Continuous long-term array analysis of seismic records observed during the 2011 Shinmoedake eruption activity of Kirishima volcano, southwest Japan

We deployed a seismic array at a site 5 km east of Shinmoedake volcano, in the Kirishima volcanic complex of southwest Japan, five days after the sub-Plinian eruption on 26 January, 2011. The array record between February and September 2011 included explosion earthquakes and episodes of weak continuous tremor during eruption periods. We estimated slownesses and back azimuths of seismic waves on a sliding 1-min window using the semblance method. The slownesses of the weak continuous tremor clustered within the range 0.2–0.8 s/km, consistent with a mix of body and surface waves. A probabilistic approach based on a grid search was used to estimate the source locations of the explosion earthquakes and weak continuous tremor. The sources of the explosion earthquakes were beneath the crater at depths of −0.5–1 km above sea level, while the source of the weak continuous tremor was beneath the northern part of Shinmoedake at depths between 1 km below sea level and 1 km above sea level. This latter region corresponds to a shallow low-resistivity layer, suggesting that hydrothermal processes are more plausible than magmatic processes as the generating mechanism of the weak continuous tremor.