The 2018 small phreatic eruption that occurred at Iwo-yama (Ebinokogen Ioyama) volcano of the Kirishima Volcanic Complex in Japan was recorded by infrasonic, seismic, and visual (video) observations within 1 km from eruption vents, which provides a good opportunity to investigate infrasound mechanisms and associated surface eruptive dynamics. The monitoring camera captured a widening of the vent about 3 h after the eruption onset, and then the infrasound stations recorded pulse-like intermittent signals. We carried out seismoacoustic analysis mainly focusing on the pulse-like infrasound, and also referred to rheological properties of mud suspension sampled from the rim of the vent to consider mechanical processes of the infrasonic pulse generation. The observed infrasonic pulses exhibited clear transitions in their waveforms and spectral contents. Based on the results of the seismoacoustic analysis and rheological measurements, we infer that the infrasonic pulse transition may have been caused by the shift of bubble bursting styles at the surface of the mud suspension, through the decrease of yield stress and viscosity due to increasing water fraction. The vent widening can be attributed to a collapse of soil around the vent due to the addition of water and the resultant weakening of the soil strength. We also observed high-frequency continuous infrasound (infrasonic tremors), which coupled into the ground and were recorded by the seismometers as well. The high-frequency infrasonic tremors had spectral features similar to those known for jet noise and might have been caused by the ash-steam mixture jet from the vent. Finally, we propose a conceptual model of the transition process of the surface eruptive activity by attributing them to the rheological change of the mud suspension with the increase of the water fraction.
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