Seismic Noise as a Window into Volcanic Unrest: Observations from Ruapehu, New Zealand in 2022.

Ruapehu is an active andesitic composite volcano that erupted twice (in 2006 and 2007), following its major eruptive sequence in 1995–1996. Those eruptions were phreatic explosions that occurred with few no precursors. In March 2022, Ruapehu entered a period of significant volcanic unrest characterized by volcanic tremor, gas emissions, and heating of the crater lake. To monitor temporal variations in the elastic properties of Ruapehu volcano during this unrest episode, we applied the method ''Moving Window Cross-Spectral Analysis". This technique revealed a $\sim$0.5$\%$ decrease in relative seismic velocity at the beginning of 2022, coinciding with a small earthquake swarm. This correlation suggests that the process triggering the swarm may also be responsible for the observed velocity drop. A similar $\sim$0.5$\%$ relative seismic velocity reduction was observed during the volcanic unrest in the North-East sector of the volcanic edifice. This seismic velocity drop appears to be a reversible process, likely driven by magmatic or fluid movement. Possible causes include the opening of fractures within the magmatic reservoir, fluid fluxes (water, gas, or magma), magmatic anomalies (magma that does not reach the surface but intrudes into the subsurface), or environmental factors (e.g. rainfall or atmospheric pressure changes) that may contribute to the formation of low-velocity zones. By mid-May 2022, volcanic tremor and crater lake temperature began to decline, most likely due to a reduction in fluid circulation, through sealing processes. However, the reduction in seismic velocity persisted, indicating potential ongoing subsurface alteration. This study highlights the importance of using ambient noise monitoring to detect seismic velocity changes during volcanic unrest. By providing potential insights into subsurface processes, this method can complement current monitoring techniques and improve eruption forecasting and hazard assessment at Ruapehu.