The movement of the bedrock under the Fukushima No.1 nuclear power plant during the Great East Japan Earthquake was larger than pre-quake estimates used by the plant’s operator in its disaster planning, according to government simulations.
Tokyo Electric Power Co. (TEPCO) maintained before the March 11 catastrophe that bedrock motion would exceed its “standard seismic motion” only once in 10,000 to 1 million years and used the figure to evaluate whether its buildings and equipment would withstand a quake.
But a new government analysis, announced Dec. 9 by the Nuclear and Industrial Safety Agency (NISA), suggests that the actual shaking on March 11 was 675 gal, in excess of the “standard seismic motion” of 600 gal at a depth of 196 meters beneath the ground posited in TEPCO’s planning. A gal is a unit of acceleration.
TEPCO still denies that the March 11 quake, rather than the subsequent tsunami, caused damage to key equipment, and maintains that the shaking at the plant was within what it was designed to withstand.