2019 年 14 巻 5 号 p. 755-765
The Esan volcanic complex (EVC), northern Japan, is an active volcanic complex. The EVC represents a potential threat owing to its close proximity to inhabited areas, and abundant phreatic deposits occur around its volcanic aprons. Eleven samples of black paleosol and charcoal were collected from trenches dug at two sites (A and B) for the purpose of 14C dating. Sites A and B are located 2.6 km and 2.2 km from the source crater, respectively, and the deposits at these sites represent a proximal facies. At least 11 (at site A) and 12 (at site B) volcaniclastic units are identified. A newly identified Holocene phreatic unit (Es-0) is dated at ca 11 ka and is older than the EsMP unit that is a Holocene lava-dome building episode at ca 9 ka. Holocene phreatic episodes Es-1 and Es-3 can be divided into three and five subunits, respectively, and include a pyroclastic surge deposit that would have posed a threat to current local communities. The sequence of Holocene eruptions includes at least 11 tephra events over 11,000 years. The material ejected during phreatic explosions is characterised by highly heterogeneous grain size. Therefore, grain size analysis of erupted phreatic material might not effectively discriminate the processes of deposition, in contrast to such analysis of magmatic ejecta. The topography of the EVC largely constrains the area of deposition of phreatic ejecta.