The government's Headquarters for Earthquake Research Promotion newly added 16 faults to a list of major active fault zones in which a magnitude-7 or larger earthquake could occur.
As a result, the total number of major active fault zones in Japan has increased to 113 from the previous 97. This is the first time since 2005 that new locations have been added to the list.
Newly included are two faults in the Kanto region, including Yamanashi Prefecture; eight in the Chugoku region; and six in the Kyushu region. Among them are the Okubo fault straddling Gunma and Tochigi prefectures; the Midorikawa fault zone adjacent to the Hinagu fault zone that moved when the Kumamoto Earthquake took place; the Shinji (Kashima) fault near Chugoku Electric Power Co.'s Shimane nuclear power plant in Matsue; and the Koshiki fault zone near Kyushu Electric Power Co.'s Sendai nuclear power plant in Kagoshima Prefecture. Both electric power firms said they had already taken the faults into account in the earthquake-resistant designs of their nuclear plants.
There are generally said to be about 2,000 active faults in Japan that could cause an earthquake within a period of 1,000 to several tens of thousands of years. Active faults that meet such requirements as being more than 20 kilometers long and having experienced a slip of more than 10 centimeters during a 1,000-year period are designated as "major active fault zones."
The headquarters reviewed the length, activity and other factors of active faults in the three regions. This revealed that 16 faults there met the conditions for major active fault zones, such as the fact that their lengths are longer than conventional estimates.
As the probability of an earthquake is not clear at other faults in the regions, the headquarters plans to survey these faults in detail later, and intends to evaluate faults in other regions.
The headquarters conducts detailed surveys for faults to designate them as major active fault zones, having local governments reflect the survey results in their disaster prevention plans and drawing the attention of local residents to earthquakes.
When a fault's length exceeds 20 kilometres, a magnitude-7 or larger earthquake could occur, such as the 1995 Hanshin Earthquake and the 2016 Kumamoto Earthquake.
"There is no predicting precisely how large an earthquake will be and when it will take place. I'd like all residents to be very careful," said Kazuro Hirahara, a professor of seismology at Kyoto University and a section chief at the headquarters.