5:15 PM - 6:45 PM
[MGI31-P09] How and what can we reveal seismic history from seafloor sediments?: Case study in the western Nankai Trough
Keywords:Nankai Earthquake, western Nankai Trough, turbidite, submarine active fault
How great earthquakes along the subduction zone recorded in sediments and what can we decipher seismic history from seafloor sediments? A total of 67 cores obtained during various cruises (KR9705, KR9911, KY12-11, KY13-08, KY13-17, KR14-E05, KY14-11) from the western Nankai Trough were used for this study. We have performed X-ray CT and MSCL (magnetic susceptibility and density) analysis for those cores and made smear slide observation to detect volcanic ashes.
Our data and observation suggest; 1) Sz-3 volcanic ash (1471 Sakurajima Eruption) and Kikai Akahoya volcanic ash (K-Ah; about 7300 years ago) were useful for correlation of cores in the study area and able to estimate accumulation rate of hemipelagic mud for those cored sites, 2) A turbidite bed was detected within Sz-3 volcanic ash in the Nankai Trough off Muroto Peninsula, which suggests Nankai Earthquake occurred a little bit earlier than the 1498 Meio Tonankai Earthquake, 3) Turbidites frequency in cores from the Muroto Trough were more larger than the historical earthquake recurrence interval, that suggest turebidies in this area provided not only earthquakes but also provided by costal current, in the other hand, 4) it was very rare or no turbidites in cores from the slope basins southward of the outer ridge, excepting one site which located near an outer thrust, 5) Tosabase Basin and sevelal other basin within the outer ridge, co-seismic turbidites corresponding to historical earthquakes and archaeological site surveys are well recorded, 6) Magnetic susceptibility, Ca/Ti profile, and X-rai CT helical images are useful to detect turbidites and characterize sedimentary structure, 7) a major normal fault movement, with 20-30cm vertical element, occurred in 3670 years ago or 3440 years ago; based on the correlation between Cores KY13-17 PC08 and PC09 obtained from both side of a normal/strike-slip fault.
Our data and observation suggest; 1) Sz-3 volcanic ash (1471 Sakurajima Eruption) and Kikai Akahoya volcanic ash (K-Ah; about 7300 years ago) were useful for correlation of cores in the study area and able to estimate accumulation rate of hemipelagic mud for those cored sites, 2) A turbidite bed was detected within Sz-3 volcanic ash in the Nankai Trough off Muroto Peninsula, which suggests Nankai Earthquake occurred a little bit earlier than the 1498 Meio Tonankai Earthquake, 3) Turbidites frequency in cores from the Muroto Trough were more larger than the historical earthquake recurrence interval, that suggest turebidies in this area provided not only earthquakes but also provided by costal current, in the other hand, 4) it was very rare or no turbidites in cores from the slope basins southward of the outer ridge, excepting one site which located near an outer thrust, 5) Tosabase Basin and sevelal other basin within the outer ridge, co-seismic turbidites corresponding to historical earthquakes and archaeological site surveys are well recorded, 6) Magnetic susceptibility, Ca/Ti profile, and X-rai CT helical images are useful to detect turbidites and characterize sedimentary structure, 7) a major normal fault movement, with 20-30cm vertical element, occurred in 3670 years ago or 3440 years ago; based on the correlation between Cores KY13-17 PC08 and PC09 obtained from both side of a normal/strike-slip fault.