As one of the major historic Japanese tsunami earthquake, there are still aspects unclear. So far the 1611 Keicho earthquake has been located to the northern end of the Japan Trench affecting especially the coasts of Eastern Honshu. However, following the findings of Nanayama (2003) and Sawai (2008), there was an unusually large tsunami earthquake offshore Hokkaido and Kuril Islands. Based on sedimentary records this event was dated to the early 17th century. Assuming that this tsunami was much larger than any instrumentally recorded earthquake today, a magnitude of at least 8.5 was assumed.