Paleoseismology Along the Japan Trench Subduction Zone: Deep-Sea Sediment Records of Earthquakes in Tohoku
Friday, 19 December 2014: 10:20 AM
After the 2011 off the Pacific coast of Tohoku Earthquake, we have intensively explored earthquake-induced records in the deep-sea basins close to rupture zones of the 2011 and past earthquakes in Tohoku, Our study area focuses on small terminal basins near the trench for the following reason. Mass transport deposits from up-slope area down into the Japan Trench axis floor during the 2011 event were documented (e. g. Oguri et al., 2013). Similarly past earthquake-induced turbidites were expected to have been deposited in a number of small basins in the trench floor and in a lower slope terrace. Here we introduce our recent results and future prospects for paleoseismology in the Japan Trench. We collected cores from areas that are generally composed of diatomaceous fine grain sediment and fine-grained turbidites. Occasionally, wide spread tephra patches/layers are intercalated in the sediment. In the trench floor, two thick turbidite units below (older) than the 2011 event deposits were identified in up to 10m long sedimentary cores. An intercalated tephra within these turbidite units indicates that turbidites were triggered by historical earthquakes in Tohoku (Ikehara et al., in prep). On the other hand, frequent occurrence of thin-bedded turbidites was identified in the lower slope terrace. Several turbidite layers can be correlated over a wide area by tephra-correlation and matching of paleomagnetic secular variations (Usami et al., in this session). Turbidite-stratigraphy from both locations, the trench and lower slope terrace shows a similar pattern, and seems to be tied to the onland tsunami deposit stratigraphy straightforwardly. These observations suggest that deep-sea turbidite records are more reliable proxy for paleoseimic-stratigraphy of the Tohoku area. Encouraged by the results obtained so far, we plan extensive investigations in the area over the next few years to document effectively the spatiotemporal distribution of earthquake records of Tohoku.