Multiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

The Noboribetsu geothermal field (area 1×1.5 km) is located in the western part of Kuttara volcano, southwestern Hokkaido, Japan. Seventy-one shallow trenches (<90 cm deep) were systematically dug within the geothermal field, in order to study the stratigraphy and distribution of the most recent phreatic fall deposit (the Nb-a deposit) erupted from the field. The Nb-a deposit (<68 cm thick) consists of altered dacitic lithic clasts in a fine-grained clay-rich matrix. The deposit overlies the Us-b tephra that was emplaced in AD 1663. Stratigraphic sections constructed from the 71 trenches indicate that the deposit is distributed within an elliptical area measuring 850×1250m across and extending NNWSSE. Isopach and maximum-grain-size isopleth maps indicate that the deposit increases in thickness and maximumgrain-size toward several explosion craters. These data suggest that phreatic eruptions, through multiple vents, occurred in the Noboribetsu geothermal field after AD 1663.