Keywords:The 2016 Kumamoto earthquakes, Fault outcrop, Striation, Fault rocks
During a rapid survey after the 2016 Kumamoto earthquakes, we found three major fault outcrops (the Kamijin, the Kawahara, and the Kuwazuru outcrop) along the Futagawa fault, which considered to moved with the earthquakes. The Kamijin outcrop is located between the places where surface ruptures of 1-2 m (dextral movement) are found. The sharp fault plane, which trends N66°E and dips 90°, juxtaposes the Aso-4 pyroclastic rock and latery formed colluvial soil. The fault displaces modern ground surface and shows dextral displacement of 2 m in maximum due to the 2016 Kumamoto earthquakes. Based on the spatial distribution of the Aso-4 pyroclastic rock and the colluvial soil, we estimate that the accumulated displacement of ~10 m after the formation of the Aso-4 pyroclastic rock.
At the Kawahara outcrop, we found a faut breccia zone of 20 cm thickness which trends N90°E and dips 72°N.