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3D Simulations of Seismo-Acoustic Coupling Over Topography

Observations from field experiments have shown that a propagating acoustic wave can generate a variety of elastic waves as it impinges upon the earth. Theoretical treatments have thoroughly examined reflections and refractions at a planar interface, but nonplanar interfaces, particularly topography, can have considerable impact on local infrasonic propagation and have not been addressed in coupling scenarios. Numerical approaches to this problem are somewhat limited in the presence of topography, and it is typically assumed that the ground surface is a rigid interface. SPECFEM3D, a spectral finite element code, can numerically simulate the coupling between acoustic and elastic waves over meshed topography. With a series of halfspace models, we investigate the effect of the angle of incidence and subsurface structure on acoustic to elastic coupling. These simulations are sourced with acoustic analogs for a variety of moment tensor sources, and we show that the general diagonal moment tensor in a fluid medium can be related to a classical acoustic quadrupole source. When the moment tensor is isotropic, this expression collapses to an exact relationship between the seismic moment and a monopole acoustic source. Modeling results show significant deviation at steep incidence angles from the commonly assumed expressions for transfer coefficients, which has implications for propagation with multiple surface interactions and larger propagation distances. Large energy transfer occurs at steeper incidence angles when the shear wave speed of the earth is less than the sound speed of the incident acoustic wave, which we attribute to increased infrasound to S-wave and surface wave coupling. This effect appears to be broadly consistent with previously published observations by Edwards et al. (2007) on seismo-acoustic coupling. For comparison, we also simulate coupling over topography using a smoothed model of Sakurajima Volcano, Japan. Using a similar methodology to the halfspace cases, we estimate energy admittance values and possible spatial relationships to topographic relief.

Presenting Author: Jordan W. Bishop

Student Presenter: Yes

Day: 4/20/2021

Time: 4:15 PM - 5:15 PM Pacific

Additional Authors