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Discriminating S-Wave Polarization Angles of Explosive and Earthquake Sources With 2D and 3D Simulations

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 S-wavefield. However, observations from nuclear tests have shown they are capable of producing significant S wave energy on both the radial and transverse components. In this study, we perform 2D and 3D numerical experiments using SPECFEM to better constrain the conditions in which an S-wavefield generated from an explosion might differ from one produced from an earthquake. We produce the S-wavefield for our explosions by placing the source location within a small region with large velocity and/or density heterogeneities. The wavelength of heterogeneities is smaller than the wavelength of the wavefield. We investigate isotropic and anisotropic anomalies with an average velocity that is the same as the background medium. Placing the source just outside the region produces a negligible S-wavefield while having the source surrounded but not within the heterogenous region also produces a much smaller S-wavefield. The relative amplitudes of the P and S wavefield depends on the size and strength of the anomalous region. Initial results in 2D demonstrates the anomalous region affects the P-S amplitude ratios of an explosion more than an earthquake of the same magnitude. The presence of strong anisotropic heterogeneities near the source can also produce a large S-wavefield for random VTI/HTI/TTI symmetries. We will expand this work into 3D simulations and analyze S-wave polarization angles as a function of azimuth and distance within realistic earth models. LA-UR-23-20265

Session: Exploiting Explosion Sources: Advancements in Seismic Source Physics [Poster]

Type: Poster

Date: 4/19/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Peter Nelson

Student Presenter: No

Invited Presentation: 

Authors