The origin of volcano-tectonic earthquake swarms

Volcano-tectonic (VT) seismicity is commonly recorded prior to and during eruptions. VT seismicity may reflect stresses induced by dike propagation, as indicated by propagating hypocenters and fault-plane solutions (FPS) reflecting regional stresses, or stresses induced by dike inflation, indicated by randomly distributed hypocenters and FPS with pressure axes rotated similar to 90 degrees to regional maximum compression. Although numerical models of dike-induced stresses indicate that both regimes should occur during dike emplacement, this has not yet been observed, according to published seismic data. Instead, similar to 90 degrees rotated FPS are observed in some cases, while propagating hypocenters mark dike formation in other cases. We suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma-wall-rock interactions. Ultimately, it may be possible to use information contained in VT seismicity to forecast changes in a volcano's behavior by establishing the characteristic stress field response for a given volcano, or through a deeper understanding of the complex relationships between VT seismicity, local crustal stresses, and the physical mechanisms of magma migration.