Understanding volcanic eruption triggers is critical towards anticipating future activity. While internal magma dynamics typically receive more attention, the influence of external processes remains less understood. In this context, we explore the relationship between seasonal snow cycles and eruptive activity at Ruapehu, New Zealand. This is motivated by apparent seasonality in the eruptive record, where a higher than expected proportion of eruptions (post-1960) occur in spring (including the two previous eruptions of 2006 and 2007). Employing recent advancements in passive seismic interferometry, we compute sub-surface seismic velocity changes between 2005–2009 using the cross-wavelet transform approach. Opposite trends in velocities are identified on and off the volcano, with stations closest to the summit recording a winter high closely correlated with the presence of snow. Inverting for depth suggests these changes occur within the upper 200–300 m. Reduced water infiltration (as precipitation falls as snow) is considered the likely control of seasonal velocities, while modeling also points to a contribution from snow-loading. We hypothesise that this latter process may play a crucial role towards explaining seasonality in the eruptive record. Specifically, loading/unloading may influence the volcanic system through increased degassing, thereby increasing the likelihood of small, gas-driven, eruptions. Our findings shed light on the complex interactions between volcanoes and external environmental processes, highlighting the need for more focused research in this area. Pursuing this line of inquiry has significant implications towards improved risk and hazard assessments at not just Ruapehu, but also other volcanoes globally that experience seasonal snow cover.

We use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre-eruptive period and the onset of the 2018 eruption of Kilauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano-tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.