Precursory tilt changes associated with a phreatic eruption of the Hakone volcano and the corresponding source model

Abstract

The 2015 unrest of the Hakone volcano in Japan, which began on April 26, generated earthquake swarms accompanied by long-term deformation. The earthquake swarm activity reached its maximum in mid-May and gradually calmed down; however, it increased again on the morning of June 29, 2015. Simultaneously with the earthquake increase, rapid tilt changes started 10 s before 07:33 (JST) and they lasted for approximately 2 min. The rapid tilt changes likely reflected opening of a shallow crack that was formed near the eruption center prior to the phreatic eruption on that day. In this study, we modeled the pressure source beneath the eruption center based on static tilt changes determined using both tilt meters and broadband seismometers. In the best-fit model, the source depth was 854 m above sea level, and its orientation (N316°E) agreed with the direction of maximum compression estimated based on focal mechanism and S-wave splitting data. The extent of the crack opening was estimated to be 4.6 cm, while the volume change was approximately 1.6 × 10 5 m 3 . The top of the crack reached to approximately 150 m below the eruption center. Because the crack was too thin to be penetrated by magma, the crack opening was attributed to the intrusion of hydrothermal water. This intrusion of hydrothermal water may have triggered the phreatic eruption. Reverse polarity motion with respect to that expected from crack opening was recognized in 1 Hz tilt records during the first 20 s of the intrusion of hydrothermal water. This motion, not the subsidence of volcanic edifice, was responsible for the observed displacement. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Monitoring ground deformation of eruption center by ground-based interferometric synthetic aperture radar (GB-InSAR): a case study during the 2015 phreatic eruption of Hakone volcano

“…5). We thus consider that the uplift started at least before 7:48, which is close to the onset (7:32) of the volcanic tremor and rapid tilt change (Yukutake et al 2017;Honda et al 2018). These results support the idea that the increase in pressure, caused by the intrusion of hydrothermal fluid, triggered the uplifting of the ground surface of the COA.…”

“…An abrupt increase in the number of earthquakes and volcanic tremors occurred at 7:32 a.m. (Japan Standard Time in this paper; JST = UTC + 9) (Yukutake et al 2017). The concurrently observed tilt change suggested formation of a NW-SE-trending crack near Owakudani (Honda et al 2018). On the same day, volcanic ashfall started at approximately 12:30.…”

Source constraints for the 2015 phreatic eruption of Hakone volcano, Japan, based on geological analysis and resistivity structure

“…The formation of the open crack was also revealed by a satellite InSAR analysis. Its upper limit (830 or 854 m asl) and the strike direction (NW-SE) deduced by both analyses were consistent (Doke et al 2018;Honda et al 2018).…”

“…On the morning of June 29 (Japan Standard Time in this study; JST = UTC + 9), an open crack formed beneath the Owakudani area, which was detected by tiltmeters and broadband seismometers (Honda et al 2018). The formation of the open crack was also revealed by a satellite InSAR analysis.…”

Special issue “Towards forecasting phreatic eruptions: examples from Hakone volcano and some global equivalents”

“…However, the extracted infrasound signal indicates an emission of highly pressurized fluid at 07:32 A.M. (JST), which is the timing of formation of the initial open crack that triggered the eruption . Honda et al (2018) observed a rapid tilt change at Hakone Volcano, which started 10 s before 07:33 A.M. (JST) and lasted for approximately 2 min. The tilt change, which occurred long before the initiation of ash dispersal at approximately 12:30 P.M. (JST), was observed not only by tiltmeters but also by broadband seismometers, which were temporarily deployed around the eruption center.…”