[M-GI25] Holocene paleoenvironment, paleoclimate, and paleohazards in the Pacific Islands

[MGI25-06] From Holocene relative sea-level changes to seismic supercycles : the case of Ishigaki in the southern Ryukyus, Japan.

*Sophie Debaecker¹、Nathalie Feuillet²、Kenji Satake³、Kohki Sowa⁴、Masaki Yamada⁵、Tetsuro Sato⁶、Atsushi Watanabe³、Ayaka Saiki³、Jean-Marie Saurel²、Mamoru Nakamura⁷、Giovanni Occhipinti²、Tsai-Luen Yu^8,9、Chuan-Chou Shen^8,9 (1.Littoral Environnement et Societes (LIENSs), UMR 7266 CNRS, La Rochelle Université, La Rochelle, France、2.Institut de Physique du Globe de Paris, Université de Paris, Paris, France、3.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan、4.Ministry of the Environment, Japan、5.Department of Geology, Faculty of Science, Shinshu University, Matsumoto, Japan、6.Department of Earth Sciences, School of Education, Waseda University, Tokyo, Japan、7.University of the Ryukyu, Okinawa, Japan、8.National Taiwan University, High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Taipei, Taiwan ROC、9.National Taiwan University, Research Center for Future Earth, Taipei, Taiwan ROC)

キーワード：Supercycle, Subduction, Microatolls, Ryukyus

Understanding the history and behavior of subduction zone earthquakes is crucial to prevent at best their possible dramatic consequences. In the Ryukyu subduction zone, recent studies revealed a weak coupling as well as aseismic events along the megathrust. However, the instrumental period is short and the current hazard assessment in this area does not consider that recurrence interval between large megathrust earthquakes can be longer. Using fossil coral microatolls, we aimed to extend our knowledge of the past behavior of the Ryukyu megathrust and thus improve its current seismic hazard estimation.
Coral microatolls grow in lagoons surrounding tropical islands with a constant growth rate and marking annual growth band in their skeleton. As their growth is limited by the water height under which they develop, they record annual variations of the relative sea-level (RSL) once they have reached the sea-surface and until their death. According to the geographical extent and size of the RSL anomalies, as well as their matches with seismic catalog, it is often possible to decipher their seismic or climatic origin.
In Nagura in western Ishigaki island, we have found an exceptional set of fossil microatolls, with ages ranging between 2 and 5 ka. They are distributed in stair-case plateaus, the older the higher, and each of them died in less than a year after a sudden and large drop of the RSL. Detailed analysis of the slice of 7 fossil corals revealed large relative emergence motions, likely from tectonic origin. Reconstruction of the Holocene RSL history of the site showed that such large RSL drops were clustered between 5 to 4 ka and 3 to 2 ka. Using elastic modeling, we showed that such events correspond to megathrust earthquakes; and some of them may be associated with traces of past tsunamis in the area. We infer that the clusters of megathrust earthquakes reveal a pattern of seismic supercycles, with a recurrence interval of at least 1 ka. The last cluster of megathrust earthquakes ended at 2 ka, which implies a strong seismic hazard in the area for the next centuries.