[S-VC31] Active Volcanism

[SVC31-P07] Temporary seismic observation at Hakone volcano and results of seismic interferometry analysis

*Mare Yamamoto¹, Shoki Akiyama¹, Hiroshi Aoyama², Takao Ohminato³, Akihiko Terada⁴, Yuta Maeda⁵, Takahiro Ohkura⁶, Takeshi Matsushima⁷, Haruhisa Nakamichi⁸, Ryou Honda⁹ (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2.Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, 3.Earthquake Research Institute, Tokyo University, 4.Volcanic Fluid Research Center, Tokyo Institute of Technology, 5.Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, 6.Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 7.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 8. Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, 9. Hot Springs Research Institute of Kanagawa Prefecture)

Hakone volcano is a Quaternary volcano located at the northern end of the Izu-Ogasawara arc and consists of a caldera and central cones. The caldera is located on the intersection area of Tanna and Hirayama faults which extend south and north of the caldera respectively, and the relationship between the pull-apart structure and volcanic activity has been discussed (e.g., Takahashi and Koyama, 1993). Recent volcanic activities have taken place around Kamiyama (Owakudani), and a small phreatic eruption occurred in 2015. The seismic velocity structure around Hakone volcano has been studied in detail by Yukutake et al. (2021) using body-wave tomography with regional earthquakes, and the fluid supply path from the lower crust to the surface has been proposed. On the other hand, due to the limitation of ray path crossings, the spatial resolution is not necessarily sufficient for structures shallower than about 6 km in depth, where shallow magma reservoir and seismically active zone are located. Therefore, we conducted a temporary seismic observation as a part of the "Earthquake and Volcano Hazards Observation and Research Program" to understand the structure of shallow fluid pathways that mainly control volcanic activities on the surface. In this presentation, we will give an overview of the temporary observations and report on the characteristics of the surface waves retrieved by seismic interferometry analysis.

In this temporary observation, we installed 17 stations inside and outside the caldera to supplement the existing seismic network operated by Hot Springs Research Institute of Kanagawa Prefecture, and we continued the observation for about a year from February 2022. The location of the temporary stations was selected within 10 km from the central cone, and mainly covers the west side of the caldera where the existing observation network is rather sparse. Except four stations where we used a cellular network for the telemetry, the other 13 stations were observed offline, and the data at all the stations are recorded continuously with a sampling rate of 200 Hz.

Using the vertical seismogram recorded at the temporary stations and the existing seismic network, we first performed seismic interferometry analysis focusing on the Rayleigh wave. We used the period band from 1 s to 10 s to compute the cross-correlation function between each station pair, aiming to estimate the structure of the shallow depth up to about 5 km depth. The obtained group velocity of the fundamental mode is generally low (around 1.2 km/s) for periods of about 3 s or less, and it shows clear normal dispersion in longer period bands. In addition, the group velocity is lower in the west side of the caldera than in the east side in all period band. The retrieved dispersion relation of the fundamental Rayleigh wave is consistent with the velocity structure by Hiraga (1987) and Yukutake et al. (2021) on average, and the difference between the east and west sides of the caldera is considered to be related to a decrease of S-wave velocity caused by fractures within the seismogenic zone (shallower than 6 km depth) just beneath the central crater.
Further analysis of three-dimensional velocity structure from the retrieved Rayleigh wave as well as the estimation of anisotropy using the Love wave may contribute to a better understanding of the fluid pathway beneath Hakone volcano.

Acknowledgments: We would like to express our sincere appreciation for the cooperation and support of all concerned in conducting this observation. We also thank the Earthquake Research Institute, the University of Tokyo, Joint Research program 2002-M-03, 04, 05, 06 for the use of their observation equipment.