Time evolution of vertical displacement during Holocene: case of kikai Island, Ryukyu Islands, Japan

KiKai island is a part of the Ryukyu Islands of southwest Japan. It is raised coral reef, where is the closest island to the Ryukyu Trench, and the marine terraces in this island record the fastest uplift rate in Japan. Uplift rate is 1.8 mm/yr since the last interglacial period. In Holocene, four marine terraces in Kikai island were generated by coseismic uplift by geological studies (i.e., Ota et. (2000)). On the other hand, in Kikai Island, GPS data of GEONET are observed steady uplift of several mm/yr, which is interpreted as the collision of the Amami Plateau, subducting northwestward beneath the Ryukyu Islands at Kikai Island, though GPS data cannot be simply extended to the past. So we try to understand history of vertical displacement during Holocene. Here, we assume that the marine terraces are mostly caused by steady uplift and small stepwise uplift. In this study, we focus on shape of plate interface, which is considering the collision of the Amami Plateau. So we realized the collision of the Amami Plateau and marine terraces using finite element method. We make subduction zone model considering bump on the plate interface due to the subducting Amami platau. We simulate history of vertical displacement in Holocene and marine terraces in Kikai island. Estimated time evolution of vertical displacement in Holocene do not constant rate and are large change in time. Viscoelastic response of upper mantle due to earthquake cycle is large effect for vertical displacement. Although, vertical displacement at Kiki Island was subsidence at just after earthquake, vertical displacement at Kikai Island is uplift at several hundred years after earthquake. The history of vertical deformation is not constant, it's very unstable through holocene. And also, we try to estimate viscosity in upper mantle by fitting marine terrace at Kikai Island. We obtain that viscosity of upper mantle is 10^20 Pas. As result, we can extract rheology information of upper mantle from marine terrace.