Researchers can't rule out Hokkaido quake triggered by major active fault zone shift

TOKYO -- The government's Earthquake Research Committee said on Sept. 11 it cannot rule out the possibility that a movement of a major active fault zone triggered the strong earthquake that hit Hokkaido on Sept. 6.

The committee, chaired by University of Tokyo professor Naoshi Hirata, had previously suggested in a Sept. 6 ad hoc meeting that a fault other than the Eastern Boundary Fault Zone of the Ishikari Lowland caused the quake, which registered a maximum 7 on the 7-point Japanese seismic intensity scale.

Meanwhile, the Geospatial Information Authority of Japan informed the research committee of a preliminary finding that the approximately 1.2 meter vertical movement of a fault zone, running north to south about 15 kilometers, was responsible for the destructive jolt.

Furthermore, analysis of an image taken by the Advanced Land Observing Satellite-2 "Daichi-2" revealed that some areas in the town of Atsuma and Mukawa, southeast of the epicenter, were uplifted by as much as 7 centimeters.

The research committee had formerly denied a connection between the Ishikari fault zone and the massive quake, as it believed that the major active zone only stretched out approximately 25 kilometers underground while the focus was about 37 kilometers underground. However with subsequent findings, they cannot currently deny the possibility that a previously unknown part of the Ishikari fault zone had moved deep underground.