Abstract
Detecting subsurface changes during volcanic unrest remains challenging when surface manifestations are weak or absent. Between March and July 2022, Ruapehu volcano (New Zealand) experienced heightened seismicity, volcanic tremor, and crater lake heating, yet no eruption occurred. To monitor temporal variations in the subsurface elastic properties during this unrest episode, we estimated relative velocity changes ( $$\varvec{dv/v}$$ d v / v ) from the coda of the correlations using the Moving Window Cross-Spectral (MWCS) method. Our results show spatially heterogeneous changes: a distinct velocity drop occurred on the NE flank, while the NW flank showed no such pattern. The network mean velocity reached $$\varvec{\sim }$$ ∼ −0.26% in the 0.1–0.3 Hz frequency band, corresponding to peak sensitivity to structural changes at depths of $$\varvec{\sim }$$ ∼ 2–6 km. A 20-day temporal lag between the NE flank and summit signals suggests a lateral-to-vertical migration of magmatic fluids toward the vent. Following the unrest, $$\varvec{dv/v}$$ d v / v recovered to background levels as the crater lake cooled. This may indicate a reversible restoration of the edifice’s elastic state and the cessation of volcanic forcing. Although environmental factors were considered, the magnitude and timing of the $$\varvec{dv/v}$$ d v / v variations are more consistent with a primary volcanic driver. This is the first $$\varvec{dv/v}$$ d v / v application at Ruapehu focusing on a non-eruptive unrest episode; it suggests that frequency-dependent depth constraints and spatial localization can effectively monitor magmatic processes in the absence of measurable geodetic deformation. As such, this approach may improve monitoring and risk assessments of volcanic hazards during similar episodes of unrest.