Seismic subsidence　near　the　source　region　of　the　1662　Kanbun　Hyuganada　Sea　earthquake;　Geochemical,　stratigraphical, chronological, and paleontological evidences in Miyazaki Plain, southwest Japan

Masakazu Niwa, Takanobu Kamataki, Hideki Kurosawa, Yoko Saito-Kokubu, and Masafumi Ikuta

 To identify the subsidence induced by an offshore earthquake, chemical compositions, fossil assemblages, and radiocarbon ages of organic materials were analyzed for Holocene sediments in the Miyazaki Plane, southwest Japan. The lower sedimentary facies (SF1) composed of sand- and pumice-rich sediments and the upper sedimentary facies (SF2) composed of silt-rich sediments were identified by stratigraphical description. Sharp facies changes and sediment disturbances are observed in the boundary between SF1 and SF2. Diatoms of brackish and marine water origins, foraminifers, ostracods, and mollusks are common in SF2, while fossils are rare, and most of them are diatoms of freshwater origins in SF1. The stratigraphical and paleontological evidences suggest an environmental change from the lower terrestrial to the upper shallow marine. The change is laterally continuous and seems to be inconsistent with Holocene uplift trends in the Miyazaki Plane. Radiocarbon ages of organic matters and the occurrence of the Sakurajima-Bunmei tephra (1471-1476 AD) show that the environmental change could be attributed to local subsidence due to the 1662 Kanbun Hyuganada Sea earthquake shown in historical records. The chemical analysis shows clear contrast between SF1 and SF2 in terms of concentrations in bulk sediments, supporting the environmental change from terrestrial to shallow marine. However, the bulk chemical composition, except total sulfur, remains rather unclear for use as marine-derived chemical markers compared to the adsorbed water, because it is mora affected by the composition of the source sediment.

Island Arc

John Wiley and Sons Australia, Ltd

10.1111/iar.12341