Historical documents indicate that the AD 1611 Keicho earthquake tsunami struck the Pacific coast of Tohoku, northern Japan. Meanwhile, geological records suggest that a large earthquake occurred along the Kuril Trench in the same century. Although historians and geologists have investigated the events intensively, understanding of those events remains insufficient because of limited evidence. As described herein, we suggest future research directions for the huge seventeenth-century tsunamis based on a comprehensive review of earlier works and simulation of the directivity of tsunamis. Our results suggest the possibility that one extremely large or two or three closely spaced large earthquakes and tsunamis occurred in a short time interval along the Kuril and Japan trenches. If this is the case, we must devote attention to the risk of a future occurrence of a Kuril Trench event because the situation after the AD 2011 Tohoku-oki tsunami might be similar to the seventeenth-century events. The key research directions to clarify the seventeenth-century events are a careful review of historical documents during several decades after the AD 1611 earthquake and accurate constraint of the event age of tsunami deposits.

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The 10 July 1804 Kisakata earthquake occurred offshore Kisakata (Akita, Japan), and widespread felt shaking was reported from Matsumae (Hokkaido) to Ohmi-Hachiman (Shiga Prefecture). The earthquake caused strong ground motions that extensively damaged areas near the epicenter, such as along the coast of Kisakata, and the resultant tsunami caused extensive damage along the coast from Kisakata to Sakata. Furthermore, Kisakata lagoon was uplifted by dislocation during the earthquake, exposing the lagoon floor. Here, we performed a field survey of the uplift distribution based on microtopographic remnants of the former shoreline of Kisakata lagoon and used historical documents to re-evaluate tsunami trace heights. Using ocean-bottom reflection profiles, we estimated a fault model for the earthquake and resultant tsunami. Our model indicates that an average of 5.6 m of slip on the fault (equivalent to an Mw 7.1 earthquake) is required to explain the observed crustal deformation and tsunami height distributions, and back correction of the modeled slip reproduced well the former shoreline of Kisakata lagoon.