1:45 PM - 2:00 PM
[SEM16-01] Magnetotelluric survey across Hokkaido
Keywords:Magnetotelluric, Resistivity structure
Many magnetotelluric (MT) resistivity surveys have been conducted in Hokkaido, where there is little electromagnetic noise (e.g., Ogawa et al., 1994; Uyeshima et al., 2001; Mogi et al., 2002; Ichihara et al., 2019). However, most previous studies were based on surveys along relatively short lines or in a restricted area. Therefore, the regional resistivity structure of the entire Hokkaido still has remained uncovered. In this study, aiming to break this situation, we conducted MT measurements along an extensive survey line crossing east-west around N43.5º in the central part of Hokkaido. Our survey line runs roughly perpendicular to the principal geological strikes of this area. In addition, it includes some exciting targets in geological and tectonic contexts, such as an active fault zone and volcanism.
We performed broadband MT measurements at 21 sites from 2019 to 2021. In addition to the data, we used the data obtained in the previous studies at four sites in the Teshikaga area from Ichihara et al. (2013), two sites near Mt. Tokachidake Volcano from the Hokkaido Research Organization (2017), and two sites near Mt. Meakandake Volcano from Inoue (2020). Based on these integrated data, we modeled a resistivity structure using the 3D inversion code (ModEM: Egbert and Kelbert (2012); Kelbert et al., 2014), taking into account the sea-land distribution.
Figure 2 shows the resistivity cross-section of our preliminary model. Several significant resistivity anomalies can be recognized. In the westernmost part of this survey line, the resistivity is high (R1) at depth greater than about 10 km. In addition, we recognize an undulation of the upper boundary of R1, suggesting the location of the reverse fault zone mentioned above. However, no significant low resistivity anomaly is found in the fault zone itself or its deeper extension.
Meanwhile, we recognize significant low resistivity zones C1 and C3 below Mt. Tokachidake and Mt. Meakandake Volcanoes, respectively. Compared with the seismic velocity structure (Koulakov et al., 2015) and the seismic attenuation structure (Kita et al., 2014), it is likely that these conductors correspond to low velocity and low Qp anomalies. Low resistivity zone C4 found in the shallow part of the eastern margin of the survey line is consistent with the conductive layer of the Neogene sediments inferred by Satoh et al. (2001). The high resistivity zones R1 to R3 seems to correspond to the seismic high-velocity anomalies, while the low resistivity zone C2 does to the seismic low-velocity anomaly.
Acknowledgments: The Hokkaido Research Organization provided MT data around Mt. Tokachidake, Hiroshi Ichihara of Nagoya University provided the data around the Teshikaga area, and Tomohiro Inoue of Hokkaido University provided the data around Mt. Meakandake. We thank T. Inoue, R. Ito, K. Shibuya, and A. Narita of Hokkaido University for their cooperation in fieldwork.
We performed broadband MT measurements at 21 sites from 2019 to 2021. In addition to the data, we used the data obtained in the previous studies at four sites in the Teshikaga area from Ichihara et al. (2013), two sites near Mt. Tokachidake Volcano from the Hokkaido Research Organization (2017), and two sites near Mt. Meakandake Volcano from Inoue (2020). Based on these integrated data, we modeled a resistivity structure using the 3D inversion code (ModEM: Egbert and Kelbert (2012); Kelbert et al., 2014), taking into account the sea-land distribution.
Figure 2 shows the resistivity cross-section of our preliminary model. Several significant resistivity anomalies can be recognized. In the westernmost part of this survey line, the resistivity is high (R1) at depth greater than about 10 km. In addition, we recognize an undulation of the upper boundary of R1, suggesting the location of the reverse fault zone mentioned above. However, no significant low resistivity anomaly is found in the fault zone itself or its deeper extension.
Meanwhile, we recognize significant low resistivity zones C1 and C3 below Mt. Tokachidake and Mt. Meakandake Volcanoes, respectively. Compared with the seismic velocity structure (Koulakov et al., 2015) and the seismic attenuation structure (Kita et al., 2014), it is likely that these conductors correspond to low velocity and low Qp anomalies. Low resistivity zone C4 found in the shallow part of the eastern margin of the survey line is consistent with the conductive layer of the Neogene sediments inferred by Satoh et al. (2001). The high resistivity zones R1 to R3 seems to correspond to the seismic high-velocity anomalies, while the low resistivity zone C2 does to the seismic low-velocity anomaly.
Acknowledgments: The Hokkaido Research Organization provided MT data around Mt. Tokachidake, Hiroshi Ichihara of Nagoya University provided the data around the Teshikaga area, and Tomohiro Inoue of Hokkaido University provided the data around Mt. Meakandake. We thank T. Inoue, R. Ito, K. Shibuya, and A. Narita of Hokkaido University for their cooperation in fieldwork.