Seismic background level growth can reveal slowly developing long-term eruption precursors – A case study at Kirishima volcano, Japan

The accelerating growth of seismic unrest before eruptions has been observed at many volcanoes and utilized for eruption forecasts. However, there are still many eruptions for which no precursory unrest has been identified, even at well-monitored volcanoes. The recent eruptions of Shinmoe-dake, an active cone of Kirishima volcano, Japan, had been another negative example of this kind. Here we present seismological evidence that the eruption preparation had been ongoing at the shallow depths beneath Shinmoe-dake for several months to a year.

We investigated the seismic background level (SBL) of eleven-year data recorded around the volcano, including two stations about 1 km from the eruptive crater. We searched for persistent weak signals, focusing on low-amplitude time windows recorded during quiet nighttime. Then the spectra of daily background noise were classified by clustering analysis. The SBL analysis successfully revealed very weak precursory tremors from more than several months before the eruption, and residual tremors to the end of the eruptive period. The precursory signals grew acceleratory in a similar way as is assumed in the material failure forecast method applied to eruption forecasts. However, their growth was significantly slower and longer compared to previous cases. Such slow and quiet preparations would not be captured by conventional seismological methods but could be a common feature at volcanoes with developed hydrothermal systems. It is also noted that the SBL monitoring is potentially useful to judge the end of an eruption period. Further studies are necessary for clarifying the source locations and mechanisms of the SBL noise.