Fluid flow of CO2-driven geyser activity as inferred from tilt and self-potential observations of the Kibedani Geyser Japan.

We conducted multi-parameter, multi-site observations at the Kibedani Geyser, Japan, where cold spring-water eruptions (20 °C) are driven by underground CO₂ pressure. Observations included video, temperature, self-potential (SP), ground tilt, and acoustic measurements. The geyser exhibits a 38-min cycle, consisting of 32 min between eruptions followed by 6 min of eruption. All data revealed changes corresponding to this cycle. Modeling of tilt data indicate that an inclined crack-shaped cavity pressure source expands between eruptions and contracts during eruption, with post-eruption re-expansion beginning ~ 2.5 min after eruption, when ground tremor has ceased. This cavity is interpreted as a bubble trap where CO₂ gas, separated from ascending spring-water, accumulates prior to eruption. SP data modeling suggests that the current source responsible for SP generation, represented by a point and crack-shaped model, is located near this cavity. The correlation between the pressure source and the SP current source suggests that groundwater inflow occurs into the pressure source. Our findings suggest that CO₂ accumulates in the cavity, uplifting water during an eruption and driving groundwater inflow into the cavity. This study highlights the effectiveness of combining tilt and SP methods for analyzing fluid-driven phenomena, providing insights into geyser activity and analogous processes such as volcanic phreatic eruptions. We anticipate that our approach, when applied not only to CO₂-driven geysers but also to hydrothermal-driven geysers, will contribute to the elucidation of mechanisms underlying phreatic eruptions.