Prehistoric Nankai Greatearthquakes: evidence of turbidites from slope basins in the western Nankai Trough

Friday, 19 December 2014
Masao Iwai and Munemasa Kobayashi, University of Kochi, Kochi, Japan
The March 2011 Tohoku earthquake (Mw=9.0) shows us that geologic evidence is very important and essential for the risk assessment of low frequency/high impact subduction earthquakes and tsunami hazards. Resolving the tempo and mode of the millennial-scale mega-earthquakes is a cardinal challenge of current paleoseimologic reserach. Coseismic turbidites give us an opportunity to reveal recurrence history and suppercyles of such prehistoric great earthquakes.
We investigated three piston cores from landward slope basins in the Nankai Trough off Muroto, southwest Japan, in order to detect Holocene paleoearthquake history. Tow of three, Cores KR9705P1 and KR9911PC1 were obtain from the Tosabae Trough, and the other Core KR9911PC2 from an adjacent landward slope basin. Each turbidities layer from those three piston cores were correlated based on the visual core descriptin, CT scan images, magnetic susceptibility, XRF-CS profiles, and grain size analysis.
Magnetic susceptibility and grain size are sensitive to detect turbidite layers. Also XRF-CS profiles are usefull to detect and characterize those turbidits. Our correlation suggest that average recurrence time interval of turbidites and/or sandy layers was ~150 years and it was almostly equivalent to those of historic earthquakes for last 7 k.y.
Tosabae trough are filling with a total of ~260m thick stratified sediments and sedimentary sequance of this isolated basin has a potential to be a paleoseismometer of the Nankai Great Earthquakes for last the 200-400 kyrs for to understand the tempo and mode of subduction earthquakes under different climatic conditions.