Mechanism of small vulcanian eruptions at Suwanosejima volcano, Japan, as inferred from precursor inflations and tremor signals

We examine precursory signals of vulcanian eruptions at Suwanosejima volcano, southwest of Japan, by deploying seismic and tilt stations close to the active crater to understand the shallow conduit processes just before explosions. Vulcanian eruptions at Suwanosejima for the period from October 2009 to 2011 are often preceded by a sudden cessation of continuous tremor associated with continuous ash emissions. The amplitude of continuous tremor decreases within about 10 s, and tilt signal starts to show uplift of the active crater. Then, the pause of continuous tremor and uplift continue for about 1-2 min, and tilt amplitudes reach a few tens of nanoradians at a distance of 400 m from the active crater. As the pause time of continuous tremor before explosion becomes longer, the maximum amplitude of explosion earthquake and uplift signal become larger. Vulcanian eruptions are also observed during periods without continuous tremor. In these cases, tilt signals capture uplifts of the active crater for about 10 min before explosions and reach a few tens of nanoradians. Cessations of volcanic gas emissions are observed before some explosions during periods without continuous tremor (Mori et al. 2011). These observations suggest that pressurization in the shallow conduit is caused by an accumulation of volcanic gases and ash through the formation of 'cap' in the conduit, possibly composed of stiffened magma due to degassing, cooling, rheological changes, or other mechanisms.