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Discriminating S-Wave Polarization Angles of Explosive and Earthquake Sources

Description: 

Distinguishing whether a recorded seismic event is an earthquake or explosion is a core problem in explosion monitoring. In a 1D flat layer model, earthquakes produce a predictable S-wavefield based off their radiation pattern while an explosion source will not produce a direct S-wavefield. However, observations from nuclear tests have shown they can produce significant S wave energy on both the radial and transverse components. In this study, we perform 2D and 3D numerical experiments using SPECFEM (2D and GLOBE) to better constrain the conditions in which an S-wavefield generated from an explosion might differ from one sourced from an earthquake. We demonstrate that anomalies in rigidity and anisotropic parameters near the source can cause an explosion to produce a large S-wave field. The key to producing an S-wavefield is that the wavelength of heterogeneities must be smaller than the wavelength of the wavefield and extremely close to the source. The S-wave radiation pattern is highly sensitive to the source’s proximity to the anomalies and moving the source sightly can produce a significantly different pattern. We do not observe the same high sensitivity near source structure has on far-field S-wave radiation pattern for earthquake sources. P-S ratios of explosive source change as a function of frequency and near source anomality size while earthquakes P-S ratios remain constant. Lastly, we compare our synthetic observations to S-wave polarization measurements that we made on nuclear tests conducted at the Nevada National Security Site and nearby earthquakes. S-wave polarization angles depend on the ratio of energy the incoming wave has on the radial and transverse components. Our initial results show that we can makestable measurements of the S-wave polarization angles for earthquakes as a function of frequency. We have also measured explosion S-wave polarization angles, but the measurements are not so common nor as robust. This method might prove useful for discriminating events of interest which have clear S-waves. LA-UR-24-20147

Session: Advancements in Forensic Seismology and Explosion Monitoring - II

Type: Oral

Date: 5/2/2024

Presentation Time: 11:15 AM (local time)

Presenting Author: Peter

Student Presenter: No

Invited Presentation: 

Authors