Report on Kusatsu-Shiranesan (Japan) — 25 April-1 May 2018
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 25 April-1 May 2018
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2018. Report on Kusatsu-Shiranesan (Japan). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 25 April-1 May 2018. Smithsonian Institution and US Geological Survey.
36.618°N, 138.528°E; summit elev. 2165 m
All times are local (unless otherwise noted)
JMA reported that on 21 April the number of volcanic earthquakes at Yugama crater (Kusatsu-Shiranesan complex) increased and deformation was also recorded. The Alert Level for the crater area was raised to 2 (on a 5-level scale) the next day. Deformation slowed on 23 April. Seismicity decreased on 23 April though continued to be somewhat elevated, and low-frequency events were recorded on 24 April. No surficial changes were noted during an overflight on 26 April.
Geologic Background. The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.