There's a god for that
Geologically Japan is split across two tectonic plates: the islands of Kyushu, Shikoku and southwestern Honshu are part of the Amurian Plate; the islands of Hokkaido and northwestern Honshu are part of the odd peninsula-like extension of the North American Plate (which is the same piece of mantle that all of Central America, United States, Canada and eastern Russia ride on). As the two are slowly turning and slipping past each other, East meets West. But these two plates have only supporting roles in the story; the major drama of tsunamis is triggered by the nearby off-shore presence of two other plates: the great Pacific Plate, which is diving under the North American Plate; and the much smaller Philippine Sea Plate, which is diving under the Amurian Plate. The northern pair of these is what has triggered the recent cataclysmic earthquake and tsunami.
The first record of this phenomenon dates to July 9th of the year 869, along the Tōhoku coast at the village of Tagajō, near Sendai. Archaeological work here has unearthed 8th and 9th century buildings buried beneath the sand and mud. In the Nihon Sandai Jitsuroku (“Annals of Japan’s Three Emperors”) a work completed in the year 901, a story is told: “Some people were trapped under houses and some fell into the open earth and even part of the castle fell down . . . water flowed up the river reaching all the way to the castle . . . the roads and land were submerged, so people could not escape to the mountains, thus 1000 people drowned . . . after that day the farmland disappeared.”