Abstract
We herein propose an alternative model for deformation caused by an eruption at Sakurajima, which has been previously interpreted as being due to a Mogi-type spherical point source beneath Minami-dake. On November 13, 2017, a large explosion with a plume height of 4200 m occurred at Minami-dake. During the 3 min following the onset of the explosion (November 13, 2017, 22:07–22:10 (Japan standard time (UTC + 9); the same hereinafter), phase 1, a large strain with changes up to 120 nstrain was detected at the Arimura observation tunnel (AVOT) located approximately 2.1 km southeast from the Minami-dake crater. After the peak of the explosion (November 13, 2017, 22:10–24:00), phase 2, a large deflation was detected at every monitoring station due to the continuous Strombolian eruption. Subsidence toward Minami-dake was detected at five out of six stations, whereas subsidence toward the north of Sakurajima was detected at the newly installed Komen observation tunnel (KMT), located approximately 4.0 km northeast from the Minami-dake crater. The large strain change at AVOT as well as small tilt changes at all stations and small strain changes at the Harutayama observation tunnel (HVOT) and KMT during phase 1 can be explained by a very shallow deflation source beneath Minami-dake at 0.1 km below sea level (bsl). For phase 2, a deeper deflation source beneath Minami-dake at a depth of 3.3 km bsl was found in addition to the shallow source beneath Minami-dake, which turned inflation after the deflation that occurred during phase 1. However, this model cannot explain the tilt change of KMT. Adding a spherical deflation source beneath Kita-dake at a depth of 3.2 km bsl can explain the tilt and strain change at KMT and the other stations. The Kita-dake source was also found in a previous study of long-term ground deformation. Not only the deeper Minami-dake source MD, but also the Kita-dake source deflated due to the Minami-dake explosion.
Highlights
The tilt and strain changes accompanying Vulcanian eruptions of Sakurajima have been interpreted as being due to the deflation of a Mogi-type source (Mogi 1958) beneath Minami-dake, because downward tilts associated with eruptions are oriented towards Minami-dake in the underground tunnels Harutayama (2.7 km northwest of the Minami-dake crater) and Arimura (2.1 kmHotta and Iguchi Earth, Planets and Space (2021) 73:70 from the deformation expected from the inflation of the Mogi-type spherical point source beneath Minami-dake found in previous studies, and the deformation at the Komen tunnel is considered to be affected by a different source.In the present study, we propose a composite source model for tilt and strain changes at ground deformation stations, taking into account the tilt change at the Komen tunnel
Observations and data At Sakurajima, tilt and strain observations have been performed by the Sakurajima Volcano Research Center (SVRC) of Kyoto University; the Osumi Office of River and National Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT); and the Japan Meteorological Agency (JMA)
We used data obtained by watertube tiltmeters and linear strainmeters at the Arimura, Harutayama and Komen underground tunnels (AVOT, HVOT and KMT, respectively) and borehole tiltmeters at Yokoyama, Amida-gawa and Komen (JMA-A, JMA-F and KOM, respectively)
Summary
The tilt and strain changes accompanying Vulcanian eruptions of Sakurajima have been interpreted as being due to the deflation of a Mogi-type source (Mogi 1958) beneath Minami-dake, because downward tilts associated with eruptions are oriented towards Minami-dake in the underground tunnels Harutayama (2.7 km northwest of the Minami-dake crater) and Arimura (2.1 kmHotta and Iguchi Earth, Planets and Space (2021) 73:70 from the deformation expected from the inflation of the Mogi-type spherical point source beneath Minami-dake found in previous studies, and the deformation at the Komen tunnel is considered to be affected by a different source.In the present study, we propose a composite source model for tilt and strain changes at ground deformation stations, taking into account the tilt change at the Komen tunnel. The tilt and strain changes accompanying Vulcanian eruptions of Sakurajima have been interpreted as being due to the deflation of a Mogi-type source (Mogi 1958) beneath Minami-dake, because downward tilts associated with eruptions are oriented towards Minami-dake in the underground tunnels Harutayama (2.7 km northwest of the Minami-dake crater) and Arimura We used data obtained by watertube tiltmeters and linear strainmeters at the Arimura, Harutayama and Komen underground tunnels (AVOT, HVOT and KMT, respectively) and borehole tiltmeters at Yokoyama, Amida-gawa and Komen (JMA-A, JMA-F and KOM, respectively). The tiltmeters and strainmeters at AVOT, HVOT, KMT and KOM were installed in directions radial and tangential to Minami-dake and the tiltmeters at JMA-A and F were in the north–south and east–west directions. The analog-to-digital resolution was 24 bits and the resolution of tilt and strain was 1 0−6 nrad
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