History of Phreatic Eruptions in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

Abstract

A 6.3-m-deep trench was dug in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan, to clarify the history of phreatic eruptions in the field. The stratigraphic section in the trench consists of (from lower to upper): the Kt-1 pyroclastic surge deposit, nine phreatic fall deposits (Nb-l to Nb-d), the B-Tm tephra, two phreatic fall deposits (Nb-c, Nb-b), the Us-b tephra, and a phreatic fall deposit (Nb-a). The 12 phreatic fall deposits (Nb-l to Nb-a) are 3-100 cm thick and consist of altered dacitic lithic fragments in a clay-rich fine-grained matrix. These deposits are inferred to have been erupted from the Noboribetsu Geothermal Field. Buried soil layers occur between the deposits. Radiocarbon dating of buried soil samples from immediately below each of the phreatic fall deposits suggests that phreatic eruptions occurred at ca. BC6450, BC 5370, BC 3980, BC 3440, BC 1990, BC 1710, BC 1280, BC 900, BC 200, AD 980, AD 1480, and after AD 1663, corresponding to an average eruption interval of 〜700 years.