In 1923 a magnitude 7.9 earthquake devastated the Tokyo area, resulting in more than 100,000 deaths. About 200 years earlier, in 1703, a magnitude 8.2 earthquake struck the same region, causing more than 10,000 deaths. These earthquakes, which occurred just south of the area hit by the March 2011 Tohoku earthquake, were produced by slip on the boundary between the subducting Philippine Sea plate and the overlying plate. To estimate the average recurrence time between earthquakes in this region, and thus learn more about earthquake hazard, scientists need to know when earthquakes occurred before 1703. There are few historical documents describing earlier earthquakes, though some records indicate that earthquakes occurred in 1293 and 1433.