Mt. Tarumae is an active volcano located in the southeast of the Shikotsu caldera, Hokkaido, Japan. Recently, crustal expansion occurred in 1999–2000 and 2013 near the summit of Mt. Tarumae, with a M5.6 earthquake recorded west of the summit on July 8, 2014. In this study, we determined hypocenter distributions and performed b-value analysis for the period between August 1, 2014 and August 12, 2016 to improve our understanding of the geometry of the magma system beneath the summit of Mt. Tarumae. Hypocenters were mainly distributed in two regions: 3–5 km west of Mt. Tarumae, and beneath the volcano. We then determined b-value distributions. Regions with relatively high b-values (∼1.3) were located at depths of –0.5 to 2.0 km beneath the summit and at depths greater than 6.0 km about 1.5–3.0 km northwest of the summit, whereas a region with relatively low b-values (∼0.6) was located at depths of 2.0–6.0 km beneath the summit. Based on comparison of the b-value distributions with other geophysical observations, it was found that the high b-value region from –0.5 to 2.0 km in depth corresponded to regions of lower resistivity, positive self-potential anomaly, and an inflation source detected in 1999–2000. Therefore, it is inferred that this region was generated by crustal heterogeneity, a decrease in effective normal stress, and change of frictional properties caused by the development of faults and fissures and the circulation of hydrothermal fluids. On the other hand, the inflation source detected in 2013 was located near the boundary between the low b-value region beneath the summit and the deeper high b-value region about 1.5–3.0 km northwest of the summit. Studies of other volcanoes have suggested that such high b-values likely correspond to the presence of a magma chamber. Based on the deeper high b-value region estimated in this study, the magma chamber is inferred to be located at depths greater than 6.0 km about 1.5–3.0 km northwest of the summit. Thus, these findings contribute to our understanding of the magma plumbing system beneath the summit of Mt. Tarumae.

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