2025 Volume 131 Issue 1 Pages 155-164
On 10 July 1804, a strong earthquake with a magnitude of 7.3 (the 1804 Kisakata earthquake) occurred in the coastal region of the Japan Sea, northeast Japan. The earthquake occurred on a reverse fault that strikes parallel to the coastline (i.e., N–S-trending) and caused significant crustal deformation and damaging tsunamis. Kisakata lagoon was uplifted during the earthquake, resulting in the exposure of the bottom of the lagoon. Tsunami inundation heights were estimated along the coast between Noshiro (Akita Prefecture) and Sanze (Yamagata Prefecture), with the maximum inundation height at Kisakata being 4–5 m. We identified tsunami deposits and reconstructed the paleoenvironment of Kisakata lagoon based on sedimentary facies and fossils. Cores (1–2 m long) were collected at 11 locations along an 800-m-long transect, nearly perpendicular to the coastline, using a Handy–Geoslicer and hand auger. We determined the thickness, grain size, and other features of the tsunami deposits. Samples for grain size analysis were collected at 1 cm intervals from the uppermost tsunami deposit layer through to the sediment underlying the tsunami deposits at each drilling site. We identified a sandy event deposit within lagoonal muds, which is well sorted and has a sharp basal contact. The thickness of the sandy event deposit decreases with distance from the coast, and the layer disappears at ~1400 m from the coastline. The sediment thickness decreases and the grain size becomes finer towards inland areas, indicative of typical tsunami deposits. Diatom analysis suggests that the lagoonal environment was affected by uplift associated with faulting, as evidenced by the significant changes in diatom species in the sediments above the tsunami deposits compared with those below.
