A number of great interplate earthquakes have occurred along the Suruga Trough and Nankai Trough, off the southwestern coasts of Japan. These earthquakes are associated with the subduction of the Philippine Sea Plate. These recurring earthquakes take place at intervals of one or two centuries. These earthquakes take place in pairs, with an example of Tōkai earthquakes taking place in eastern Japan, and an example of Nankai earthquakes taking place in western Japan. The Hakuho earthquake is the oldest recorded Nankai earthquake.[2]
In this earthquake, the sea reportedly "swallowed up" 500,000 shiro of cultivated land in Tosa Province. The Japanese unit shiro is equivalent to 15.13 acres.[4] The governor of this province reported the sinking of many ships by a "high-rising great tide" which accompanied the earthquake. He was describing a tsunami, using the then-used term "oshio".[4]
Regarding the flooding, the Nihon Shoki reports that the local people had never experienced a disaster of this type.[3]
A corresponding Tōkai earthquake may have occurred in 684, but is not clearly recorded in primary sources. The next known Nankai earthquake took place in 887.[2]
