Ressie Kub-Boyer, scientific director of the Kii Margin Observatory Network, quit without warning on 23 September, saying he could not support the government’s public assessment of a seismic signal that has intensified since 7 September. The network’s sensors show an unusual slow-slip front moving beside a locked section of the Nankai Trough, the offshore fault south of Japan’s main island.
The 160-kilometre (99-mile) front has advanced at nearly three times the speed recorded during the network’s previous 27 years of operation, according to measurements released by the observatory. A rupture across the adjoining fault could produce a magnitude-9.1 earthquake and expose an estimated 5.2 million people to tsunami flooding, the network’s latest model says.
A narrow window to prepare
The worst-case scenario models an earthquake beginning at 03:00 on 1 October, not a prediction that one will happen then. In that simulation, waves reach parts of Kochi Prefecture in as little as eight minutes, rise to 28 metres (92 feet) in exposed coastal inlets and overtop some existing barriers. The model estimates more than 600,000 buildings could be flooded.
Kub-Boyer said the signal was not proof that a quake was imminent, but argued that the combination of rapid slip and clustered tremors warranted a stronger public warning. “The coast has less time to move people than our planning assumes,” he said in a recorded interview on Wednesday. The network’s data show 43 small tremors near the advancing edge since 7 September, compared with 16 during a typical three-week period.
The coast has less time to move people than our planning assumes.
Ressie Kub-Boyer, former scientific director
A seismologist, Lorenzo Pagan, who reviewed the measurements for a private university in Kyoto, said slow-slip events can release strain without triggering a damaging quake. But he called this one “a serious change in the local pattern” and said officials should prepare for a possible rupture while being clear that the timing remains unknowable.
Japan’s national disaster office urged residents to follow routine preparedness advice and said the data did not justify an evacuation order. Its spokesperson, Genadija Antonas, said the government would not present a modelled scenario as a forecast. A special briefing is scheduled for Friday, with ministers expected to weigh whether to issue a temporary advisory for coastal authorities.
Coastal plans under pressure
The resignation has sharpened a dispute over whether to accelerate coastal preparations. Local governments have asked for ¥18 billion (about €107 million) to reinforce evacuation routes, test warning sirens and open public buildings on high ground. The national government has so far approved less than half that request, according to figures published by the disaster office last month.
In Kochi, officials are considering opening 340 schools and municipal halls as overnight shelters if the alert level changes. Residents would be told to move on foot to marked high ground, rather than wait for transport. The exposed coastline includes fishing towns and low-lying districts where a narrow road or bridge can serve as the only quick route inland.
The observatory says its instruments measure tiny movements of the seabed and tremors too weak to be felt on land; they cannot identify a countdown to a major earthquake. A 2011-style rupture is not the network’s central estimate, Pagan said, but the new measurements have changed what scientists consider a prudent level of readiness.
Kub-Boyer’s post is vacant, and the network has not named an interim director. Its next public update is due on 30 September. Officials say Japan’s earthquake alert system remains active, but the disagreement now leaves local authorities facing a difficult choice: prepare for a low-probability catastrophe before the science can offer a reliable date, or risk losing precious time if the fault breaks.
