Complex patterns of multiple diffusive source areas for high-frequency volcanic tremor on Mt. Etna

In this study we present results from an unprecedented, dense, high-resolution seismic de- ployment on Mt. Etna that reveal rarely observed high-frequency tremor, predominantly in the 12–15Hz band. Such signals are typically overlooked at volcanoes due to strong scat- tering and rapid attenuation, which restrict their detection to near-source regions. By com- bining multi-array beamforming with a three-dimensional grid search based on shifted and stacked cross-correlation functions, we identify three principal tremor source regions across the summit. The most persistent source coincides with degassing activity at Bocca Nuova Crater (BNC), where synthetic tests constrain tremor and impulsive degassing events to shallow depths (∼ 270–470m). Temporal changes in apparent slowness further indicate alternating downward and upward propagating tremor sources beneath BNC. In addition, two further source regions are detected to the southwest and northwest of BNC. As no degassing is known in these areas, a direct fluid-driven relationship is not expected. Instead, the spatially diffuse character of these sources, suggests these tremor episodes may be the result of numerous low-amplitude, small stress-drop events closely spaced in time and thus appear- ing as tremor. This hypothesis is in agreement with previous numerical and laboratory studies associating volcanic tremor with quasi-brittle failure of the weak volcanic material unrelated to fluid migration, but linked to small stress level changes driven by slow deformation of the edifice. We carry out synthetic tests showing the successful recovery of multiple superimposed tremor sources embedded in strong noise despite imperfect station coverage. Our findings highlight the diversity of processes generating high-frequency volcanic tremor. Furthermore, we detect a temporal correlation between the different source regions in terms of intensifying and subsiding activity, which may be indicative of interconnected shallow tremor source regions. These results emphasize the complexity of volcanic tremor generation and un- derline the importance of high-resolution seismic networks for improving our understanding of the origin of volcanic tremor.