The magnitude earthquake 9.0 in Japan in March shifted the ocean floor sideways by more than 20 metres, an instrument on the seabed shows.
The figure was recorded by the Japan Coast Guard. The BBC reports it exceeds the displacement suggested by some models built only from data gathered on land.
An upwards movement of 3 metres was registered by the same instrument.
The data underlines the colossal nature of the magnitude 9.0 quake which triggered a tsunami that devastated parts of northeast Japan.
''The scale is almost double that estimated only from the terrestrial data,'' Dr Mariko Sato told BBC News.
The quake occurred close to the Japan Trench, a tectonic plate boundary where the dense rock of the Pacific Ocean floor is being pulled down beneath Japan as it moves westwards towards Eurasia.
The Coast Guard had positional equipment in five locations on the seabed.
The quake occurred well out to sea, 130km from the city of Sendai in the Miyagi Prefecture, at a depth of 32km.
According to the latest police figures, almost 15,000 people are now known to have died in the disaster. A further 11,000 are still missing.