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Japan’s government is heeding a key message from last year’s Tohoku earthquake and tsunami: the underwater faults that encircle the country can unleash much greater devastation than previously anticipated. Last week, the cabinet’s disaster-management division briefed local officials on simulations that raise the spectre of waves even larger and more destructive than those last March, sending the officials scrambling to rethink their tsunami defence plans.

The estimates come from a government-appointed team of scientists led by Katsuyuki Abe, a tsunami expert and emeritus professor of the University of Tokyo. The team updated the nation’s main tsunami model by increasing the magnitude of the largest expected earthquake to incorporate recent massive quakes, including the magnitude-9.0 Tohoku and the magnitude 8.8 that hit Chile in 2010, causing a tsunami that reached as far as Japan. The result: for most locations, the size of future tsunamis could far outstrip previous estimates, which were made in 2003. The town of Kuroshio in Kōchi prefecture was steeled for a maximum 14.1-metre wave; it now faces the threat of a 34.4-metre inundation. Likewise, the offshore island of Niijima has to prepare for a 29.7-metre wave, rather than 5.4 metres (see ‘Making waves’).

Ninety cities and towns must consider how to withstand a tsunami wave of higher than 10 metres, whereas just ten locales were expecting this from the older simulations. Twenty-three have been told to prepare for a tsunami of 20 metres or more, a threat none had previously anticipated. The team also raised the estimates of the risks posed by the largest earthquakes, with the number of towns and cities expecting the maximum level of ground shaking — 7 on Japan’s intensity scale — raised from 35 to 153.

The reappraisal focuses on the Nankai trough, an offshore fault south of Honshu, Japan’s main island, that regularly produces large quakes, including the magnitude-8.7 Hoei quake of 1707 that set the ceiling for the 2003 estimates. The latest simulations use the Tohoku quake, which was roughly three times more powerful than Hoei, as the maximum. They also take account of recent studies that used the thickness of sediment layers deposited by past tsunamis to estimate the size and frequency of major earthquakes and tsunamis that happened before accurate measurement methods existed. The 2003 models, which predicted much smaller tsunamis than those that followed the Tohoku earthquake, have been criticized for not including such sedimentation data (see Nature 483,141–143; 2012).

The updated model also considers ‘large slip’ areas, in which extensive crust movement can make parts of tsunamis particularly hefty. Such areas were to blame for the unexpectedly high waves in parts of Japan last year. The team considered how coastal regions would be affected in 11 tsunami scenarios, each of which had large slip areas in different places along the Nankai trough.

Magnitude-9 events are expected to occur very rarely, perhaps once every millennium, says Kenji Satake, a tsunami expert at the University of Tokyo and a member of the team that updated the models. How much preparation is needed for such rare, devastating events remains a matter of debate, but the models have put local governments in a tight spot. Yukihiko Nakamura, head of earthquake preparedness in the Kōchi prefecture, says that the briefing was not detailed enough to give administrators a clear path forward.

“We need to know how high the water will come at different points. Will it be 34 metres everywhere?” The government has promised a fine-grained simulation and inundation maps that detail water levels by the end of May.

Even that information may not help officials to determine how to prepare for such devastating events. Some towns are installing additional loudspeakers to warn citizens of an approaching tsunami, but they might give no more than a few minutes’ warning. Nakamura says it is unreasonable to think that people could seek shelter — by climbing evacuation towers tens of metres tall, for example — within that time.

Officials in some regions are also making plans to move city offices to higher ground, and considering measures such as building large underground shelters or relocating large numbers of homes — both of which are costly options that are beyond the capacity of local or prefectural governments. A cabinet working group is discussing how the central government could help.

Before last year’s devastating earthquake, “we had been preparing for a magnitude-8 quake they said would have 60% chance of coming in the next 30 years”, says Nakamura. “But after Tohoku, we have to be prepared for the unexpected, too.”