天然記念物に指定された2016年熊本地震の地表地震断層と火山域での横ずれ変位速度の時間的変化

2016年熊本地震の地表地震断層の一部が天然記念物として保存された。本稿では，保存された三箇所の地形を記載し，活断層の位置との関係を検討した。潮井神社と堂園に出現した地震断層は，最大級の2m程度の変位を示し，変動地形学的に認識された活断層線の近くに出現したが，福原では全く予見できない場所に地震断層が現れた。本稿では，さらに，阿蘇山の側火山の噴出物である高遊原溶岩が布田川断層によって470～510m程度横ずれしていることを報告した。布田川断層の過去約9万年間の横ずれ平均変位速度は5.2～5.7mm／年と算定され，地震断層の変位量と過去約1.5万年間の古地震から推定される変位速度とに大きな齟齬がある。数万年間で変位速度が異なる例は我が国では知られておらず，火山噴出直後は断層周辺の温度や歪みの状態がその後と異なっていた可能性が考えられた。火山周辺で算出される変位速度では算定される時間的な違いを考慮する必要があると考えられる。

The surface ruptures of the 2016 Kumamoto Earthquake with a 7.2 magnitude appeared along the northern part of the Futagawa–Hinagu fault zone. The ruptures located at three sites in Mashiki Town in 2018 were designated as a precious natural treasure by the Agency of Cultural Affairs, Japan. Herein, we describe the topography of the displacement along the surface ruptures at the designated areas and examine the relationship between the active fault distribution by tectonic geomorphological interpretation and the surface ruptures that appeared in 2016. At Shioi Shrine and Dozon, surface ruptures with a displacement of approximately 2 m appeared near the location of the active fault line, which was previously known as a fault scarp, displacing the Aso-2 pyroclastic flow and the Takayubaru lava flow. From the viewpoint of tectonic geomorphological interpretation, we could not estimate the surface rupture location at the Fukuhara site, even after the earthquake. The right lateral displacement of the Takayubaru lava flow, which erupted from the Omine pyroclastic cone of the Aso lateral cone in approximately 90 ka, is up to 470-510 m along the Futagawa fault. The mean slip rate during 90 ka is approximately 5.2-5.7 mm/year. This value is much faster than that of this fault in previous studies and shows one of the fastest faults on Japanese islands. Moreover, the slip rate was faster than that calculated based on the displacement along the surface rupture in 2016 and the recurrence interval of the paleo-earthquake during 15 ka. The change in the slip rate of the active fault over a few 10,000 years was not known in the Japanese islands. This suggests that the temperature or stress along the fault plane just after the eruption is different from that in recent geological time. In this case, the timing of the sediment deposition with the slip rate estimation would be critical in evaluating the seismic hazards of the active fault distributed near the volcano.