Fault model of the 1804 Kisakata earthquake (Akita, Japan)

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of the uplift distribution based on microtopographic remnants of the former shoreline of Kisakata lagoon and used historical documents to re-evaluate tsunami trace heights.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.