Estimation of Vent Radii From Video Recordings and Infrasound Data Analysis: Implications for Vulcanian Eruptions From Sakurajima Volcano, Japan

We estimated the vent radius within Showa Crater of Sakurajima volcano from ejection velocity and flow rate of gas‐particle mixtures. The ejection velocity was calculated from video recordings, and the flow rate and volume from infrasound data. Based on the assumption that the vent shape does not change during an explosion, the vent radius was estimated from 201 impulsive Vulcanian eruptions at Showa Crater, yielding values of 6.4–42.3 m (median 23.8 m), which is comparable with the width of fresh lava capping the vent, as photographed from a helicopter. Long‐term changes in vent radius (i.e., over several months) show a relationship with magma accumulation within a reservoir 2–5 km beneath the crater. If the top of the conduit is assumed to be cylindrical, then the vertical extent of the gas‐rich zone is estimated to be 120 m, which may reflect the depth of gas accumulation and buildup of significant overpressure. Plain Language Summary A volcanic vent is a hole through which ash and volcanic bombs are ejected during an eruption. Beneath the vent, a gas‐rich pressurized chamber often forms, particularly before cannon‐like explosions. Observing the vent is commonly difficult because of increasing danger when approaching the crater. Accordingly, we present a method to estimate the vent size using eruption videos and infrasound data collected away from the crater. This method was applied to eruptions from Showa Crater of Sakurajima volcano in Japan. The estimated vent size is similar to the actual vent size, as observed from a helicopter. Interestingly, the size of the vent changed over time, increases/decreases in vent size were synchronous with phases of ground deformation, suggesting that the vent size is related to the rate of magma accumulation within the reservoir. The results enhance our understanding of volcanic eruption mechanisms. Key Points Vent radius is estimated by video recordings and infrasound data analysis Long‐term changes in vent radius were observed over 3.5 years A gas‐rich zone beneath the vent has a maximum vertical extent of 120 m, assuming it has a cylindrical shape