Microstructure analysis of marine seismogenic turbidites in Kumano forearc basin

Abstract:

An elongated depression was located in an ENE-WSW direction between the southern margin of the forearc basin and the outer ridge off Kumano. A terminal basin that captures all sediments supplied from outside is developed within this depression, making it an adequate site to study paleoseismology using seismogenic turbidites.

Previous study results reveal the Cs-137 and Pb-210 that the upper 17-cm mud layer was deposited immediately after the 2004 off Kii Peninsula earthquakes (Ashi et al., 2015, JpGU Meeting abstract). We herein investigate the characteristics of marine seismogenic turbidites based on various measurements including their compositions, X-ray CT images, and anisotropy of magnetic susceptibility (AMS). We observed a very thin fine-grained sand layer of 6 mm thick at 17 cm below seafloor and a volcanic ash layer of 15 cm thick at 5.1 m. The X-ray CT image shows seven silty clay laminations thinning upwards at 6 -15 cm below seafloor with homogeneous clay based silt above it, and several foraminifera-enriched layer below 1.7 m. The AMS parameters decrease upwards in the interval showing parallel/cross laminations and the lowest value is measured in the overlying silt layer. Moreover, the paleocurrent directions showed the NW-SE flow direction. These results indicate that the upper 17 cm layer beginning from the very fine-grained sand can be interpreted to be formed by a low density reflected gravity flow between the SE and NW dipping slopes of the basin. Structural observations by X-ray CT scanner reveal characteristic structures yielding various orientation oblique to bedding plane at the mud layer 17 cm below seafloor, suggesting that the structure is likely formed by coseismic deformation accompanied by the earthquake in 2004 or earlier ones. Magnetic fabrics derived from AMS measurements and the structure observed by X-ray CT scanner also agree to this picture.