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This study focuses on strain change observations with a precision of 10 −9 associated with the 2011 Shinmoedake eruptions in Japan, using vault-housed extensometers installed approximately 18 km northwest of the Shinmoe-dake crater. The extensometers recorded major strain changes of 10 −7 during three sub-Plinian eruptions and subsequent magma effusion. Our analysis indicates that these extensometer records provide a reasonable estimation of the parameters of an isotropic point source that can explain eruption-related ground deformation. The extensometers also recorded subtle strain changes of 10 −9 prior to the three sub-Plinian eruptions. Time series data indicate that changes in strain at these rates are generally only observed immediately before explosive eruptions, suggesting that these strain changes are precursors to sub-Plinian eruptions. The source of these subtle strain changes is likely to be shallower than the magma chamber associated with these eruptions. The precursory strain changes might have been caused by a pressure increase and a subsequent pressure decrease under the volcano. One possible scenario that can explain these pressure changes is the accumulation of volcanic gases at depth, causing an increase in pressure that was eventually released during gas emissions from the crater prior to the explosive eruptions.

Abstract. The Shinmoe-dake volcano in southwest Japan, which produced its first major eruption in 52 yr on 26 January 2011 but had been quiescent since 1 March, re-erupted on 13 March. It was only two days after the occurrence of the M = 9.0 Tohoku-oki earthquake in eastern Japan. The coincidence of the two events raises the question of whether the earthquake triggered the volcanic activity. As a provisional but rapid assessment of this question, we examined highresolution strain data at a site located 18 km from Shinmoedake. In terms of the Tohoku-oki earthquake, three points can be drawn from the strain data: (1) static strain changes were less than 0.05 × 10 −6 , which is too small to trigger an eruption; (2) the amplitudes of dynamic strain changes are on the order of 10 −6 , which may trigger seismicity or volcanic eruption; and (3) strain rates were not accelerated, which indicates no significant change in magma pressure. Comparing these results with reports of other eruptions coincident with seismic events, and considering a scenario in which a seismic event triggered an eruption, we tentatively conclude that the eruption on 13 March was not a triggered event. However, this conclusion may be revised after analyzing seismic data.

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