After the urgent field survey from April 15, Professor Shinji Toda, together with other researchers, went to Kumamoto again to continue their research from April 22 to 25. As a result, a normal faulting rupture was also discovered in the Futagawa fault zone, and it indicates that an oblique slip occurred deep underground but a strike-slip fault and a normal fault running parallel approximately two kilometers apart on the earth’s surface were identified.

According to the survey and analysis of the research team including Professor Toda, it can be estimated that the fault branched from the underground to the surface, and the oblique slip underground was dissolved by both the strike-slip and dip-slip on surface. This is technically called slip partitioning (Figure 2). Though there have been several reports of this occurrence in earthquakes overseas, there is a high possibility that it is the first time such a case was recognized in Japan. The idea that the Futagawa fault zone has a normal fault element, is consistent with the fact that the fault zone is situated on the southern edge of the Beppu-Shimabara rift zone.

*Normal fault is a fault where the ground moves vertically under extensional strain.

During the field survey conducted on April 24, a normal fault stretching for more than approximately two kilometers was found on the west side of Tawarayama mountain in Nishihara Village (Figure 1, Pictures 1-3). Dip-slip from the normal fault had a maximum of 1.5 meters, with the north-west side dropping. This is not a sliding cliff resulting from landslides, but a normal fault continuing from the earthquake source fault which caused the earthquake.

On the other hand, in Nishihara Village, a right-lateral strike-slip fault crosses the body of the Ogiribata Dam, causing an approximately 1.5 meter right-lateral strike-slip (Pictures 4, 5). Therefore, it can be said that a right-lateral strike-slip and a normal fault are running parallel, approximately two kilometers apart (Figure 2).

Figure 1

Figure 2

Photo 1

Photo 2

Photo 3

Photo 4

Photo 5

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