Expert indicates that an active fault line in the undersea section of the Henoko Base construction zone could pose danger


October 25, 2017 Ryukyu Shimpo

By Ikue Nakaima


It was discovered on October 24 that there exists an active fault line in the undersea section of the construction zone for the Henoko Base in Nago.

In the “Map of the undersea section” figure, part of the data collected by the “Alternative Facilities Construction Committee” formed by the Ministry of Defense in 2000, there is a depression on the map in an area close to 50 meters in length.

Dr. Yuzo Kato, professor emeritus at the University of the Ryukyus (specializing in petrology), said, “If the depression was formed in a relatively recent era, there is a chance that the seafloor fault line can grow larger.”

Already, on the island portion of the area plotted out for the new base, there is the “Henoko Fault,” and the “Sukku Fault.”

The continuation of these faults extend to a steep valley and incline on the seafloor.

Furthermore, the depression disclosed by the Ministry of Defense is overlapping. The fault appears to show evidence of earthquakes in the past, meaning that as an active fault there is a good probability of earthquakes in the future, casting doubt on the suitability of the location for a military base.

October 25 is also the six-month anniversary of the start of seawall construction.


According to Ministry of Defense data, the band of seafloor that stretches from Nagashima to the Chuhi reef known as the “Ryukyu Group” contains a depression of over 50 meters.

The Ministry of Defense also writes that “the depression can be thought to come from a fault at its base,” although it gives no indication for when the fault could become active.


In response to Ryukyu Shimpo’s October 24 coverage of the existence of an active fault in the construction zone, the Okinawa Defense Bureau (ODB) commented that, “Depending on the source, an obvious active fault in the Northern Area cannot be confirmed or denied.

” With regards to the safety of the seafloor in the construction zone, the ODB responded that they, “are still investigating, and cannot definitively say at this time.”


The Ryukyu Group where this 50-meter depression was found is a band of earth that contains Ryukyu Limestone, and was formed no earlier than a few hundred thousand years prior.

Prof. Kato commented, “Since this band of earth has formed over a more recent era, there is a chance that it could see movement hereafter, which leads to the possibility of an active fault as well.”


The two land-based faults, (Henoko and Sukku) are classified as active in the document, “Nago, Yanbaru Geology,” published by the Nago City Department of Education. An active classification means that the faults have experienced movement no later than a few hundred thousand years prior, to which Prof.

Kato also says means that “These two faults as well could become active.”


From February through April the ODB conducted a geological survey over the undersea section of the construction zone on the large-scale multipurpose vessel Poseidon, although the results have not been released.


Prof. Kato also mentioned that, “In order to refute the possibility of an active fault Japan needs to release and present the supporting data immediately.

” He also noted that the undersea construction zone contained much of the soft, cavity-filled Ryukyu Limestone.

Prof. Kato noted the danger of this as well, stating, “Even if the base is carefully constructed, if the fault directly below were to experience movement, the facilities would be destroyed.”


(English translation by T&CT and Sam Grieb)


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