[S-CG48] 地殻深部のマグマ供給系の解明

[SCG48-P03] 地震波速度構造から明らかになった箱根火山のマグマ溜まりとマグマ供給系

*行竹 洋平¹、安部 祐希¹、本多 亮¹、酒井 慎一² (1.神奈川県温泉地学研究所、2.東京大学地震研究所)

キーワード：Magma reservoir、Velocity structure、Hakone volcano

Delineation of the characteristics of magmatic fluid such as melt and water beneath volcanoes has been challenging. In this study, we obtained a detailed picture of the magma reservoir beneath the felsic caldera volcano of Hakone, central Japan using a velocity model obtained from a dense seismic observation network. To obtain a highly resolved velocity structure, we used 2,279 earthquakes that occurred in and around Hakone volcano, applying the double-difference tomography method (Zhang and Thurber, 2003). We could resolve the velocity structure for both P and S waves with a spatial resolution of 3km up to the depth of 10km beneath Hakone volcano. In the shallow region of the caldera, a high-velocity region associated with a solidified magma where many earthquake swarms occur was estimated, and a significantly low-velocity region exists beneath the lower depth limit of seismicity, deeper than 6 km. The upper part of this low-velocity region is characterized by a low Vp/Vs zone that was interpreted as a water-rich region developed by the dehydration from the magma reservoir located beneath it. The temperature in this water-rich zone was estimated to be above 350°C regarding seismogenic depth. The upper depth of the magma reservoir was determined to be 9 km deep, where the high Vp/Vs was estimated. The volume fraction of melt was estimated as less than 10%, suggesting a highly crystallized mush structure. The low-velocity zone was estimated beneath the shallow magma reservoir at a depth of 15 km and the deep extension of the source region of deep low-frequency earthquakes. The activities of deep low-frequency earthquakes are synchronized with the shallow volcanic activity, such as the crustal expansion due to the inflation of pressure source at the depth around 6km and the earthquake swarms (Yukutake et al., 2019). The results could suggest the pathway of magma transport from the deep part of the volcano to this shallow reservoir and the upper-ward migration of magmatic fluid to the shallow reservoir is an important factor to cause the volcanic activities.

Acknowledgment
We thank the Earthquake Research Institute of Tokyo University, the National Research Institute for Earth Science and Disaster Prevention, the Japan Meteorological Agency, allowing us to use their waveform data. We used arrival time data from the unified earthquake catalog of the Japan Meteorological Agency. This work was funded from JSPS KAKENHI Grant Number 18K03816, ERI JURP 2017-M-01, 2017-M-03 and 2020-S-04. We thank H. Zhang, C. H. Thurber for providing the tomoDD and simul2000 code.

References
Yukutake, Y., Abe, Y., Doke, R., 2019. Deep Low-Frequency Earthquakes Beneath the Hakone Volcano, Central Japan, and their Relation to Volcanic Activity. Geophys. Res. Lett, doi:10.1029/2019gl084357.
Zhang, H., Thurber, C.H., 2003. Double-difference tomography: The method and its application to the Hayward Fault, California. Bull. Seismol. Soc. Am. 93, 1875-1889, doi:10.1785/0120020190.