Wide swaths of the Pacific coastline stretching from Honshu to Shikoku may be hit by tsunami over 20 meters high if a newly feared megaquake occurs in the Nankai Trough, a Cabinet Office panel warned Saturday.
The new warning comes after the panel revised its 2003 estimate to reflect new findings from the March 2011 earthquake and tsunami that devastated the Tohoku region's coastline last year.
The 2003 report said no areas would see tsunami higher than 20 meters. The updated report is based on the assumption that the earthquake will have a magnitude of 9.0.
The tidal waves generated by the Nankai Trough temblor would slam areas from Kanto to Kyushu, with waves of up to 34.4 meters likely in Kuroshio, Kochi Prefecture, and between 10 and 20 meters in parts of Shizuoka, Kochi and Miyazaki prefectures.
Urban areas of Tokyo would see tsunami up to 2.3 meters high, but the village of Niijima in the Izu Island chain, which is administered by Tokyo, could face deadly waves up to 29.7 meters high, the panel said.
Strong tremors would continue for around 3 minutes, and some areas in Shizuoka and Wakayama prefectures could be hit by tsunami even as they continued shaking, it said.
The panel will continue to study the potential extent of the coastal inundation while the government re-examines its emergency measures based on the new estimates.
Meanwhile, a research team under the Education, Culture, Sports, Science and Technology Ministry said Friday that a massive inland quake threatening urban areas of Tokyo and parts of Kanagawa Prefecture will top the Japanese seismic intensity scale at 7.
The quake, which will originate directly below the northern part of Tokyo Bay, will register as an upper 6 in surrounding areas, including Chiba, Kanagawa and Saitama prefectures, the research team said.
Professor Kazuki Koketsu of the University of Tokyo's Earthquake Research Institute, which heads the ministry team, urged Tokyoites to prepare for the disaster.
"There is not much difference between the intensity scales of upper 6 and 7," he warned.
An earthquake in the upper 6 range can trigger landslides, crack apart the ground and cause weak wooden structures to collapse, while sturdier wooden houses and buildings made of reinforced concrete are at risk of collapsing from quakes with an intensity of 7, according to the Meteorological Agency.