Shinmoe-dake in the Kirishima volcano group located in southwestern Japan started to erupt on January 19, 2011 and the eruption developed to Sub-Plinian eruptionson January 26 and 27. Crustal deformations associated with the eruption, including pre-eruptive inflation, co-eruptive deflation, and post-eruptive inflation, were detected by DInSAR and GPS measurements. Geodetic information revealed by two different methods compensate each other and exhibit good agreement. The centers of these deformations for each period almost overlap and are located about 5km west-northwest of the Shinmoe-dake crater. Assuming that the deformation source indicating magma chamber at a depth of 7.5km was fixed at the same location and depth, volumes of the source are estimated to have increased 1.7×10⁷m³ for the pre-eruptive period, decreased 1.5×10⁷m³ for the co-eruptive period, and increased 8.0×10⁶m³ for the post-eruptive period. The co-eruptive volume decrease is comparable to the pre-eruptive volume increase and the emitted volume. However, the post-eruptive volume increase is obviously small and ceased after December 2011. The magma chamber has now returned to about 50% of the eruptive volume of the 2011 eruption. If a volume increase start again at a similar rate, it will be about 10 months until the next 2011-size eruption.

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Since even small amounts of tephra fall can have a significant negative impact on infrastructure, a rapid estimate of the eruptive mass of magma, which is one of the most important indicators of the scale of eruption, is required. Shinmoedake volcano, part of the Kirishima volcanoes, Japan, began erupting on 19 January 2011 after a dormant period of 300 years. Sub-plinian eruptions occurred on 26-27 January, followed by vulcanian eruptions and eruptions of ash fall thereafter. Here, we report the results of a preliminary study that sought to estimate the eruptive mass and the distribution of ash fall based on observations of tephra fall within 2 weeks of an eruption at Shinmoedake. In cooperation with municipal authorities, we observed temporal changes in the amounts of ash erupted during volcanic activity at Shinmoedake volcano from 24 February 2011 to 29 February 2012. Observations were made at 35 sites in five municipalities (Miyakonojo City, Takaharu Town, Kirishima City, Kobayashi City, and Ebino City). The observations enabled estimates of the eruptive masses of ash from seven eruptive events. The eruptive masses are between 1×10⁶kg and 2×10⁸kg, with the largest being the eruption of 13 March 2011. We found positive correlations between (1) the eruptive mass and the height of volcanic cloud determined by radar echo and between (2) the eruptive mass and the height of cloud that reported by the Japan Meteorological Agency. The former correlation is consistent with a relatively well-known geophysical relationship between the maximum height of a convection current and the amount of thermal output pulse. The empirical correlation may allow us to easily and rapidly estimate the eruptive mass from the observed height of the volcanic cloud top.

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Direct current (DC) electrical and controlled source audio-frequency magnetotelluric (CSAMT) surveys were performed over the Hiyoriyama Cryptodome in Kuttara Volcano, Hokkaido, Japan. Both surveys were performed on the same survey line across the cryptodome in a NE-SW orientation. Two-dimensional joint inversion of the DC electrical and CSAMT data revealed the underground resistivity structure at depths less than 400m beneath the cryptodome. The resistivity structure suggests that the cryptodome comprises a dacite intrusion of 150m wide and 80m thick (20-50Ω・m), and overlying pyroclastic deposits that are 10-30m thick (＞100Ω・m). The dacite intrusion is underlain by a convex-shaped, low-resistivity layer (＜5Ω・m) that is interpreted to be smectite-rich, altered pyroclastic deposits that have been subjected to low-temperature (＜200℃) hydrothermal alteration. The low-resistivity layer is underlain by a slightly higher-resistivity layer (10-30Ω・m) that is interpreted to be altered pyroclastic deposits that were subjected to higher-temperature (＞200℃) hydrothermal alteration in a relatively deep, hot region near the conduit of the cryptodome.

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