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Leveraging Seismic Particle Motion of Air-to-ground Coupled Waves to Investigate the Structure of the Shallow Subsurface

Description: 

When low-frequency acoustic (infrasound) waves impinge on the Earth’s surface, they can couple into the solid earth and produce measurable ground motion on seismometers. Analysis of seismic particle motion is often used to help identify these air-to-ground coupled waves and can also aid in determining the backazimuth of the infrasound wave. Literature regarding acoustic-seismic coupling often assumes that pressure-induced particle motion is retrograde elliptical, however we show that prograde motion is often observed. Here we explore how to use particle motion to uncover information about the shallow subsurface and the incident acoustic wave at stations with co-located infrasound sensors and 3-component seismometers. We adapt the Normalized Inner Product method (Meza-Fajardo et al., 2015) to quantify the sense of retrograde or prograde motion of air-to-ground coupled waves in the time-frequency domain. We also analyze how the particle motion ellipse is tilted at different frequencies, which may inform identification of the recorded, transmitted wave (body waves, guided waves, etc.). From observations of coupling events, we find that different stations can exhibit broadband retrograde motion, broadband prograde motion, and frequency-dependent particle motion. Previous literature suggests that prograde motion occurs when the subsurface shear wave velocity is less than the acoustic wave speed. However, we also observe prograde motion at low frequencies (0.02 Hz), which are sensitive to structure at depths (> 5 km) where velocities are likely far greater than the speed of sound. To further understand the mechanisms for prograde motion of air-to-ground coupled waves, we model the seismic ground motions induced by pressure waves coupling into a layered subsurface using a matrix propagator technique. We aim to replicate the particle motions observed in real data from chemical and volcanic explosions to understand how the shallow subsurface affects the varying aspects of particle motion.

This work was supported by the Nuclear Arms Control Technology Program at Defense Threat Reduction Agency (DTRA). Cleared for release.

Session: Advancements in Forensic Seismology and Explosion Monitoring - IV

Type: Oral

Date: 4/17/2025

Presentation Time: 05:15 PM (local time)

Presenting Author: Logan

Student Presenter: Yes

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