2:30 PM - 2:45 PM
[MTT37-04] Transient crustal deformation around Hakone volcano and its new activity image inferred from dense GNSS observations
Seismic swarm activity has occurred at Hakone Volcano every few years since 2001, and the associated inflation of the volcanic edifice has been repeatedly observed by tiltmeter and GNSS (e.g., Daita et al., 2009; Kobayashi et al., 2018; Harada et al. 2018, etc.). The inflation of the edifice has been explained by a Mogi source at a depth of about 5-7 km and shallow open cracks with the upper edge near 0 m above sea level. On the other hand, crustal deformations that conventional models could not explain have been observed for the 2008-2009 and 2019 unrests, for example. In this presentation, we report crustal deformation since July 2023, revealed by a dense GNSS observation network consisting of GEONET stations and Hot Springs Research Institute, Japan Metrological Agency, and Softbank Corp. stations.
In the 2023 unrest, the time sequence of the event was observed to start with a significant northeastward displacement around the Ashigara Plain, east of Hakone volcano, in July, and then, during August, a significant displacement within and west of the Hakone caldera, which could be interpreted as an expansion of Hakone volcano. This sequence is noteworthy because transient crustal deformation beneath the Ashigara Plain seems to have triggered that of Hakone Volcano. Although the cause of the transient crustal deformation in the Ashigara Plain is not definitive, the significant displacement in July can be explained as a low-angle fault slip at more than 10 km depth on the east side of Hakone volcano.
Doke et al. (2020) noted the existence of a shear deformation zone in the northeastern Izu Peninsula. They considered the existence of detachment in the middle or lower crust (Seno, 2005) as a possible cause. The low-angle fault slip in the deep part may suggest a relationship with such structures. Moreover, previous studies often indicate that Hakone volcano is controlled by regional tectonics (Takahashi et al., 1999; Kobayashi et al., 2006; Doke et al., 2021, etc.) because the activity of the volcano is correlated with the timing of faulting events in this area. The observed event in 2023 is a result that supports this.
The crustal deformation observed in and around Hakone volcano in 2019 also shows a significant displacement near the Ashigara Plain, and the conventional model could not fully explain the displacement (Doke et al., 2019), which implies a mechanism similar to the 2023 event. Furthermore, looking back to past unrests, relatively high-velocity east-west baseline extension is observed for the 2001 and 2015 events, which can clearly be explained by the Mogi source, whereas 2008-2009, 2017, 2019, 2021, and 2023 events have relatively low velocity. This may reflect source location and geometry differences in both events and requires re-examining the past data. In addition, the more prominent occurrence of low-velocity events after the 2015 phreatic eruption may have been caused by the failure of caprock and sealing structures during the phreatic eruption, which prevented the accumulation of pressure in the underground (Mannen et al., 2021).
GNSS data from the SoftBank observation network were provided by SoftBank Corp. and ALES Corp. to the “Consortium to utilize the SoftBank original reference sites for Earth and Space Science” under the associated contract. We also used GNSS data from GSI and JMA.
In the 2023 unrest, the time sequence of the event was observed to start with a significant northeastward displacement around the Ashigara Plain, east of Hakone volcano, in July, and then, during August, a significant displacement within and west of the Hakone caldera, which could be interpreted as an expansion of Hakone volcano. This sequence is noteworthy because transient crustal deformation beneath the Ashigara Plain seems to have triggered that of Hakone Volcano. Although the cause of the transient crustal deformation in the Ashigara Plain is not definitive, the significant displacement in July can be explained as a low-angle fault slip at more than 10 km depth on the east side of Hakone volcano.
Doke et al. (2020) noted the existence of a shear deformation zone in the northeastern Izu Peninsula. They considered the existence of detachment in the middle or lower crust (Seno, 2005) as a possible cause. The low-angle fault slip in the deep part may suggest a relationship with such structures. Moreover, previous studies often indicate that Hakone volcano is controlled by regional tectonics (Takahashi et al., 1999; Kobayashi et al., 2006; Doke et al., 2021, etc.) because the activity of the volcano is correlated with the timing of faulting events in this area. The observed event in 2023 is a result that supports this.
The crustal deformation observed in and around Hakone volcano in 2019 also shows a significant displacement near the Ashigara Plain, and the conventional model could not fully explain the displacement (Doke et al., 2019), which implies a mechanism similar to the 2023 event. Furthermore, looking back to past unrests, relatively high-velocity east-west baseline extension is observed for the 2001 and 2015 events, which can clearly be explained by the Mogi source, whereas 2008-2009, 2017, 2019, 2021, and 2023 events have relatively low velocity. This may reflect source location and geometry differences in both events and requires re-examining the past data. In addition, the more prominent occurrence of low-velocity events after the 2015 phreatic eruption may have been caused by the failure of caprock and sealing structures during the phreatic eruption, which prevented the accumulation of pressure in the underground (Mannen et al., 2021).
GNSS data from the SoftBank observation network were provided by SoftBank Corp. and ALES Corp. to the “Consortium to utilize the SoftBank original reference sites for Earth and Space Science” under the associated contract. We also used GNSS data from GSI and JMA.