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2D Elastic models of the seismic cycle. All models are performed in an elastic half space with λ= μ = 32 Gpa. (a) Deformation at the surface induced by motion along different portion of the plate interface in front of Yoron during the seismic cycle. Dashed grey curves and dark grey area: vertical deformation rate (mm/year) promoted by backslip along the dislocation Dint‐Yor‐1 (dashed grey line on the cross‐section of the slab beneath) during the interseismic period. Black curves and light grey areas: vertical deformations rate (mm/year) promoted by backslip along the dislocation Dint‐Yor‐2 (black line on the cross‐section of the slab beneath) during the interseismic period. Values in % indicate the coupling rate (100% full coupling, 50% half coupling). Dashed red curve: vertical deformation at the surface (m, right scale) promoted by slip of 0.8 m on the plate interface between 0 and 30‐km depth (above the coupling zone, red line on the cross‐section of the slab beneath) during an earthquake or a slow slip event. Blue curve: vertical deformation at the surface (m) induced by a slip of 2 m in the transition zone (blue line on the cross‐section of the slab beneath). White dots: values of interseismic deformation rates (mm/year) measured in Yoron at different time period (see Table 3). Pink square: average value of likely sudden subsidence motion (cm) recorded in Yoron in 1934, 1965, or 1968 and 1986 or 1987 (see main text). Slab depth from the SLAB1.0 model (see text), topography from GeomapApp database (http://www.geomapapp.org). (b) As in a but for Okinawa with a different geometry of the slab and different values of interseismic rates (see Table 3). Dashed red curves: vertical deformation at the surface (m) induced by a slip of 0.8 and 1 m in the shallower part of the plate interface between 0 and 20 km (D‐Ok‐cos, dashed line on the cross‐section of the slab beneath). White square: value of the likely sudden subsidence recorded in Itoman after 1906 until 1933. Pink square: value of the subsidence recorded in Itoman between 1933 until 1950. Green dots: value of the GPS vertical motion in Naha, Okinawa (http://www.sonel.org/, Wöppelmann & Marcos, 2016). Dashed green curves: vertical deformation during interseismic period for a coupling rate of 10% or less (see text). The black dashed curve below 30‐km depth on the cross‐section of the slab represents the dipping plate.
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This study focuses on Okinawa and Yoron islands, in order to better understand tectonics in the Ryukyu Arc related to the subduction zone. We used coral microatolls—known for their centimetric accuracy in the record of relative sea‐level (RSL) changes—to reconstruct RSL changes over the last century from living microatolls. A fossil microatoll in Y...
Citations
... The change of the RSL can be sudden, as during an earthquake for example, or progressive at timescales ranging from few years to several decades. As they grow in the intertidal zone, very close to the sea surface, microatolls are very sensitive to interannual variability of the sea-level changes due to La Nina, El Nino, The Indian Ocean Dipole or North Atlantic Oscillation events at the scale of oceanic basins (Meltzner & Woodroffe, 2015;Taylor et al., 1987;Weil-Accardo et al., 2020;Zachariasen, 1998 and references therein). The small and temporary decreases of the sea-level due to these variations are recorded, over a wide region, in tide gauges and as die-downs of climatic origin in microatolls. ...
... Coral microatolls have been used in climate studies to quantify the glacio-isostatic adjustment (Wan et al., 2020;Woodroffe et al., 2012), evaluate the impact of climatic anomalies such as El Nino climatic oscillations (Song et al., 2012;Woodroffe & McLean, 1990), or estimate the Holocene sea-level changes in the South China Sea and offshore Australia (Leonard et al., 2013;Yu et al., 2009). Coral microatolls were also used for tectonic purposes mainly to document the seismic cycle of the megathrust in several subduction zones such as Vanuatu, Indonesia, Myanmar, Lesser Antilles, Central Ryukyus, Japan and in transform zones such as in Haiti (Meltzner et al., , 2010(Meltzner et al., , 2017Natawidjaja et al., 2006;Philibosian et al., 2012;Shyu et al., 2018;Sieh et al., 2008;Taylor et al., 1980Taylor et al., , 1982Weil-Accardo et al., 2020;Weil-Accardo, Feuillet, Jacques, Deschamps, Beauducel, et al., 2016;Weil-Accardo, Feuillet, Jacques, Deschamps, Saurel, et al., 2016;Zachariasen et al., 1999Zachariasen et al., , 2000. ...
