Spatiotemporal variations of ⁴⁰K in atmospheric fallout caused by volcanic eruptions: A case study for Kyushu Island,...

This study focused on ⁴⁰K in atmospheric-fallout monitoring data from various locations across Kyushu Island, which hosts numerous active volcanoes, including Sakurajima Volcano, one of Japan's most active volcanoes. It evaluated and verified the relationship between the fallout levels of the naturally occurring radionuclide ⁴⁰K and volcanic activity in the region. Volcanic ash contains several percent potassium, and ashfall leads to an increase in the ⁴⁰K fallout as an atmospheric deposit. A statistically significant positive correlation was found between the monthly number of explosive eruptions at Sakurajima and the monthly ⁴⁰K fallout in Miyazaki City, located 80 km away from Sakurajima, over 35 years. This finding demonstrates that Sakurajima's activity consistently causes ashfall in Miyazaki City, although no statistical data on ashfall exists. Ashfall levels there are approximately 10-20 g m^-2 per month during peak periods, estimated from ⁴⁰K fallout levels and the known potassium content in volcanic ash from Sakurajima. Furthermore, while large ⁴⁰K fallout amounts are sometimes observed on short time scales at the monitoring station in Kyushu, correlating the timing of these significant events with the eruption history of volcanoes within Kyushu led to the conclusion that most significant increases in ⁴⁰K fallout are attributable to ashfall from eruptions of specific volcanoes. Observed ⁴⁰K fallout data can reveal unrecorded volcanic ashfall events and be used to estimate ashfall amounts.