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[SCG55-P02] Vertical crustal deformation of Yonaguni Island in recent decades estimated from microatolls
Keywords:crustal movement, Ryukyu, microatoll, Yonaguni Island
Last interglacial marine terraces are distributed at 18 to 30 m elevations and uplifted at 0.1 to 0.2 mm/yr in Yonaguni Island (Omura et al., 1994). The elevations of beach rocks in the northern and western parts of the island, which were formed approximately 1500-500 BP, are also mid-tidal to high-tidal zone (Omoto, 2003). Applying the uplift rates estimated from the terraces to the age of formation of the beach rocks, the amount of uplift since 500 and 1500 BP is 0.05 to 0.3 m. This is consistent with the elevation of the beach rock. On the other hand, from the GSI GNSS continuous observations, Yonaguni Island has been uplifted at an average rate of 7.5 mm/yr since 1998, when GNSS observations began. To resolve the discrepancy between the geomorphological and geodetic uplift rates of Yonaguni, we examined the vertical movement of the island over recent decades using coral micro-atolls.
The study was conducted at three sites in the northern, southern, and western parts of Yonaguni Island. The shape of the present micro-atolls was measured. The micro-atolls at Naama Beach in the western part and the present-day micro-atolls at Katabaru Beach in the southern part both had a Hat-shaped shape with a nearly flat top but with a gradual decrease in elevation at the periphery. The western micro-atolls had margins ranging from 20 to 30 cm wide with a gradual decrease in elevation of approximately 10 cm toward the outside. The elevation of the westernmost margin is -1.06 m (TP). An ring-shaped raised structure is observed on the flat top of the larger micro-atolls. The southern micro-atoll is 30 cm wide with a gradual decrease in elevation toward the outside of the micro-atoll of about 12 cm. The elevation of the most marginal edge is -1.11 m (TP).
The shapes of these micro-atolls indicate that a gradual lowering of sea level has recently occurred in the Yonaguni Island. Micro-atolls in the Sakishima Islands, other than Yonaguni Island, show no evidence of a 10 cm sea-level drop in the last 20 years (Debaecker et al., 2023). Therefore, if we attribute this change entirely to tectonic activity, Yonaguni Island would have been recently uplifted by approximately 10 cm. Since micro-atolls were not sampled and only their shapes were investigated, the growth history is still being determined. Therefore, we used the average growth rate (0.77 cm/yr) of micro-atolls in the Sakishima Islands (Debaecker et al., 2023) to estimate the onset of crustal movement based on the location of the change in top elevation. This would indicate that the sea level lowered 15 to 23 years ago. This is close to the afterslip's start, just below Yonaguni Island (Nakamura, 2009). The afterslip began around March 2002 and continued for approximately five years. According to the GSI GNSS record at Kubura, the uplift due to the afterslip was about 8 cm, which is almost the same value as the uplift estimated by micro-atolls. Therefore, this uplift is due to the afterslip that started in 2002.
The top of the micro-atoll is nearly flat except for the peripheral areas. This indicates that the uplift of Yonaguni Island was temporary and that the vertical crustal movement before the afterslip would have been slight. Since the diameter of the micro-atoll is about 2 to 3 m, the shape of the apex may reflect the recent sea-level change of about 80 to 120 years. In other words, Yonaguni Island would have been little uplifted during the recent 80-120 years, except for the 2002 afterslip.
Possible causes of vertical crustal deformation on Yonaguni Island include normal fault activity on and around the island and viscoelastic effects from the rifting of the Okinawa Trough (Iwasa & Heki, 2018). However, slow earthquakes caused by subduction would also contribute to this crustal movements.
The study was conducted at three sites in the northern, southern, and western parts of Yonaguni Island. The shape of the present micro-atolls was measured. The micro-atolls at Naama Beach in the western part and the present-day micro-atolls at Katabaru Beach in the southern part both had a Hat-shaped shape with a nearly flat top but with a gradual decrease in elevation at the periphery. The western micro-atolls had margins ranging from 20 to 30 cm wide with a gradual decrease in elevation of approximately 10 cm toward the outside. The elevation of the westernmost margin is -1.06 m (TP). An ring-shaped raised structure is observed on the flat top of the larger micro-atolls. The southern micro-atoll is 30 cm wide with a gradual decrease in elevation toward the outside of the micro-atoll of about 12 cm. The elevation of the most marginal edge is -1.11 m (TP).
The shapes of these micro-atolls indicate that a gradual lowering of sea level has recently occurred in the Yonaguni Island. Micro-atolls in the Sakishima Islands, other than Yonaguni Island, show no evidence of a 10 cm sea-level drop in the last 20 years (Debaecker et al., 2023). Therefore, if we attribute this change entirely to tectonic activity, Yonaguni Island would have been recently uplifted by approximately 10 cm. Since micro-atolls were not sampled and only their shapes were investigated, the growth history is still being determined. Therefore, we used the average growth rate (0.77 cm/yr) of micro-atolls in the Sakishima Islands (Debaecker et al., 2023) to estimate the onset of crustal movement based on the location of the change in top elevation. This would indicate that the sea level lowered 15 to 23 years ago. This is close to the afterslip's start, just below Yonaguni Island (Nakamura, 2009). The afterslip began around March 2002 and continued for approximately five years. According to the GSI GNSS record at Kubura, the uplift due to the afterslip was about 8 cm, which is almost the same value as the uplift estimated by micro-atolls. Therefore, this uplift is due to the afterslip that started in 2002.
The top of the micro-atoll is nearly flat except for the peripheral areas. This indicates that the uplift of Yonaguni Island was temporary and that the vertical crustal movement before the afterslip would have been slight. Since the diameter of the micro-atoll is about 2 to 3 m, the shape of the apex may reflect the recent sea-level change of about 80 to 120 years. In other words, Yonaguni Island would have been little uplifted during the recent 80-120 years, except for the 2002 afterslip.
Possible causes of vertical crustal deformation on Yonaguni Island include normal fault activity on and around the island and viscoelastic effects from the rifting of the Okinawa Trough (Iwasa & Heki, 2018). However, slow earthquakes caused by subduction would also contribute to this crustal movements.