The Earthquake Research Institute, the University of Tokyo has announced the geological characteristics of both fault zones after the Kumamoto earthquake occurred near the Nunagawa and Hinagu fault zones, which are the representative active faults in Kyushu.It was created to aid discussions at the government's Earthquake Research Committee, and mentions the relationship between the Kumamoto earthquake and both fault zones.
According to the Earthquake Research Institute, the University of Tokyo, the Hinagu Fault Zone extends 81 km from the vicinity of Kiyama, Mashiki Town, Kumamoto Prefecture, through Ashikita Town, to the southern part of the Yashiro Sea.Based on the characteristics of the geological structure, it is divided into the Takano / White flag section (16 km), the Hinagu section (40 km), and the Yashiroumi section (30 km) in order from the north.
The Hinagu section has a clear right lateral shift of valleys and ridges along the fault, while the Takano-Shirahata section, which is one of the areas where this earthquake occurred, has an almost vertical fault plane.The Futagawa fault zone is a structural boundary that decreases to the north, the Hinagu section has a structure that decreases to the west, and the Takano-Shirahata section has a complicated underground structure.The difference in the fault structure for each section is thought to be due to the difference in maturity.There have been many paleoearthquake surveys in the Hinagu Fault Zone, and it is said that the latest paleoearthquake occurred 1 to 1,200 years ago in the Takano-White Flag section.The latest activities are believed to be older in the other two sections.
The Earthquake Research Institute said that most of the Kumamoto earthquakes appear to occur along the Takano-Shirahata section of the Hinagu fault zone, and while determining the exact location of aftershocks, the distribution of faults that appeared on the ground surface was investigated. , It is necessary to clarify the nature of the earthquake.Furthermore, since there is data that part of it occurs in the Futagawa fault zone, it is necessary to clarify this point as well.
The Japan Meteorological Agency regards the magnitude 14 earthquake that occurred on the 6.5th as the foreshock and the magnitude 16 earthquake that occurred on the 7.3th as the mainshock.The Earthquake Research Committee evaluates that the foreshock is due to the activity of the Hinagu fault zone and the mainshock is due to the activity of the Futagawa fault zone.