Photo/IllutrationThe main keep, at center, of Matsumoto Castle needs to be reinforced for protection against a possible strong quake, as do the “Inui Kotenshu” on the right and the “Tsukimi Yagura” at left foreground in Matsumoto, Nagano Prefecture, on May 18. (Wataru Miura)

MATSUMOTO, Nagano Prefecture--Matsumoto Castle, one of Japan's most famous and imposing fortresses, is at risk of partial collapse if a strong earthquake jolts the area.

The government-designated national treasure could be severely damaged by a quake with an intensity of upper 6 or higher on the Japanese scale, which has a maximum level of 7, a seismic diagnosis revealed.

The city's education board has decided to set up a review committee in July at the earliest to plan construction work as soon as possible to reinforce what is one of just 12 castles in Japan that have their original wooden keeps still standing. It is hoped the work will start in fiscal 2019.

The seismic check-up report warned that the beams on the third and fifth floors of “Daitenshu” (main keep) and the second floor of “Inui Kotenshu” (Inui small keep) were at risk.

The Itoigawa-Shizuoka Tectonic Line is a major fault that cuts Nagano Prefecture into two and is believed capable of producing an earthquake with an intensity of upper 6 or even higher.

The pillars holding up “Tsukimi Yagura” (moon-viewing tower) would also be in danger of collapse in a strong quake.

On the positive side, five keeps would not be significantly damaged by a jolt with an intensity of about 5, according to the seismic diagnosis results.

The new committee is expected to design the reinforcement plan as well as the evacuation guidelines for sightseers in the event of a major tremor.

“It will take some years at the longest to complete the reinforcement construction against earthquakes,” said an official of the management office of the castle.

Whether visitors should be banned from entering the keeps during construction will be discussed by the committee.

The popular landmark is one of two original wooden castles that consist of five keeps, along with Himeji Castle in Japan, and attracted 990,000 visitors in fiscal 2016 alone.

The seismic diagnosis was conducted from 2014 to March this year based on the Guidelines for Assessing Seismic Resistance of Important Cultural Properties issued by the Cultural Affairs Agency.