Characteristics of tephra fall from eruptions at Sakurajima volcano, revealed by optical disdrometer measurements

We investigated the main characteristics of tephra fall from the eruptions at Sakurajima volcano in southern Kyushu, Japan, by using an optical disdrometer for simultaneous measurement of particle size and fall velocity. Measurements were carried out for 2 years at one station located 2.5 km south of the active crater. This made it possible to detect 76 tephra fall events, with the detection rate of the tephra events from these south-directed eruption clouds being about 25%. From the relationship between particle diameter and fall velocity, we were able to estimate morphological parameters of volcanic particles. Measurements of temporal change in the tephra fall for each minute revealed that both the tephra diameter and fall velocity gradually decrease with time during each event, where the ranges of the diameter and velocity for the events with south-directed eruption clouds were 0.336–2.04 mm and 1.40–6.24 m s−1, respectively. By using temporal changes in the diameter and velocity, we estimated travel time and distance for the tephra falling from a release point in the eruption cloud to the station, and found a positive correlation between the travel distance and column height for the south-directed clouds, with a determination coefficient of 0.498 for linear fitting. The comparison between tephra data and seismic data revealed that episodic increases in the tephra diameter and fall velocity were correlated with a drastic increase in amplitude of explosion earthquake or tremor. These results indicate that disdrometer measurements can provide valuable information about the quantitative features of the tephra fall and are useful for detecting changes in the magnitude of eruption if a number of suitably placed instruments are deployed.