吐喝喇列島・小宝島における完新世サンゴ礁段丘上の低崖地形

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Numerous active faults in the Ryukyu Islands have been identified. However, relatively few studies and reports on the Holocene activity of these faults have been reported. Kodakara Island, which is part of the volcanic inner arc of the central Ryukyus, is fringed by well-developed Holocene-raised coral reef terraces, Terraces I, II, III in ascending order, that were formed by co-seismic uplifts. We found two continuous low scarps across the Holocene terraces. We verified whether low scarps are tectonic in origin or not on the basis of detailed field surveys such as mapping and measurements around the scarp, affirmations from inhabitants, and previous earthquake records. The low scarps are usually associated with ground fissures. The vertical displacements of T I and T II are 1.5-1.7m and 0.7m, respectively, which indicate the accumulation of displacement. These characteristics indicate that the scarps may have formed by normal faulting during the Holocene. However, based on the presence of trees (Ficus microcarpa) with recently altered orientations beside the scarps, affirmations from inhabitants, and previous earthquake records, it seems likely that the scarps formed during seismic events that have occurred over the past three to four decades. Moreover, the timings of the scarp formations correspond to swarm earthquake activities, and their focus is not located on Kodakara Island, but near the island. This indicates the scarps have been formed as a result of swinging during recent swarm earthquake activities, not by faulting. Hence, we define the scarps as a landslide scarp associated with the characteristics of normal fault morphology. Our results imply that the area in the vicinity of Kodakara Island is currently tectonically active and that the scarps are likely to be displaced in future swarm seismic events.
2009-04-25

吐喝喇列島・小宝島における完新世サンゴ礁段丘上の低崖地形

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