Osozawa Soichi, Ito Hisatoshi
Institute of Geology and Paleontology, Faculty of Science, Tohoku University, Sendai, 980-8578, Japan.
Kawaoso Molecular Bio-Geology Institute, Sendai, 982-0807, Japan.
Sci Rep. 2023 Aug 28;13(1):14026. doi: 10.1038/s41598-023-41031-6.
We conducted a detailed geological survey of the Sendai region, covering an area of 100 × 50 km. Our survey focused on accurately mapping river terraces, identifying the source volcanoes responsible for intercalated tephras, and locating the Nagamachi-Rifu fault and associated faults. The river terraces were observed and categorized based on their elevation relative to the present river channels. These terraces are predominantly found on the hanging wall of major reverse faults. Each terrace comprises fluvial gravels at the lower levels and eolian loam intercalated with local and regional tephras at higher levels, with the contact age corresponding to the time of emergence. To determine the ages of the terrace gravels, we employed a combination of zircon U-Pb dating, sedimentation rate calculations, and extrapolation techniques. This allowed us to establish the abandonment or emergent dates of the terraces. The formation of these terraces coincided with periods of fault activity, including hanging wall uplift, footwall subsidence, and fault vertical displacement, effectively monitoring the co-seismicity of the Nagamachi-Rifu fault. While we cannot predict the exact timing of future events, it is crucial to remain vigilant regarding the potential occurrence of a significant earthquake triggered by these fault activities.
我们对仙台地区进行了详细的地质调查，调查范围覆盖面积为100×50千米。我们的调查重点是精确绘制河流阶地地图，确定造成夹在其中的火山灰层的源火山，以及定位长町-利府断层和相关断层。根据河流阶地相对于当前河道的海拔高度对其进行了观测和分类。这些阶地主要分布在主要逆断层的上盘。每个阶地在较低层由河流砾石组成，在较高层则夹有与当地和区域火山灰层相间的风成壤土，其接触年代对应于阶地出现的时间。为了确定阶地砾石的年代，我们采用了锆石U-Pb测年、沉积速率计算和外推技术相结合的方法。这使我们能够确定阶地的废弃或出现日期。这些阶地的形成与断层活动时期相吻合，包括上盘隆起、下盘沉降和断层垂直位移，有效地监测了长町-利府断层的同震情况。虽然我们无法预测未来事件的确切时间，但对于这些断层活动引发重大地震的潜在可能性保持警惕至关重要。
