5:15 PM - 6:45 PM
[HCG23-P02] Study on paleoseismic history using turbidites - Example at the base of the continental slope off Cape Muroto
Keywords:turbidite, Nankai Trough, landslide, Nankai megathrust earthquakes
Paleoseismic studies using turbidite require comparison of samples from multiple sites to evaluate whether the sediments are associated with earthquakes and to estimate the location of the epicenter. In the case of the Nankai Trough, isolated sedimentary basins on the landward slope of the subduction zone, which are not connected to submarine canyons, are suitable for the study because flooding effects do not need to be considered (Ikehara, 2001, Jour. Geography). However, because these basins are less undulating, submarine cables are often laid, and the number of sites where core sampling is restricted has increased intensely in recent years. Therefore, it is necessary to include the sites where sediments are supplied from the land in the study of the seismic history of the Nankai Trough. On the other hand, the frequency of turbidite layers in the basins far from the continental slope is very small after the last glacial period (Omura et al., 2012, Sediment. Geol.), and it is not possible to study seismic events using turbidite. Therefore, it is necessary to study turbidite at the site close to the continental slope where the influence of sediment supply from rivers is small. This presentation reports a paleoseismic study using turbidites from three nearby sites at the continental shelf base offshore east of Cape Muroto.
The continental shelf is continuous from ENE to WSW off east of Cape Muroto. There is no large river along the coast, and the only None submarine canyon is located in the vicinity of Cape Muroto. In this study, we used core samples (PC11 from Hakuho Maru KH-17-2 cruise and PC02 and PC03 from Shinsei Maru KS-22-3 cruise) from three sites located about 30 km northeast of Cape Muroto. The PC02 sampling point is located about 4 km offshore from PC11 at the base of the slope, and was planned to capture depositional changes with distance from the continental slope. The analysis was performed by X-ray CT scanning and elemental concentration analysis using an X-ray fluorescence core logger. Radiocarbon dates of planktonic foraminifera and total organic carbon were obtained using a single-stage accelerator mass spectrometer at AORI, UTokyo. All core samples were terminated in volcanic ash layers, and K-Ah tephra were identified based on the morphology and refractive index of volcanic glass, although there was minor contamination on PC02 and PC03 during deposition (by Kyoto Fission Truck Co., Ltd.).
The identification of the turbidite layers was based on X-ray CT images and elemental concentrations of X-ray fluorescence core loggers, since they are not easily identified by the naked eye due to the fine grained muddy turbidites. The number of turbidite layers was lowest in PC02, which is farther from the continental slope, possibly due to less turbidity current reaching the site or oversight of fine-grained turbidite. Correlation of turbidites between cores was generally possible based on X-ray CT values and magnetic susceptibility, and the age-depth model using radiocarbon dates allowed almost all turbidite layers in PC02 and PC03 to be correlated with PC11. PC11 is the fastest after about 2,000 years ago, and PC02, which is farther from the base of the shelf slope, is faster than PC11 before about 4,000 years ago. The channel from the None submarine canyon, now located 2 km south of PC02, may have once been in close proximity to the channel or may have had a greater sediment supply than at present. However, the number of turbidites in PC02 is lower than in PC11 in all stratigraphic levels, suggesting that the frequency of depositional events through the submarine canyon has not changed.
The Tosa-Bae Basin, 55 km south of this site, is not connected to landward slopes or submarine canyon and is not affected by flooding. Therefore, turbidites can be interpreted as seismogenic, and a recurrence interval of about 215 years has been reported, which is consistent with the interval of plate boundary earthquakes in the Nankai Trough (Iwai et al., 2004, Memoirs of Geol. Soc. Japan). The depositional interval of the turbidite layer at site PC11 in this study is estimated to be about 197 years, which is not significantly different from the value for the Tosa-Bae. Although the study site is located at the base of a continental slope and the influence of flooding cannot be excluded, we believe that it can provide useful information for estimating seismic events.
This study was conducted as a part of the commissioned research for the disaster prevention measures for nuclear facilities (study on the evaluation method of paleoseismic history in the ocean). X-ray CT scanning and X-ray core logger analysis were conducted by the International Marine Core Laboratory, Kochi University, Japan.
The continental shelf is continuous from ENE to WSW off east of Cape Muroto. There is no large river along the coast, and the only None submarine canyon is located in the vicinity of Cape Muroto. In this study, we used core samples (PC11 from Hakuho Maru KH-17-2 cruise and PC02 and PC03 from Shinsei Maru KS-22-3 cruise) from three sites located about 30 km northeast of Cape Muroto. The PC02 sampling point is located about 4 km offshore from PC11 at the base of the slope, and was planned to capture depositional changes with distance from the continental slope. The analysis was performed by X-ray CT scanning and elemental concentration analysis using an X-ray fluorescence core logger. Radiocarbon dates of planktonic foraminifera and total organic carbon were obtained using a single-stage accelerator mass spectrometer at AORI, UTokyo. All core samples were terminated in volcanic ash layers, and K-Ah tephra were identified based on the morphology and refractive index of volcanic glass, although there was minor contamination on PC02 and PC03 during deposition (by Kyoto Fission Truck Co., Ltd.).
The identification of the turbidite layers was based on X-ray CT images and elemental concentrations of X-ray fluorescence core loggers, since they are not easily identified by the naked eye due to the fine grained muddy turbidites. The number of turbidite layers was lowest in PC02, which is farther from the continental slope, possibly due to less turbidity current reaching the site or oversight of fine-grained turbidite. Correlation of turbidites between cores was generally possible based on X-ray CT values and magnetic susceptibility, and the age-depth model using radiocarbon dates allowed almost all turbidite layers in PC02 and PC03 to be correlated with PC11. PC11 is the fastest after about 2,000 years ago, and PC02, which is farther from the base of the shelf slope, is faster than PC11 before about 4,000 years ago. The channel from the None submarine canyon, now located 2 km south of PC02, may have once been in close proximity to the channel or may have had a greater sediment supply than at present. However, the number of turbidites in PC02 is lower than in PC11 in all stratigraphic levels, suggesting that the frequency of depositional events through the submarine canyon has not changed.
The Tosa-Bae Basin, 55 km south of this site, is not connected to landward slopes or submarine canyon and is not affected by flooding. Therefore, turbidites can be interpreted as seismogenic, and a recurrence interval of about 215 years has been reported, which is consistent with the interval of plate boundary earthquakes in the Nankai Trough (Iwai et al., 2004, Memoirs of Geol. Soc. Japan). The depositional interval of the turbidite layer at site PC11 in this study is estimated to be about 197 years, which is not significantly different from the value for the Tosa-Bae. Although the study site is located at the base of a continental slope and the influence of flooding cannot be excluded, we believe that it can provide useful information for estimating seismic events.
This study was conducted as a part of the commissioned research for the disaster prevention measures for nuclear facilities (study on the evaluation method of paleoseismic history in the ocean). X-ray CT scanning and X-ray core logger analysis were conducted by the International Marine Core Laboratory, Kochi University, Japan.