P/SV Amplitude Ratios of Shallow Isotropic Explosions and Earthquakes Could Be Indistinguishable at Local Distances: Insights from Single-Station Waveform Simulations

Abstract

Shear waves play a key role in seismic discrimination between explosions and earthquakes due to their different source mechanisms. However, shear waves are often observed in field explosions with unexpectedly large amplitude, and their generation mechanism is still a significant unresolved question in seismology. Many explanations have been proposed, including the asymmetry of explosive sources, and heterogeneity and/or anisotropy of the Earth’s subsurface. However, it has not been well understood whether source or velocity structure can independently and sufficiently explain the shear waves generated by explosions. Theoretically, tangential SH waves can be converted and scattered from vertical and radial SV waves due to anisotropy and heterogeneity. Thus, it is essential to understand the generation of SV waves by explosions. In this study, we utilize the frequency–wavenumber algorithm and 1D layered velocity models to simulate waveforms of isotropic explosions and double-couple earthquakes at local distances (<20 km). Our results suggest that explosions and earthquakes may generate comparable SV waves if both occurred within a near-surface velocity gradient zone. The earliest SV waves by explosions appear to originate from the near-source region. It implies that P/SV amplitude ratios of explosions and earthquakes could be indistinguishable under certain circumstances.