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[SVC31-04] Variation with time and depth of b value of volcanic earthquakes associated with the 2008 Shinmoe-dake eruption
Keywords:volcanic earthquake, volcanic eruption, b-value, Shinmoe-dake, Kirishima Volcano, seismic swarm, phreatic eruption
Kirishima volcanoes are a group of volcanoes that erupt at the same time from multiple craters such as Shinmoe-dake, Ohachi and Ioyama, and are currently one of the most active volcanoes in Japan. The latest eruptive activity of Shinmoe-dake started on August 22, 2008 after 16 years dormancy. Subsequently, it erupted on March 30, April 17, May 27, June 27 and 28, and July 5 and 10, 2010. In 2011, the eruption started on January 19 and was followed by sub-Plinian eruption on 27 January. Eruptive activity gradually ceased since February 2 and moved to Vulcanian activities. It erupted in October 2017 and in March 2018. Eruptions continued until June 27. In this study, we clarify the characteristics of the seismic swarm activity that occurred just before the 2008 eruption and discuss the thermal fluid activity and formation of the conduit that caused the eruption.
Seismic activity around Shinmoe-dake is low before the 22 August eruption, where a seismic swarm started 3 days before the eruption. After the eruption, seismic activity became low, but the activity became higher again in December 2009 and continued until the January 2011 eruption. Fig.1 shows the time variation of hypocenter depth during earthquake swarms. The seismic swarm began on the 19th at a depth about 1-2 km below sea level just below the summit crater. The hypocenter region gradually expanded to the shallower area, reaching near the surface on the 22nd, and active seismic swarm continued until the eruption. The upper edge of the seismic region raised by about 1 km per day. After the eruption, earthquakes continued to occur at depths shallower than 1 km below sea level.
The measured b-value obtained from the frequency distribution of the magnitude was 2.0 for the seismic swarm just before the 2008 eruption, and 0.6 to 0.9 for other periods. The b-value, which represents the characteristics of the seismic field, is considered to be about 0.7 to 1.1 for tectonic earthquakes and over 2 for volcanic earthquakes. In this case, the large b-value is considered to reflect the state change in the thermal fluid that caused the eruption. Furthermore, Fig.2 and Fig.3 show the change of b-value during the seismic swarm occurrence period. Fig.2 shows the frequency distribution divided into the first half (solid diamonds) and the second half (solid squares) of the seismic swarm occurrence period. The b-values are 1.9 and 2.1, respectively, and there is almost no change. Fig.3 shows the frequency distribution divided into areas deeper than 1 km above sea level (solid circles) and shallower than 1 km (horizontal bars). The b-values are 2.1 and 1.4, respectively. In other words, the seismic area below the summit crater is divided into regions: the area with a large b-value (2.1) deeper than 1km above sea level and the area with a small b-value (1.4) shallower than 1km.
The above is interpreted as follows. Just before the 2008 eruption, the thermal fluid that caused the eruption became active and opened a passage while rising. Corresponding to the eruption process, active seismic activity occurred in a region deeper than 1 km above sea level below the crater, resulting in a large b-value. Several small eruptions that occurred in 2010 did not show seismic activity with large b-value immediately before. This can be interpreted that the eruption occurred without changing the state of the surrounding seismic field because the passage had already opened.
Seismic activity around Shinmoe-dake is low before the 22 August eruption, where a seismic swarm started 3 days before the eruption. After the eruption, seismic activity became low, but the activity became higher again in December 2009 and continued until the January 2011 eruption. Fig.1 shows the time variation of hypocenter depth during earthquake swarms. The seismic swarm began on the 19th at a depth about 1-2 km below sea level just below the summit crater. The hypocenter region gradually expanded to the shallower area, reaching near the surface on the 22nd, and active seismic swarm continued until the eruption. The upper edge of the seismic region raised by about 1 km per day. After the eruption, earthquakes continued to occur at depths shallower than 1 km below sea level.
The measured b-value obtained from the frequency distribution of the magnitude was 2.0 for the seismic swarm just before the 2008 eruption, and 0.6 to 0.9 for other periods. The b-value, which represents the characteristics of the seismic field, is considered to be about 0.7 to 1.1 for tectonic earthquakes and over 2 for volcanic earthquakes. In this case, the large b-value is considered to reflect the state change in the thermal fluid that caused the eruption. Furthermore, Fig.2 and Fig.3 show the change of b-value during the seismic swarm occurrence period. Fig.2 shows the frequency distribution divided into the first half (solid diamonds) and the second half (solid squares) of the seismic swarm occurrence period. The b-values are 1.9 and 2.1, respectively, and there is almost no change. Fig.3 shows the frequency distribution divided into areas deeper than 1 km above sea level (solid circles) and shallower than 1 km (horizontal bars). The b-values are 2.1 and 1.4, respectively. In other words, the seismic area below the summit crater is divided into regions: the area with a large b-value (2.1) deeper than 1km above sea level and the area with a small b-value (1.4) shallower than 1km.
The above is interpreted as follows. Just before the 2008 eruption, the thermal fluid that caused the eruption became active and opened a passage while rising. Corresponding to the eruption process, active seismic activity occurred in a region deeper than 1 km above sea level below the crater, resulting in a large b-value. Several small eruptions that occurred in 2010 did not show seismic activity with large b-value immediately before. This can be interpreted that the eruption occurred without changing the state of the surrounding seismic field because the passage had already opened.