Multiple scattering of seismic waves in a heterogeneous magmatic system and spectral characteristic of long period volcanic earthquakes

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