Regional differences of the crustal movements and the duration of the more active tectonics during the Holocene time in the Ryukyu Islands

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Ryukyu Islands are an island arc, situated between Kyushu and Taiwan. The Philippine Sea plate has subducted under the Eurasia plate at the Ryukyu Trench, and the Okinawa Trough has rifted in the Quaternary period ; the Ryukyu Islands have been stressed by the subduction of the Philippine Sea plate and the expansion of the Okinawa Trough. Consequently, the Ryukyu Islands have demonstrated the active island arc in the Holocene time. On the other hand, the Ryukyu Islands are surrounded by coral reefs so that the emerged Holocene reefs are dated by using of the C-14 method for unaltered aragonitic fossil corals. In addition, tsunami deposits which consist of Holocene corals can be dated by the same method. These tsunami deposits have been transported to the Ryukyu Islands probably due to submarine earthquakes. Following islands are selected to investigate in this study : Amami, Toku, Okinoerabu, Okinawa, Iheya, Izena, Tokashiki, Zamami, Aka, Kume, Tonaki, Miyako, Tarama and Yaeyama Islands. On the basis of coastal geomorphology of limestone shores and many dates of fossil corals in these islands, the following results may be summarized.1. The eustatic sea-level fall occurred 4200 and 3000 years BP.2. Holocene tsunamis generated off the South Ryukyus about 220, 500, 1000, 2000, 2400, 3800 and 4400 years BP. probably caused by submarine earthquakes in the Submarine Deep-Sea Terrace between the Ryukyu Trench and the South Ryukyus.3. The more active tectonics in the Holocene period may have occurred about 1700-2400 years BP. due to the intensive activity of the Philippine Sea plate.4. Regional differences of the Holocene tectonics in the Ryukyu Islands are due to the interaction between the subduction of the Philippine Sea plate and expansion of the Okinawa Trough.