Abstract
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.
Highlights
Shinmoe-dake is one of the most active volcanos of the Kirishima volcano group, a cluster of 20 volcanoes in southern Kyushu, Japan (Imura and Kobayashi 1991)
We investigated the spatial and temporal distributions of b-value to constrain the causes of volcanic activity and the magma plumbing system in and around Shinmoedake, Kirishima volcano, Japan, using the VT earthquake catalogue constructed by the JMA
It is inferred that this region of relatively high b-value was generated by crustal heterogeneity related to pressure sources beneath the summit
Summary
Shinmoe-dake is one of the most active volcanos of the Kirishima volcano group, a cluster of 20 volcanoes in southern Kyushu, Japan (Imura and Kobayashi 1991). Earthquake frequency–magnitude distributions were first developed in Japan by Ishimoto and Iida (1939). Computed b-values are often higher for volcanic areas than for typical tectonic earthquake sequences (e.g., Frohlich and Davis 1993; Wyss et al 1997; McNutt 2005). Many studies have demonstrated the effectiveness of calculating the spatial and temporal distributions of b-values in interpreting volcanic structure (e.g., Wiemer and Benoit 1996; Wiemer and McNutt 1997; Wiemer et al 1998; McNutt 2005; Bridges and Gao 2006; Farrell and Smith 2009). We anticipate that a detailed image of the magma plumbing system can be obtained in and around Shinmoe-dake based on a spatiotemporal analysis of the b-value, as a dense network of permanent seismic stations exists on and around the volcano
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