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[SSS08-07] Paleo-seismological survey on surface rupture with small amount of displacement associated with the 2016 Kumamoto earthquake
Keywords:2016 Kumamoto earthquake, Active fault, Trench survey, Tephra, Aso volcano
Detailed surface ruptures around Miyaji were reported in Ishimura et al. (in press). Small surface failures (Vertical offset: less than 10 cm, Right lateral offset: less than 5cm) were recognized and we excavated a 10-m-long, 5-m-wide, 3-m-deep trench on the northern trace. On the upper trench wall, we identified continuous soil sequence, including Ojodake scoria (OjS) and Nakadake N2 scoria (N2S). On the lower trench wall, we identified fluvial deposits containing much Kishimadake scoria (KsS). Tephra ages of KsS, OjS, and N2S are 4.0 ka, 3.6 ka, 1.5 ka, respectively (Miyabuchi, 2009).
We read two faulting events on the trench wall. The latest event is the 2016 event associated with the 2016 Kumamoto earthquake. A fault cuts up to the bottom of cultivation soil cultivated after the 2016 Kumamoto earthquake and its vertical separation is 10-15 cm. The penultimate event is estimated between N2S and OjS. Branch fault cuts OjS, but is covered by N2S. Vertical separation using layers below the OjS is 25-35 cm. Its value is twice of the 2016 event.
Penultimate event age is corresponding to the ages of open cracks on the archeological site in Aso valley (Kumamoto Prefectual Board of education, 2010), paleoseismicity on the Futagawa fault and Hinagu fault zone (Okamura et al., 2017; Takahashi et al., 2017), indicating that penultimate event at the our trench site is related with the activity of Futagawa fault or Hinagu fault zones.