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.
Plain Language Summary Volcanic regions are among the most complex areas of the Earth's crust, characterized by strong heterogeneity and intense seismic activity. Seismic signals in these environments are strongly influenced by multiple scattering caused by the volcano's complex internal structure. To investigate how scattering affects waveforms and spectra, we used a physics‐based model to simulate seismic wave propagation through a realistic magmatic system. The model includes features such as vertically aligned dykes and horizontally layered sills, mimicking the distribution of magma and partially melted rocks. Our results show that waves traveling through such heterogeneous media are able to generate stable spectral peaks. While most of these peaks arise from complex propagation paths, some can be linked to partial resonances near the source, especially in low‐velocity zones associated with magma intrusions. These localized resonances appear as standing waves and persist across multiple nearby recordings, producing surface signal characteristics similar to those from fluid‐filled crack models. Their presence underscores the importance of using realistic simulations to interpret seismic data in volcanic settings. Moreover, changes in spectral content over time may indicate evolving distributions of magma or partially molten rock at depth.
Key Points We compute synthetic seismograms in a physics based model of heterogeneous volcano magmatic system Strong scattering and interference of waves propagating along complex paths result in multiple narrow peaks in the signal spectra Some spectral peaks remain stable across multiple receivers and are associated with localized resonances near partially molten regions