[SSS16-P10] Paleoseismic history and horizontal slip of the Futagawa–Hinagu fault zone based on paleoseismological and tectonic geomorphological survey at Hirata, Mashiki Town, Kumamoto Prefecture
Keywords:2016 Kumamoto earthquake, surface rupture, trenching survey, horizontal slip
On the 16 April, 2016 Kumamoto earthquake (Mj7.3), ~31km-long right-lateral surface ruptures appeared along the previously mapped Futagawa-Hinagu fault zone. According to the evaluation by the Earthquake Research Committee of the Headquarters for Earthquake Research Promotion (2013) before the Kumamoto earthquake, the slip rate of the fault zone was 0.1-0.3 mm/yr, and the recurrence interval was 8,100-26,000 years. However, some trenching surveys conducted along surface ruptures after the Kumamoto earthquake showed that the timing of the most recent faulting event before the Kumamoto earthquake was 1,000-4,000 yrBP. Moreover, the horizontal slip rate of the fault zone is 1.5-3.7 mm/yr, and the recurrence interval is 500-2,000 years in the northeastern part of the fault zone based on the investigation of the fault outcrop. On the other hand, it is not enough to examine the paleoseismic history and the slip rate in the central part of the fault zone. Then, we conducted the paleoseismological and tectonic geomorphological study in Hirata, Mashiki Town located in the central part of the fault zone in September, 2019, in order to examine the paleoseismic history and to reveal the horizontal slip rate.
The study site is the formerly cultivated land located east of the Hiratanaka Community Center in Mashiki Town. In the south of the study site, hills consisting of Aso-1 to Aso-4 pyroclastic flow deposit are distributed, and the formerly cultivated land is located at the fan surface formed the outlet of the dissected valley. The dissected valley is dextrally offset about 5m. At the formerly cultivated land, left-stepping surface ruptures appeared associated with the Kumamoto earthquake. The fence dextrally offset about 1.2m in the Hiratanaka community center site. In order to reveal the three-dimensional structure of the strata, we excavated two trenches (trench A and B).
On the trench A wall, surface soil layer, alternation of gravel and silt, and silt layer were mainly exposed. The alternation strata are distributed only on the north side of the wall and sloped to the north. Since it contains many amphiboles, it is considered to be a constitute deposit of fan surface originating from Aso-4. The upper layer of the alternation strata has a 14C age of 10,408-10,229 calBP. The silt layer is distributed only on the south side of the wall. On the trench A wall, the main fault that separates the alternation strata and silt layer and three secondary faults that displace the alternation strata. The main fault has a flower structure branching into several lines in the upper part on the west wall, and the branched fault continues to the opening crack recognized on the surface.
On the wall and the bottom of the trench B, alternation of gravel and silt and fault which bordered the layer and gravel layer covering the alternation strata were exposed. The alternation strata sloped to the north as in the trench A, and the thickness of each single layer was 0.2-0.4 m. It is considered to be the alternation strata exposed on the trench A wall based on the sedimentary facies. It is considered that the gravel layer covering the alternation strata is a debris flow sediment, and it is a sediment which constitutes the fan surface together with the alternation strata. The bottom of the alternation strata has a 14C age of 5,270 -4,871 calBP on trench B wall.
Based on the deformational features of exposed strata of trench A and B, we identified at least two faulting events, including the Kumamoto earthquake after about 10,000 yrBP. Based on the sedimentary and faulting structure and 14C age of the strata, the timing of the penultimate event is considered to be 4,800-10,400 yrBP. In the trenching site, any events after 4800 yrBP cannot be recognized because any strata after 4800 years ago has not been deposited.
On the other hand, the amount of the dextral offset of the alternation strata is at least 2.9m based on the planar structure of the alternation strata. Since this amount of offset is larger than the amount of offset in the Kumamoto earthquake (Approximately 1.2 m), it seems to have faulted multiple times. This survey is being continued even at the time of the manuscript submission, and the horizontal slip rate of the fault zone will be announced on the day.
The study site is the formerly cultivated land located east of the Hiratanaka Community Center in Mashiki Town. In the south of the study site, hills consisting of Aso-1 to Aso-4 pyroclastic flow deposit are distributed, and the formerly cultivated land is located at the fan surface formed the outlet of the dissected valley. The dissected valley is dextrally offset about 5m. At the formerly cultivated land, left-stepping surface ruptures appeared associated with the Kumamoto earthquake. The fence dextrally offset about 1.2m in the Hiratanaka community center site. In order to reveal the three-dimensional structure of the strata, we excavated two trenches (trench A and B).
On the trench A wall, surface soil layer, alternation of gravel and silt, and silt layer were mainly exposed. The alternation strata are distributed only on the north side of the wall and sloped to the north. Since it contains many amphiboles, it is considered to be a constitute deposit of fan surface originating from Aso-4. The upper layer of the alternation strata has a 14C age of 10,408-10,229 calBP. The silt layer is distributed only on the south side of the wall. On the trench A wall, the main fault that separates the alternation strata and silt layer and three secondary faults that displace the alternation strata. The main fault has a flower structure branching into several lines in the upper part on the west wall, and the branched fault continues to the opening crack recognized on the surface.
On the wall and the bottom of the trench B, alternation of gravel and silt and fault which bordered the layer and gravel layer covering the alternation strata were exposed. The alternation strata sloped to the north as in the trench A, and the thickness of each single layer was 0.2-0.4 m. It is considered to be the alternation strata exposed on the trench A wall based on the sedimentary facies. It is considered that the gravel layer covering the alternation strata is a debris flow sediment, and it is a sediment which constitutes the fan surface together with the alternation strata. The bottom of the alternation strata has a 14C age of 5,270 -4,871 calBP on trench B wall.
Based on the deformational features of exposed strata of trench A and B, we identified at least two faulting events, including the Kumamoto earthquake after about 10,000 yrBP. Based on the sedimentary and faulting structure and 14C age of the strata, the timing of the penultimate event is considered to be 4,800-10,400 yrBP. In the trenching site, any events after 4800 yrBP cannot be recognized because any strata after 4800 years ago has not been deposited.
On the other hand, the amount of the dextral offset of the alternation strata is at least 2.9m based on the planar structure of the alternation strata. Since this amount of offset is larger than the amount of offset in the Kumamoto earthquake (Approximately 1.2 m), it seems to have faulted multiple times. This survey is being continued even at the time of the manuscript submission, and the horizontal slip rate of the fault zone will be announced on the day.