Volcanic seismic swarms, clusters of earthquakes without a distinct mainshock, are commonly linked to magma and fluid movements rather than tectonic stress transfer. Magma and hydrothermal fluid migration can perturb the ambient stress field and trigger volcano-tectonic (VT) seismicity on surrounding structures, either through dike propagation or inflation and associated stress changes. We analyze nine seismic catalogs from eight volcanic systems, including two unrest periods at Campi Flegrei, to investigate the spatial organization of swarm activity. Using a standardized declustering method and stacking radial distance distributions from the largest-magnitude event in each swarm, we identify a consistent two-regime spatial pattern across most volcanoes: an approximately uniform density at short distances and an exponential decay at larger scales. The transition scale ₀ varies among volcanoes and may reflect local structural or physical constraints, while the decay slope decreases with volcano size. Santorini represents an exception, suggesting site-specific influences. These empirical regularities provide new observational benchmarks for physical models of swarm dynamics and may inform the development of improved forecasting tools for volcanic seismicity.
Godano et al. (Thu,) studied this question.