三陸海岸ならびに仙台平野における東北地方太平洋沖地震に起因した津波堆積物中のヒ素ならびに重金属類の起源

摘要

A total of 129 samples of tsunami deposits were collected immediately after the 2011 off the Pacific coast of Tohoku Earthquake at 5 ㎞ intervals over the 250 ㎞ length of the tsunami inundation zone. They were analyzed for 10 major rock-forming elements, arsenic, and base metals (Cu, Zn, Pb, and Cd) with EDXRF. Statistical analysis indicates that the populations of arsenic and heavy metals-with the exception of Cd，which had values below the detection limit-comprise two log-normal distributions divided by a threshold value: 89 ppm Cu, 245 ppm Zn, 48 ppm Pb, and 21 ppm As. Spider diagrams of their anomalous values normalized by average marine sediments in Sendai bay indicate a characteristic pattern in each mineralization province. They are coincident to river sediment anomalies detected before the tsunami and are also compatible with the production of disused mines located in the background area. We conclude that almost all base metals and arsenic anomalies in the tsunami deposits distributed on the Sanriku coast and Sendai Plain by the 2011 off the Pacific coast of Tohoku Earthquake are affected by specific geological and mineralogical properties.