Abstract
Previous studies of the major eruption at Shinmoe-dake volcano, Japan, in January 2011 suggested that gradual injection of magma from a deep source into a shallow reservoir began in December 2009 and led to the major eruption. To investigate the initial phase of this injection event, we examined extensometer data from the Isa Observatory, ~ 18.5 km from the summit of Shinmoe-dake, and discovered a strain change event that spanned about 3 days in December 2009. The size of the strain change is comparable to those observed during each sub-Plinian eruption in 2011. The source of the rapid strain change appears to be deeper than the estimated location of the magma reservoir that directly supplied magma to the 2011 eruption sequence. These observations suggest that rapid injection of magma from the deep magmatic plumbing system in December 2009 triggered the continuous ascent of additional magma from depth, which in turn drove the climactic eruptions in January 2011. Extensometers also recorded two rapid strain change events of the same order of magnitude and with similar characteristics in December 2006 and August 2008; however, noticeable inflation of the edifice was not detected immediately following either event. This suggests that transient injection of magma into a shallow reservoir is not always followed immediately by a gradual recharge process.
Highlights
Most monitored volcanoes show detectible unrest before major eruptions
Petrological studies suggest that the gradual inflation was driven by a continuous upward supply of high-temperature magma, from the deeper magmatic plumbing system to the shallow reservoir, which resulted in a major eruption through a turnover process (Tomiya et al 2013)
This observation suggests that rapid pressurization, on a time scale of a few days, occurred beneath Shinmoe-dake at the beginning of the period of gradual inflation of the magma reservoir inferred from GNSS data (Nakao et al 2013)
Summary
Most monitored volcanoes show detectible unrest before major eruptions. An unrest state is most clearly detectible in ground deformation (e.g., Nooner and Chadwick 2016; Morales Rivera et al 2017), but can include changes in seismicity (e.g., Johnson et al 2010) and migration of earthquake swarms (e.g., Nakada et al 1999; Chiba and Shimizu 2018). The strain change is characterized by extension on E2 and contraction on E1, which we expect when ground deformation is generated by a subsurface pressure source in the azimuthal direction of Shinmoe-dake This observation suggests that rapid pressurization, on a time scale of a few days, occurred beneath Shinmoe-dake at the beginning of the period of gradual inflation of the magma reservoir inferred from GNSS data (Nakao et al 2013). The anomaly in August 2008 has the same characteristics, each component returns to its original value after rain on 22 and 23 August (Fig. 7b) Their respective timings correspond to changes in observed volcanic activity at Shinmoe-dake: the first roughly corresponds to a subtle change in the GNSS baseline trend (Fig. 2 of Nakao et al 2013); the second corresponds to the start of a period of phreatic eruptions (Fig. 5 of Nakada et al 2013). The threshold value is half the strain rate at the beginning of the edifice inflation that led to the major eruption in 2011
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