In an earlier report TEPCO had estimated that the tsunami that hit Fukushima Daiichi was at least seven meters high. Later the company increased its estimate to ten meters at the Daiichi plant and 12 meters at Daini.
TEPCO now believes that a 14-meter tsunami hit both plants. The Daiichi plant was required to be able to withstand a tsunami of up to 5.7 meters. At Daini, ten kilometers along the coast, the design basis (the maximum the plant was designed to withstand) was 5.2 meters.
Ground accelerations from earthquake
WNN reports that TEPCO has released more detailed data of the ground acceleration rates caused by the magnitude 9.0 earthquake. They indicate tremors that exceed the design basis (the maximum acceleration the buildings were designed tio withstand) in one dimension. (Note 1 gal = 0.01 m/s², 981 gal = acceleration due to the Earth's gravity (g))
At the Daini plant, ground accelerations ranged from 186 gal in the vertical plane at Unit 1 to 277 gal from north to south at Unit 3, as recorded by sensors in the reactor building foundation. The range of design basis figures ranges from 415 gal to 512 gal.
At Daiichi the maximum acceleration for Unit 3 recorded was 507 gal from east to west, which exceeds the design basis of 441 gal. Other readings were below design basis, although east-west readings at Unit 6 of 431 gal approached the design basis of 448 gal.