[SSS08-P22] Comparison of GPR survey results and trench survey results at the Matoishibokujo I Fault
Keywords:GPR survey, trench survey, Matoishibokujo I Fault, the 2016 Kumamoto Earthquake, the northwest of the outer rim of the Aso caldera
Result & Discussion: Profile of GPR (using Noggin Plus manufactured by Sensors & Software Inc. with the central frequency of 250 MHz) survey at the planned site of trench survey in Matoishibokujo I Fault site showed clear reflecting plane about 3~4 m deep on the south side (subsidence side) of the fault. The reflecting plane was inclined to a deeper part as it approached the fault. This indicates that sedimentary layer (pyroclastic material or alluvium) located on the hanging wall side of half-graben-shaped normal fault pointed out by Fujiwara et al. (2016) is cumulatively subsiding associated with the normal fault movement. As a result of trench survey at the fault part, a pyroclastic layer presumed as Aso-4 pyroclastic flow deposits (89Ka) appeared around 3~4 m deep. Therefore, it is presumed that the clear reflecting plane of the GPR profile corresponds the upper surface of Aso-4. Although the GPR survey could not detect the obvious structure of the fault, there is a possibility that the fault locates at a position where a clear reflection plane corresponding to the pumice layer is disconnected.
Summary and Challenges: As a result of GPR survey across the Matoishibokujo I Fault where surface displacement occurred due to the Kumamoto Earthquake, the sedimentary layer structure inclined toward the fault around 3~4 m deep was detected. The structure indicates cumulatively subsidence associated with the movement of the half-graben-shaped normal fault. I will try to interpret the result of GPR survey in more detail by comparing the detail result of trench survey in the future.
Acknowledgements: The authors would like to express the deepest appreciation to Prof. Yasuhiro Suzuki at Nagoya Univ. for borrowing GPR survey devices. The authors would like to express the deepest appreciation to Dr. Keitaro Komura at the Central Research Institute of Electric Power Industry, students of Kaneda Laboratory at Chiba Univ., students of Department of Geography, Graduate School of Science and Technology, Nihon Univ. and Prof. Hiroshi Yagi at Yamagata Univ. for assistance with the trench survey and discussion. The part of this work was supported by JSPS KAKENHI Grant Number JP17K01234.
References: Fujiwara et al. (2016): Earth, Planets and Space, 68:160; Kyushu active structure research group ed. (1989): Active structure in Kyushu. University of Tokyo Press, 553p; Nakata and Imaizumi (2002): Digital Active Fault Map of Japan, University of Tokyo Press; Suzuki et al. (2017): 1:25,000 Active Fault Map “Aso”. GSI of Japan.