... In addition to having recorded the same large and long-lasting sea-level variations described above, the microatolls recorded other smaller and shorter-lived events that are more classic climatic die-downs (Meltzner & Woodroffe, 2015;Taylor et al., 1987;Weil-Accardo et al., 2020;Zachariasen, 1998 and references therein; Figure 9). ...
The seismic hazard related to megathrust earthquakes in the Ryukyus (southern Japan) is poorly constrained as no large earthquake has been reported there. The Meiwa tsunami impacted the coasts of the Yaeyama and Miyako islands in 1771 but its origin is still debated. Global Navigation Satellite Systems measurements indicate that strain is accumulating along the plate interface but the observation period is short. It is thus crucial to gain information on the seismic potential of the megathrust. The islands of the Ryukyu archipelago are located in a tropical region and surrounded by reefs where numerous microatolls are growing. They preserved the record of variations of the relative sea‐level in their skeleton. We mapped seven sites over five islands and subsequently selected and sampled eight slabs of modern microatolls. The corals have emerged slowly at a rate of 0.7–2.8 mm/yr due to the long‐term interseismic loading on the megathrust up to 40 km in deep. The coupling rate estimated from elastic back‐slip models ranges between 10% and 100%. We also identified multi‐decadal relative sea‐level changes of a few cm/yr, likely due to very long duration slow‐slip events (SSE) along the shallow or deep parts of the megathrust. Those SSEs occur each 10–40 years and have accommodated 50% of the convergence rate in the last 250 years. Our study provides new constraints on the seismic cycle of the Ryukyu megathrust and on the seismic hazard in this region and suggests that a large megathrust earthquake could occur in the area in the future.
... provide evidence of interseismic subsidence of the island and one instance of coseismic subsidence related to the Mw 71,946 subduction earthquake (Weil-Accardo et al., 2016). Studies of coral microatolls in the Antilles and elsewhere have also revealed discrepancies between deformation patterns recorded over different time scales, likely related to variability of the megathrust seismic behavior (e.g., Meltzner et al., 2015;Philibosian et al., 2014;Philibosian et al., 2022;Weil-Accardo et al., 2020. Overall, these observations of vertical deformation along the Lesser Antilles arc over multiple time scales imply stronger long-term interplate coupling than inferred from geodetic data. ...
In the context of increasing evidence of plate interface coupling variability in subduction zones, there is a need to extend the short time window given by instrumental data and to gather data over multiple time and spatial scales. We hence investigated the long‐term topography on Barbuda island, located in the northern part of the Lesser Antilles, west of the Caribbean subduction zone. Following pioneering work using a set of marine terraces on the eastern side of the island, we performed the first U‐Th dating on 10 corals in growth position from the lowest terrace, for which the highest relative sea‐level (RSL) indicator is found at 9 ± 1 m above the mean sea level. We find that this terrace corresponds to the Last Interglacial (LIG) (ages between 122.8 ± 0.3 ka and 128.1 ± 0.3 ka) and we estimate a paleo RSL of 7 ± 2 m above the current mean sea level. The present elevation of the LIG shoreline on Barbuda might imply tectonics as an additional mechanism to eustatic sea level, mantle dynamic topography and glacial isostatic adjustment. East‐west morphological asymmetry of Barbuda and difference in LIG shoreline elevation between Barbuda and Antigua suggest a regional tectonic process. As with the proposed westward tilting from the forearc to the volcanic arc of the Guadeloupe archipelago, vertical deformation on Barbuda could be related to plate‐scale subduction processes. Long‐term uplift of Barbuda might be related to the accumulation of residual coseismic deformation not fully recovered by interseismic subsidence and the corresponding seismogenic segment would extend below the Moho.
