Okamoto Kyosuke, Asanuma Hiroshi, Nimiya Hiro
Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, 2-2-9, Machiikedai, Koriyama, Fukushima, 963-0298, Japan.
Research Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8567, Japan.
Sci Rep. 2021 Apr 16;11(1):8372. doi: 10.1038/s41598-021-86775-1.
Subsurface structure survey based on horizontal-to-vertical (H/V) spectral ratios is widely conducted. The major merit of this survey is its convenience to obtain a stable result using a single station. Spatial variations of H/V spectral ratios are well-known phenomena, and it has been used to estimate the spatial fluctuation in subsurface structures. It is reasonable to anticipate temporal variations in H/V spectral ratios, especially in areas like geothermal fields, carbon capture and storage fields, etc., where rich fluid flows are expected, although there are few reports about the temporal changes. In Okuaizu Geothermal Field (OGF), Japan, dense seismic monitoring was deployed in 2015, and continuous monitoring has been consistent. We observed the H/V spectral ratios in OGF and found their repeated temporary drops. These drops seemed to be derived from local fluid activities according to a numerical calculation. Based on this finding, we examined a coherency between the H/V spectral ratios and fluid activities in OGF and found a significance. In conclusion, monitoring H/V spectral ratios can enable us to grasp fluid activities that sometimes could lead to a relatively large seismic event.
基于水平向与垂直向（H/V）谱比的地下结构调查被广泛开展。该调查的主要优点是使用单个台站就能方便地获得稳定结果。H/V谱比的空间变化是众所周知的现象，并且已被用于估计地下结构的空间波动。预计H/V谱比会有时间变化，这是合理的，特别是在诸如地热田、碳捕获与封存场地等预计有丰富流体流动的区域，尽管关于时间变化的报道很少。在日本的奥久津地热田（OGF），2015年部署了密集的地震监测，并且持续监测一直保持一致。我们在OGF观测了H/V谱比，发现它们出现了反复的暂时下降。根据数值计算，这些下降似乎源自局部流体活动。基于这一发现，我们研究了OGF中H/V谱比与流体活动之间的相关性，并发现了其显著性。总之，监测H/V谱比能够使我们掌握有时可能导致相对较大地震事件的流体活动。
