5:15 PM - 6:30 PM
[MIS15-P07] Distributions and occurrences of sandy tsunami deposits in caves located along the East Coast of Ishigaki Island, Okinawa Prefecture
Keywords:Ishigaki Island, Tsunami deposit, Cave, Archaeological site, speleothem
Many "tsunami-ishi" boulders are distributed along the east coast and coastal lowlands of Ishigaki Island, Yaeyama Islands, Okinawa Prefecture. Detailed investigations have revealed that the boulders were moved and settled by several tsunami events of different ages. The trench excavated near the coast also revealed several sandy tsunami deposits containing marine fossils. These boulders and deposits suggest that tsunamis originating from the Ryukyu Trench repeatedly struck the east coast of Ishigaki Island.
Along the coasts of the Yaeyama Islands, many limestone caves are located in marine terraces composed of limestone of the Ryukyu Group. These limestone caves are isolated from weathering of the surface environment and maintain a stable environment, which results in good preservation of sediments and fossils. As a result, these caves often include artifact remains and human bones and are sometimes recognized as archaeological sites. The speleothems found in the caves may also contain records of events such as tsunamis. In the cave deposits of the Shirahosaonetabaru Cave Site, a historic site located within the New Ishigaki Airport, a few tens of centimeters thick sandy gravel bed containing many corals and other marine fossils is presumed to be tsunami-derived deposits. This sandy graved bed was deposited about 1,800 years ago, and the cave entrance that in-flowed this sediment is estimated to have existed at an elevation of about 40 m. Therefore, it is considered evidence of a large-scale tsunami, comparable to the "Okinawa Sakishima Tsunami".
We investigate the cave sediments along the east coast of Ishigaki Island for artifacts and fossils, and tsunami event deposits in the cave sediments. In this presentation, we will show the distribution of the tsunami deposits in the caves. This work was supported by JSPS KAKENHI Grant Number JP20K04138.
Along the coasts of the Yaeyama Islands, many limestone caves are located in marine terraces composed of limestone of the Ryukyu Group. These limestone caves are isolated from weathering of the surface environment and maintain a stable environment, which results in good preservation of sediments and fossils. As a result, these caves often include artifact remains and human bones and are sometimes recognized as archaeological sites. The speleothems found in the caves may also contain records of events such as tsunamis. In the cave deposits of the Shirahosaonetabaru Cave Site, a historic site located within the New Ishigaki Airport, a few tens of centimeters thick sandy gravel bed containing many corals and other marine fossils is presumed to be tsunami-derived deposits. This sandy graved bed was deposited about 1,800 years ago, and the cave entrance that in-flowed this sediment is estimated to have existed at an elevation of about 40 m. Therefore, it is considered evidence of a large-scale tsunami, comparable to the "Okinawa Sakishima Tsunami".
We investigate the cave sediments along the east coast of Ishigaki Island for artifacts and fossils, and tsunami event deposits in the cave sediments. In this presentation, we will show the distribution of the tsunami deposits in the caves. This work was supported by JSPS KAKENHI Grant Number JP20K04138.