5:15 PM - 7:15 PM
[SCG50-P03] Reconstructing relative sea-level changes on Kikaijima Island from coral microatolls: Implications for megathrust earthquake potential along the Ryukyu Trench
Kikaijima Island, located in the central Ryukyu Islands, is surrounded by well-developed Holocene uplifted coral reef terraces. These marine terraces are considered to have formed through coseismic uplift events of approximately 1–2 meters occurring every 1,500–2,000 years (Sugihara et al., 2003). However, recent GNSS observations indicate that the interseismic uplift rate is comparable to or exceeds the long-term average uplift rate inferred from last interglacial marine terraces (~1.8 mm/yr). Consequently, an alternative hypothesis has been proposed that the Kikaijima’s marine terraces resulted from steady uplift combined with minor sea-level fluctuations, rather than large coseismic events (Shikakura, 2014). To investigate this hypothesis, we collected slab samples from both living and fossil microatolls on Kikaijima to reconstruct the relative sea-level (RSL) history over the past 300 years.
Microatolls are colonies of massive corals, such as Porites, that grow in shallow reef environments. Since their upward growth is limited by the lowest tide level, their growth histories provide detailed annual records of minor RSL changes. On Kikaijima, microatolls are commonly found in small, sheltered coves. For this study, we extracted and dated cross-sectional slabs from a 145 cm radius living microatoll from the eastern Kadon coast and a 270 cm radius fossil microatoll from the northern Onozu coast to analyze their growth histories. Additionally, we generated 3D photogrammetric models of four living microatolls and reconstructed RSL changes non-destructively using coral growth simulations.
U-Th and radiocarbon dating results indicate that the Kadon living microatoll records RSL changes dating back to approximately CE 1900, while the Onozu fossil microatoll preserves records spanning CE 1650 to CE 1900.
A comparison between the living microatoll and recent tide gauge records from Amami Oshima Island revealed a strong correlation between microatoll diedown events and sea-level drops. The differences between these records suggest an average uplift rate of 1.5–2.0 mm/yr on Kikaijima in the last 50 years. Although moating effects might have influenced the Onozu fossil microatoll, its elevation suggests a maximum uplift rate of 1.53 mm/yr from CE 1650 to the present.
The uplift rates inferred from microatolls are comparable to the long-term average uplift rates estimated from marine terraces, strongly suggesting that the previously hypothesized cyclic uplift-subsidence pattern has not occurred. Additionally, the vertical displacement rates from GNSS observations (+4.7–4.8 mm/year during CE 2000–2020) is greater than the estimated average uplift rates from the microatolls, indicating that vertical movement rates may have undergone significant fluctuations over multi-decadal timescales.
Part of this study is included in Nankai Trough Earthquake Disaster Mitigation Research Project commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through JAMSTEC. Additionally, this research was partially supported by JSPS KAKENHI Grant Number JP19K01165.
Microatolls are colonies of massive corals, such as Porites, that grow in shallow reef environments. Since their upward growth is limited by the lowest tide level, their growth histories provide detailed annual records of minor RSL changes. On Kikaijima, microatolls are commonly found in small, sheltered coves. For this study, we extracted and dated cross-sectional slabs from a 145 cm radius living microatoll from the eastern Kadon coast and a 270 cm radius fossil microatoll from the northern Onozu coast to analyze their growth histories. Additionally, we generated 3D photogrammetric models of four living microatolls and reconstructed RSL changes non-destructively using coral growth simulations.
U-Th and radiocarbon dating results indicate that the Kadon living microatoll records RSL changes dating back to approximately CE 1900, while the Onozu fossil microatoll preserves records spanning CE 1650 to CE 1900.
A comparison between the living microatoll and recent tide gauge records from Amami Oshima Island revealed a strong correlation between microatoll diedown events and sea-level drops. The differences between these records suggest an average uplift rate of 1.5–2.0 mm/yr on Kikaijima in the last 50 years. Although moating effects might have influenced the Onozu fossil microatoll, its elevation suggests a maximum uplift rate of 1.53 mm/yr from CE 1650 to the present.
The uplift rates inferred from microatolls are comparable to the long-term average uplift rates estimated from marine terraces, strongly suggesting that the previously hypothesized cyclic uplift-subsidence pattern has not occurred. Additionally, the vertical displacement rates from GNSS observations (+4.7–4.8 mm/year during CE 2000–2020) is greater than the estimated average uplift rates from the microatolls, indicating that vertical movement rates may have undergone significant fluctuations over multi-decadal timescales.
Part of this study is included in Nankai Trough Earthquake Disaster Mitigation Research Project commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through JAMSTEC. Additionally, this research was partially supported by JSPS KAKENHI Grant Number JP19K01165.