Catastrophic sector collapse at Usu volcano, Hokkaido, Japan: failure of a young edifice built on soft substratum

Abstract

Catastrophic sector collapse is a destructive volcanic process that occurs during the growth history of a volcano. Understanding the timing and causal mechanisms of sector collapse at individual volcanoes is essential for evaluating long-term volcano evolution and associated hazards. This paper describes the lithofacies, tephrostratigraphy, and radiocarbon dating of a debris avalanche deposit at Usu volcano, Japan, based on detailed field geological surveys, and proposes that uplift of soft substratum played an important role in destabilizing the young volcanic edifice. Usu is a Quaternary basaltic to andesitic stratovolcano that has an amphitheater at the summit. The basement of the volcano has been uplifted by 300–400 m relative to the surrounding area. The debris avalanche deposit is > 0.3 km3 in volume and displays well-preserved hummocky topography. The hummocks are composed of andesite blocks derived from the stratovolcano, and non-welded pyroclastic flow and fluvial deposits derived from the basement of the stratovolcano. Tephrostratigraphy and radiocarbon dating of charcoal samples from the debris avalanche deposit indicate that the sector collapse occurred at ~ 16 ka. The age of sector collapse, combined with previous geochronological data, enables reconstruction of the eruptive history of Usu as follows: (1) andesitic explosive eruption at 18–19 ka, (2) stratovolcano growth between 18 and 16 ka, (3) sector collapse at ~ 16 ka, (4) hiatus in volcanic activity for ~15,000 years, (5) rhyolitic Plinian eruption in AD 1663, and (6) dacitic dome-forming eruptions after AD 1663. This eruptive history indicates that the sector collapse occurred within ~3000 years of the onset of stratovolcano growth, much sooner than at other volcanoes where the timing of sector collapse is known. We infer that the sector collapse of such a young volcanic edifice was caused by a combination of dome-like uplifting/tilting (forced folding) of soft substratum, overloading of the volcanic edifice on the soft substratum, a NE–SW-trending fault system that deeply cuts the substratum, and possible magma intrusion within the edifice and associated earthquakes. Usu provides an excellent example of edifice instability that is characterized by uplift of soft substratum. Our new view of causal mechanisms of the sector collapse provides invaluable information for hazard assessment of Quaternary volcanoes.