[S-VC34] 火山・火成活動および長期予測

[SVC34-P17] カルデラ西側と東側の阿蘇4火砕流堆積物の古地磁気

*安田 裕紀¹ (1.東京大学地震研究所)

キーワード：阿蘇4、古地磁気方位、対比

The Aso-4 tuff sheet was emplaced at ~90 ka during a VEI 8 super-eruption from Aso caldera, central Kyushu, Japan. The Aso-4 sheet was dispersed and are preserved radially all around, but particularly west and east of, the caldera; to the west, they reach to the coasts of Shimabara and Amakusa, indicating that the Aso-4 pyroclastic currents may have traveled across the sea. Although the Aso-4 sheet to the east of the caldera and the widespread Aso-4 co-ignimbrite ash have been paleomagnetically correlated, there is little paleomagnetic data for the sheet to the west. In this study, the paleomagnetic directions of the western and eastern Aso-4 sheets were determined to establish their correlation. The new paleomagnetic directions obtained from pumice and scoria blocks in the nonwelded part of the western sheet (4 sites; Dec=350°–359°, Inc=38°–48°) are comparable to those for the nonwelded and welded portions of the eastern sheet (5 sites; Dec=351°–358°, Inc=39°–47°), demonstrating that the western and eastern sheets are coeval. These new directions are also consistent with the directions reported in the literature for the Aso-4 sheet and its co-ignimbrite ash. Remanent directions of pumice samples from the nonwelded tuff at Shimabara, ~80 km from the caldera center, are well clustered within the site (α₉₅=4.1°), which imply that the pyroclastic currents were hot enough to emplace deposits that were >600°C after travelling several tens of kilometers over the sea. The western and eastern Aso-4 sheets have been correlated and distinguished from the previous tuff sheets from Aso caldera by the presence of hornblende that is absent in the others, and this correlation is further strengthened by the new paleomagnetic results.