Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan.
Department of Earth Sciences, National Central University, Jhongli, 320, Taoyuan, Taiwan.
Sci Rep. 2023 Apr 12;13(1):5954. doi: 10.1038/s41598-023-32875-z.
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, Japan, have 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 other cases reported in the literature. Such slow and quiet eruption preparations would not be captured by conventional seismological methods. We expect that long-term SBL analyses on proximal seismic data will help detect early precursors, even at seismically quiet volcanoes, and will also help towards judging the end of an eruptive period.
在许多火山中都观察到了喷发前地震活动加速增长的现象，并将其用于喷发预测。然而，仍有许多火山喷发没有发现前兆活动，即使是在监测良好的火山也是如此。日本新燃岳火山的最近喷发就是这种情况的另一个负面例子。在这里，我们提供了地震学证据，表明在新燃岳火山浅部以下的深处，喷发准备已经持续了几个月到一年。我们研究了记录在火山周围十一年的数据的背景地震水平（SBL），包括两个距离喷发口约 1 公里的台站。我们搜索了持续的微弱信号，重点关注在安静的夜间记录的低振幅时间窗口。然后通过聚类分析对每日背景噪声的频谱进行分类。SBL 分析成功地揭示了在喷发前几个月就出现了非常微弱的前兆震颤，并且在喷发期结束时仍有残余震颤。前兆信号的增长方式与应用于喷发预测的材料失效预测方法中假设的方式相似，呈加速增长。然而，与文献中报道的其他情况相比，它们的增长速度要慢得多，持续时间也更长。这种缓慢而安静的喷发准备过程不会被传统的地震学方法所捕捉到。我们期望对近场地震数据进行长期的 SBL 分析将有助于检测早期前兆，即使在地震安静的火山也是如此，并且还有助于判断喷发期的结束。
