Seismic reflection survey is used as an effective method to evaluate the activity of submarine active faults. By combining sediment core samples, activity intervals and average displacement rate are clarified with accuracy comparable to that of onshore trench survey. However, seismic reflection survey sees the underground information as a two-dimensional profile in the vertical direction. So, it is difficult to detect the displacement of strike-slip fault as compared with the normal fault or the reverse fault. The authors have been studying the detection of horizontal displacement at high density of high- resolution geologic exploration using sub bottom profiler at the Yatsushiro-sea which located southwest section of Hinagu fault zone in western part of Kyushu. In this paper, we consider the geological structure of this area by using the data obtained so far. Also, in the 2016 Kumamoto earthquake, a part of the fault zone was active, and an earthquake of Mw 7.0 scale occurred, and the epicenter distribution expanded in the NE-SW direction. In addition to these aftershocks, surface deformation is observed along with the main shock. As a result of comparing such observed data with the data obtained by exploration of the marine area and examining the relationship, although there is a difference in scale between the two structures, morphological similarity is recognized.

1 Introduction

On April 14, 2016, an earthquake of Mw 6.2 occurred at a depth of about 10 km in Kumamoto Prefecture, and this earthquake was named "The 2016 Kumamoto Earthquake" by the Japan Meteorological Agency (Fig. 1). Thereafter, Mw 6.0 and Mw 7.0 earthquakes occurred in the same area. The earthquakes of Mw 6.2 and Mw 6.0 are estimated to be due to the Hinagu Fault Zone while the Mw 7.0 earthquake is due to the activity of the Futagawa Fault Zone (HERP, 2016). The epicenter distribution after the Kumamoto earthquake in 2016 spreads along both fault zones, but noticeable activities are not observed in the middle part of the Hinagu Fault Zone to the southwestern part. From now on, it is necessary to understand the geological structure including the marine area in the southwest section in order to evaluate the activity and linkage of the Hinagu Fault Zone. Yagi et al. (2016) performed high-resolution seismic survey to reveal the activity history of the southwest section of Hinagu Fault Zone and also, challenged three-dimensionalization using total 277 reflection profile records in the Yatsushiro-sea which the junction of the central section to the south-west section of the Hinagu Fault Zone. As a result, it is pointed out that there were, at least, four activities since the last glacial period in view of the presence of ridge-like rise, channel-shaped features, local dragging deformation of sedimentary layer, etc. developed along the seabed active fault.

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