Spatial and temporal distributions of b-value in and around Shinmoe-dake, Kirishima volcano, Japan

Kirishima Shinmoe-dake, located in southern Kyushu, Japan, erupted in October 2017 after 6 years of quiescence. Analysis of volcanic gas indicates that magma upwelling from depth affected this eruption. Shinmoe-dake's recent eruptive history also includes phreatic eruptions in 2008 and 2010 and a magmatic eruption in 2011. We examined spatial and temporal variations in b-values to investigate the magma plumbing system in and around Shinmoe-dake and its possible effects on unrest. A region with relatively high b-values (b = 1.5) is located at depths of − 1.0 to 2.0 km below sea level beneath the summit. It is likely that this anomalous region was generated by crustal heterogeneity, as it colocates to pressure sources. We investigated temporal variations in b-values from January 2007 to October 2017 in two regions: beneath the summit and 7.0–8.0 km northwest of the summit; the latter corresponds to the region above the locations of pressure sources related to the 2011 and 2017 eruptions. An increase in the b-value (b = 1.4) beneath the summit was observed beginning in early 2009, followed by a subsequent decrease (b = 0.9) immediately before and during the 2011 eruption. Similar temporal changes in the b-value were also observed beneath the summit before the 2017 eruption. From these results, we can infer that the increase and subsequent decrease in b-value express the activation of small cracks, due to the generation of hydrothermal fluids, and the development of cracks, which produces higher magnitude earthquakes, respectively. Meanwhile, a decrease in b-value (b = 0.6) was observed in the region northwest of the summit during the 2011 and 2017 eruptions. Thus, it is possible that the decreases in b-value in this region result from the activation of small cracks and the development of these crack systems and/or a change in the stress field near the magma chamber, which produce higher magnitude earthquakes. .