Abstract
The event of Ulleung Oki (U-Oki) tephra, distributed across the southwestern Japan sea, originated in an explosive eruption of alkali magma from the caldera in Ulleung island in the Japan sea. However, it is suggested that some unknown alkaline ash fall events, which are probably from the Ulleung island, had occurred besides U-Oki tephra between AT and K-Ah. The Ta/Sc ratio in the alkaline tephra from continental volcano has significantly different characteristics with the general tephra erupted by island arc volcanism. We have already carried out INAA determination of 12 trace elements from five cores in the southwest Japan Sea. The analysis of these cores had revealed some peaks in the vertical distribution of Ta / Sc ratio in the upper and lower parts of U-Oki layer, as well as the strong peak at U-Oki layer. We also detected K-Ah cryptotephra layer by the ratio of other trace elements from four cores. We are going to confirm it with using EPMA.
