering due to shallow crustal heterogeneity depends on source depth and could be responsible for the observed difference in coda wave amplitude and duration between earthquakes and mining explosions. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S11E0417V
- Keywords:
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- 7219 Seismic monitoring and test-ban treaty verification;
- SEISMOLOGY;
- 7294 Seismic instruments and networks;
- SEISMOLOGY;
- 7299 General or miscellaneous;
- SEISMOLOGY