新潟焼山における火山噴出物の古地磁気研究と噴火史の検討

摘要

Precise dating of the products of volcanic eruptions is crucial for conducting research and mitigation of volcanic disasters, and an effective method other than 14C dating is required. In this study, we applied the geomagnetic dating method (Hirooka, 1971) to investigate the eruption history of Mt. Yakeyama in Niigata Prefecture, Japan. Mt. Yakeyama, one of Japan's many active volcanoes, has caused several volcanic hazards in recorded history. Volcanic products that were believed to have erupted in the 14th and 18th centuries yielded geomagnetic ages that were consistent with previously reported ages (Hayatsu, 2008). On the other hand, volcanic products that were formerly assigned dates from the 9th to 10th century yielded geomagnetic dates of the 13th century. Furthermore, a volcanic product from the "Bouboudake stratum," whose age was hitherto unknown, yielded a geomagnetic date from around the 14th century. These results show that the paleomagnetic method is effective for determining the age of volcanic products and/or reexamining formerly estimated ages obtained by other methods such as ~(14)C and stratigraphic dating. By examining the volcanic products at Mt. Yakeyama using the ages estimated in this study, we can determine that the volcanic products from the 13th and 14th centuries have a large distribution, suggesting high volcanic activity at Mt. Yakeyama in the 13th and 14th centuries.