Multiple Scattering of Seismic Waves in a Heterogeneous Magmatic System and Spectral Characteristics of Long Period Volcanic Earthquakes

The spectral stability commonly observed in volcanic tremor signals is usually interpreted as reflecting a stable source mechanism. In this study, we investigate the role of seismic wave propagation within a magmatic plumbing system derived from thermoelastic simulations, using 2D elastic numerical simulations based on the Spectral Element Method. The modeled medium is grounded in the most recent understanding of the thermo-mechanical effects of magma injections into crustal rocks. Our wave propagation simulations demonstrate that such structures generate strong seismic wave scattering. We identify two primary mechanisms responsible for spectral stability and for generating a characteristic spectral signature: the interference of multiply scattered waves along the source-receiver paths, and the trapping of waves within the volcanic structure. In the latter case, we show that wave trapping can lead to local resonance and that its spectral signature appears clearly in the coda of volcanic signals. The observed link between frequency content and the elastic and scattering properties of the source region implies that structural changes may be characterized through the study of the spectral characteristics of volcanic recordings and their variations. Overall, our findings emphasize the fundamental importance of multiple seismic wave scattering in volcanic environments.