Marine regression induced by the 2024 earthquake extended the shorelines up to ≈ 200 m. In Minazuki Bay, shoreline extension occurred in almost all sections of the bay and was substantial, especially in the midsection (Fig. 2). The shoreline extension ranged between 0 and 197.4 m (Fig. 2b), resulting in the area of the Minazuki Bay coastal plain increasing by 0.33 km2 (Fig. 2a). Marine regression also extended the total length of the shoreline by 310.1 m. Consequently, the Minazuki and Igisu fishing ports dried up (Fig. 2).
In Yoshiura Bay, the extent of the marine regression was similar to that in Minazuki Bay. Shoreline extension occurred in most sections of the bay, with the exception of the rocky seashores at both ends of the bay. This expanded the coastal plain by 0.13 km2 (Fig. 3a). The maximum extension distance of the shoreline was 146.2 m, which occurred in the midsection of the bay (Fig. 3b). This maximum was less than that in Minazuki Bay (Fig. 2b, 3b). However, the Yoshiura fishing port also dried up due to the shoreline extension (Fig. 3), indicating that the changes in the Yoshiura Bay coastal landforms were also remarkable. The marine regression extended the total length of the shoreline by 66.7 m.
Photographs of Minazuki Bay support the changes observed in the orthophotos. A panorama photo of Minazuki Bay taken before the 2024 earthquake shows two rubble breakwaters in the center and left-hand side of the bay (Fig. 4a). After the uplift-induced marine regression, the center breakwater was located on the new shoreline and consequently integrated into the coastal plain (Fig. 4b). In panorama photos taken before and after the 2024 earthquake, the Igisu fishing port surrounded by the left-hand breakwater can be seen (Fig. 4a, b). The Igisu fishing port completely dried up due to marine regression, and became part of the coastal plain (Fig. 4c, d). The Minazuki fishing port on the opposite shore also dried up after the 2024 earthquake (Fig. 4e). It has been estimated that the uplifts in the Igisu and Minazuki fishing ports reached 4.1 and 3.3 m, respectively, based on a previous sea surface trace [19].
The uplift-induced marine regression resulted in the reshaping of the two river mouth morphologies. Before the 2024 earthquake, a sandy beach connected the Minazuki River and the shoreline (Fig. 5a). After the marine regression, the Minazuki River was forced to flow over the uplifted extended coastal plain (Fig. 5b). Importantly, the surface of the new coastal plain formed by the uplift was not completely covered with sand; it partly consisted of rocks (Fig. 5a, b). Most of the undersea rocks, captured in dark colors before the 2024 earthquake (Fig. 5a), were uplifted above sea level after the 2024 earthquake (Fig. 5b). After the marine regression, the Minazuki River flowed across this uplifted rocky area (Fig. 5a, b). In the vicinity of the lower end of the rocky area, a new sandy beach formed by the uplift prevented flow down the Minazuki River (Fig. 5b). Consequently, the Minazuki River was diverted toward its right bank (Fig. 5b-d). This uplift-induced change in the river mouth morphology extended the total length of the Minazuki River by 282.3 m (Fig. 5b), which is greater than the maximum extension distance of the shoreline (Fig. 2b).
Similarly, a sandy beach connected the Igisu River and the sea before the 2024 earthquake (Fig. 6a), but the river mouth after the marine regression was located in the rocky area (Fig. 6b, c). Thus, the total length of the Igisu River was extended by 120.6 m (Fig. 6b). After the 2024 earthquake, the Igisu River eroded its bed and banks, leading to sediment discharge, including the sandy beach (Fig. 6c).