Seismo‐Acoustic Evidence for Meteoric Water Modulation of Hydrothermal Fluid Discharge

Abstract Deciphering the signal of external processes on active hydrothermal dynamics represents a critical challenge in understanding volcanic unrest. At the Pisciarelli hydrothermal site, located within the densely populated Campi Flegrei caldera (Italy), we investigate the role of meteoric water influx in modulating shallow hydrothermal fluid discharge. Our analysis reveals a persistent seismic tremor generated at shallow depths, whose amplitudes fluctuate noticeably and closely follow rainfall patterns. Hydrothermal activity also emits distinct acoustic signals related to boiling and steam venting. Importantly, we observe an inverse relationship between tremor and acoustic amplitudes: as tremor increases, acoustic emissions decrease. The influx of cold water is also linked to a rise in hydrothermal water level, indicating increased conduit pressure, which is supported by ground tilt data showing conduit inflation. We propose a conceptual model in which pressurization cycles in the shallow hydrothermal system result from the interplay between steam condensation, meteoric water influx, and steam outgassing. These pressurization cycles may promote seismicity, as evidenced by the seasonal patterns observed in earthquake rates. However, these seasonal effects are secondary to the dominant, accelerating multi‐year trend of increasing earthquake rates due to volcanic unrest. Our findings demonstrate that seismo‐acoustic monitoring provides a sensitive tool for detecting transient changes in hydrothermal dynamics, offering valuable insights into the interpretation of volcanic unrest signals. This, in turn, contributes to more refined volcanic hazard assessments. Although developed at Campi Flegrei, this approach holds potential for application to other active hydrothermal systems worldwide.