Mechanisms of sinkhole formation and hydrothermal dynamics at the Oana Crater on Azumayama Volcano, Northeast Japan

In 2021, a sinkhole emerged at the base of the Oana Crater on Azumayama Volcano in northeastern Japan, prompting investigation into its potential link to volcanic activity. Our study examines the phenomenon of sinkhole formation within active volcanic craters. Through meticulous field observations, we provide detailed insights into the sinkhole’s characteristics and explore its underlying causes. By examining the site’s history, we uncover predisposing factors contributing to sinkhole formation. Changes in heat distribution beneath the crater, notably from the northwest to the northeast side, are estimated, affecting the activity of nearby fumaroles. This redistribution of thermal energy triggers significant changes, including the emergence of molten sulfur and heightened fumarole activity. Further analysis reveals the sinkhole’s connection to past hydrothermal activity, notably a similar event in 1966, which served as a conduit for subsurface thermal energy. We assume that weakening of surface geology by hydrothermal fluids and volcanic gases facilitated the sinkhole’s formation in 2021. Notably, while past sinkhole formations at Azumayama Volcano preceded eruptions, the 2021 event occurred independently of mud eruptions but indicates a potential shift in thermal energy pathways within the shallow subsurface. Our findings emphasize the importance of studying local topographical changes within volcanic edifices to elucidate fluids behavior and contribute to understanding eruption mechanisms.