Abstract Coseismic changes in water levels and discharges are well-known phenomena worldwide. Reliable hypotheses to explain these hydrological changes qualitatively have been proposed using various data and analyses. However, there are few quantitative assessments of these changes, which require hydrological simulators. A dense monitoring network revealed coseismic river and groundwater level changes induced by the 2016 Mw 7.0 Kumamoto crustal earthquake associated with many post-seismic fracture systems. In this study, we reproduced these changes using a physically based, fully distributed, integrated watershed modeling tool (GETFLOWS). In particular, the formation of new fracture systems is key to understanding the hydrological changes observed after the earthquake, which were incorporated into our model by changing parameters. Previous studies suggested that the observed water level drop immediately after the earthquake is explained by the drawdown of surface and aquifer waters to fill open spaces in the upper crust along the fracture systems generated under extensional stress field. After the initial water drop, water levels rose in the long-term because additional water sources were released from surrounding mountains due to the coseismic increase in the permeability of the soils and rocks around the compressional fracture systems. These hypotheses were validated in our model by adjusting the depths of fracture systems. Consequently, we constructed the three-dimensional distribution of these property changes over the study area. Furthermore, the model calculation yielded the volumes of missing and released mountain waters in the studied watershed (ca. 106 and 108 m3, respectively). This study demonstrates the utility of an integrated watershed modeling tool for investigating coseismic hydrological changes in active hydrological systems quantitatively.

中文翻译：

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使用数值模型定量评估 2016 年熊本地震引起的区域水流系统变化

摘要 水位和流量的同震变化是世界范围内众所周知的现象。已经使用各种数据和分析提出了定性解释这些水文变化的可靠假设。然而，对这些变化的定量评估很少，这需要水文模拟器。密集的监测网络揭示了与许多地震后断裂系统相关的 2016 年 Mw 7.0 熊本地壳地震引起的同震河流和地下水位变化。在这项研究中，我们使用基于物理的、完全分布式的、集成的流域建模工具 (GETFLOWS) 重现了这些变化。特别是，新断裂系统的形成是理解地震后观察到的水文变化的关键，这些变化通过改变参数被纳入我们的模型。先前的研究表明，地震后观察到的水位立即下降可以解释为地表水和含水层的水下降，以填充上地壳中沿拉伸应力场产生的裂缝系统的开放空间。在最初的水下降之后，由于压缩断裂系统周围土壤和岩石渗透性的同震增加，从周围山脉释放了额外的水源，水位长期上升。通过调整裂缝系统的深度，这些假设在我们的模型中得到了验证。因此，我们构建了研究区域内这些属性变化的三维分布。此外，模型计算得出了研究流域中缺失和释放的山地水域的体积（约 106 和 108 立方米，分别）。这项研究证明了综合流域建模工具在定量调查活动水文系统中的同震水文变化方面的实用性。