Phreatic volcanic eruption preceded by observable shallow groundwater flow at Iwo-Yama, Kirishima Volcanic Complex, Japan

Abstract It is difficult to forecast phreatic eruptions because they are often characterised by an abrupt onset at shallow depths beneath volcanoes. Here we show that temporal changes in the tilt, tremor, and horizontal electric field have occurred repeatedly near vent of a small eruption Iwo-Yama, Kirishima Volcanic Complex, Japan. Such geophysical were observed 13 times, with one these events occurring immediately before 2018 eruption. These observations suggest hydrothermal intrusions, which as tilt tremors, commonly induce near-surface cold groundwater flow, electric-field changes. Near-surface flows towards active vent, potentially inhibiting However, explosive occur when intrusion flow depleted. The key controlling occurrence can be monitored using measurements.