11:15 〜 11:30
[SEM12-08] Deep Three-Dimensional Electrical Conductivity Structure of Mt. Tokachi and Its Surrounding, Central Hokkaido, Japan
キーワード：magnetotellurics、electrical conductivity、Mt. Tokachi、magmatic-hydrothermal system、three-dimensional inversion
Mount Tokachi (Tokachidake) is an active volcano in the southern part of the Taisetsu-Tokachi volcanic chain, stretching ca. 50 km northeast-southwest across central Hokkaido. It experienced three magmatic eruptions in the 20th century and is considered likely to erupt again in the future. Information on the subsurface magma-plumbing system is essential to evaluate future medium- to long-term eruptive activity. However, there are few previous studies on the deep structure of this area, especially on the electrical conductivity at depths deeper than 10 km. Therefore, this study aims to elucidate the conductivities down to 50 km deep by using broadband MT in and around Mt. Tokachi and its surrounding areas.
We newly deployed our MT survey at 23 sites in 2022 and 2023 to the south and north of the central part of the EW survey line of Iwama (2022, Master thesis) across central Hokkaido. We performed a 3D inversion analysis using the MT impedance from a total of 50 stations together with the existing data. We found a conductor (C3) about 20-40 km deep beneath Mt. Tokachi. It seems to dip northward, although the extent is still being determined, especially on the northeast side, where we have no observation. On the other hand, we recognized a conductive layer in the shallow part below and around Mt. Tokachi and Mt. Biei at depths of several kilometers. However, the resolution of the shallow part needs to be improved with a finer model mesh to discuss it in detail. Another high conductive anomaly was also imaged at a 5-10 km depth on the east side of Mt. Biei. It appears connected to the above-mentioned deep C3, but the detail is still questionable because we have no observation directly above the anomaly. Previously, Iwama (2022) also reported a deep conductor corresponding to C3. In the present study, the planar configuration of the survey deployment made a sensitivity test possible and supported the existence of the anomalous body, especially for shallower than ca. 30 km.
It seems that C3 is an extensive conductor lying deep below the entire volcanic chain rather than localized directly beneath Mt. Tokachi. We currently see active volcanoes only on the northern and southern margins of C3, each accompanied by deep low-frequency earthquakes. C3 also corresponds well with the distribution of seismic low-velocity anomalies (Koulakov et al., 2014; Niu et al., 2018) and roughly lies between the Conrad and Moho discontinuities. Based on these observations, we suspect that C3 is an extensive partial melt zone common to the overlying magmatic systems of the Taisetsu-Tokachi volcanic chain.
Acknowledgments: In the inversion modeling of this study, we partly used the MT data of Takahashi et al. (2017), which the Hokkaido Research Organization provided. Deepest thanks to K. Aoyama, S. Nishikawa, and A. Takeda for their assistance in the fieldwork.
We newly deployed our MT survey at 23 sites in 2022 and 2023 to the south and north of the central part of the EW survey line of Iwama (2022, Master thesis) across central Hokkaido. We performed a 3D inversion analysis using the MT impedance from a total of 50 stations together with the existing data. We found a conductor (C3) about 20-40 km deep beneath Mt. Tokachi. It seems to dip northward, although the extent is still being determined, especially on the northeast side, where we have no observation. On the other hand, we recognized a conductive layer in the shallow part below and around Mt. Tokachi and Mt. Biei at depths of several kilometers. However, the resolution of the shallow part needs to be improved with a finer model mesh to discuss it in detail. Another high conductive anomaly was also imaged at a 5-10 km depth on the east side of Mt. Biei. It appears connected to the above-mentioned deep C3, but the detail is still questionable because we have no observation directly above the anomaly. Previously, Iwama (2022) also reported a deep conductor corresponding to C3. In the present study, the planar configuration of the survey deployment made a sensitivity test possible and supported the existence of the anomalous body, especially for shallower than ca. 30 km.
It seems that C3 is an extensive conductor lying deep below the entire volcanic chain rather than localized directly beneath Mt. Tokachi. We currently see active volcanoes only on the northern and southern margins of C3, each accompanied by deep low-frequency earthquakes. C3 also corresponds well with the distribution of seismic low-velocity anomalies (Koulakov et al., 2014; Niu et al., 2018) and roughly lies between the Conrad and Moho discontinuities. Based on these observations, we suspect that C3 is an extensive partial melt zone common to the overlying magmatic systems of the Taisetsu-Tokachi volcanic chain.
Acknowledgments: In the inversion modeling of this study, we partly used the MT data of Takahashi et al. (2017), which the Hokkaido Research Organization provided. Deepest thanks to K. Aoyama, S. Nishikawa, and A. Takeda for their assistance in the fieldwork.