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

Abstract Long‐Period (LP) volcanic earthquakes are characterized by long duration codas and spectra containing pronounced spectral peaks. These spectral characteristics are often attributed to source effects, such as resonances of fluid‐filled cracks. In this paper, we present the results of numerical simulations of seismic wave propagation showing that the main signal features of LP earthquakes (long duration and spectral peaks) can arise from multiple scattering in strongly heterogeneous volcanic media. We consider seismic sources located within a highly heterogeneous volcanic plumbing system created through multiple injections of magmatic dykes and sills into surrounding crustal rocks. The resulting structure contains batches of fully melted rocks and, as a consequence, strong contrasts in elastic properties. By computing wave propagation in this medium, we show that the heterogeneous structure generates strong scattering of seismic waves, whose interference leads to multiple peaks in the signal spectra. Some of these peaks are common to multiple receivers and are produced by local resonances near sources located in areas where the medium is particularly heterogeneous. Although arising within irregular‐shaped magma batches, these local resonances resemble the fluid‐filled crack model, implying their frequencies are linked to near‐source structure. Meanwhile, many spectral peaks are observed only for specific source‐receiver pairs, implying they result from interference along specific paths and should not be interpreted as signatures of near‐source structure and processes. Our results show that separating path and source effects for seismo‐volcanic signals in realistically heterogeneous media is delicate, and not all spectral features should be attributed to the latter.