The moment is described in the Sandai Jitsuroku, a Japanese history book more than a thousand years old. As the earthquake shook the land, a “luminous phenomenon” filled the night sky and illuminated the coastline. Then the tsunami came. The seawater spread across the Sendai plain, flooding the old castle town and killing about a thousand people. It was named the Jōgan tsunami after the emperor at the time. The year was 869. In 2001, a team of paleontologists and engineers examined sand locked in the strata below the Sendai plain, stretching more than two miles in from the coast. There are three layers, evenly spaced — a geological clue that a major flooding event takes place here every 800 to 1,000 years. “More than 1,100 years have passed since the Jōgan tsunami,” wrote the researchers. “Given the reoccurrence interval, the possibility of a large tsunami striking the Sendai plain is high.”