5:15 PM - 7:15 PM
[SEM15-P10] Investigating the cause of local geomagnetic field changes within the summit crater area of Tarumae Volcano
Keywords:Tarumae Volcano, anomalous phase response, geomagnetic field
Tarumae Volcano is one of the post-caldera volcanoes of the Shikotsu volcanoes in southwestern Hokkaido. It began eruptive activity ca. 9ka (Furukawa & Nakagawa, 2010). Within the summit crater area, the Japan Meteorological Agency (JMA) and Hokkaido University have repeated annual survey of geomagnetic total field to monitor the volcanic activity since 1999. These surveys aim to detect variations related to volcanic activity on timescales of months to years, and they reported re/demagnetization correlated with ground deformation of such timescales (e.g., Hashimoto et al., 2018).
In such repeated surveys, it is generally assumed that geomagnetic variations of extra-terrestrial origin are uniform across the area. However, continuous observations and recent temporary surveys by the JMA have shown that the long-period variations (from hours to a day) can differ by several nT even among closely spaced sites. Although such site-dependent differences are generally small and have not been a major concern, they are pronounced at Mt. Tarumae, resulting in errors in detection of volcano-related changes and influencing the source modeling. To gain insight into the causes of these local anomalies, we carried out EM field observations within the summit crater area.
From August 21 to September 3, 2024, we deployed our instruments at three sites within the summit crater area, measuring two and three components of the E-field and H-field, respectively. We used the Mag-13 fluxgate magnetic sensors (Bartington Inst.) and the Elog-MT data-loggers (NT-System Design). Initially, visual inspection of the time series plots for each component proved insufficient to identify site-specific features. Consequently, we calculated MT response functions to investigate whether any differences emerged among the observation sites. This approach was motivated by the hypothesis that the volcano’s heterogeneous subsurface might cause induced current channeling in a specific region, and their secondary geomagnetic fields could cause localized geomagnetic variations.
Since our data was partly contaminated with noise in the E-field, we applied remote-reference processing using the field data from the Noto Peninsula provided by Disaster Prevention Research Institute, Kyoto University. We calculated apparent resistivity and phase using the BIRRP code (Chave and Thomson, 2004). One of the three sites exhibited out-of-quadrant phase (POQ) responses in the YX component at long periods. Such a feature resembled the effect of an inclined conductive column proposed by Inoue and Hashimoto (2024). However, data quality is not very high due to artificial noise, leaving our results inconclusive.
According to the resistivity model presented by Yamaya et al. (2009), a very conductive part is inferred right beneath the summit lava dome. Airborne magnetic surveys have also indicated a tilted, columnar re-magnetizing region beneath the dome (Shibuya, 2022). Furthermore, broadband MT surveys suggest the presence of a conductive zone extending from Tarumae Volcano toward the depths in the northwest (Yamagiwa, 2022). If these anomalies are inter-connected, they could be a structure that concentrates induced currents, potentially leading to the POQs in long-period bands and localized geomagnetic variations. However, this hypothesis has yet to be verified. Future efforts should include forward modeling to simulate how such distinctive structures reproduce the observed POQ, followed by a detailed comparison with the observational data.
Acknowledgments: We express our sincere thanks to Profs. R. Yoshimura and M. Hata of Kyoto University for providing the fluxgate sensors. We extend our deepest gratitude to T. Fujita, N. Fujiwara, Y. Fukuyasu, Y. Hirado, N. Izumi, A. Onishi, S. Nakaya, Y. Tsutsui, and H. Ogawa for their assistance in the fieldwork.
In such repeated surveys, it is generally assumed that geomagnetic variations of extra-terrestrial origin are uniform across the area. However, continuous observations and recent temporary surveys by the JMA have shown that the long-period variations (from hours to a day) can differ by several nT even among closely spaced sites. Although such site-dependent differences are generally small and have not been a major concern, they are pronounced at Mt. Tarumae, resulting in errors in detection of volcano-related changes and influencing the source modeling. To gain insight into the causes of these local anomalies, we carried out EM field observations within the summit crater area.
From August 21 to September 3, 2024, we deployed our instruments at three sites within the summit crater area, measuring two and three components of the E-field and H-field, respectively. We used the Mag-13 fluxgate magnetic sensors (Bartington Inst.) and the Elog-MT data-loggers (NT-System Design). Initially, visual inspection of the time series plots for each component proved insufficient to identify site-specific features. Consequently, we calculated MT response functions to investigate whether any differences emerged among the observation sites. This approach was motivated by the hypothesis that the volcano’s heterogeneous subsurface might cause induced current channeling in a specific region, and their secondary geomagnetic fields could cause localized geomagnetic variations.
Since our data was partly contaminated with noise in the E-field, we applied remote-reference processing using the field data from the Noto Peninsula provided by Disaster Prevention Research Institute, Kyoto University. We calculated apparent resistivity and phase using the BIRRP code (Chave and Thomson, 2004). One of the three sites exhibited out-of-quadrant phase (POQ) responses in the YX component at long periods. Such a feature resembled the effect of an inclined conductive column proposed by Inoue and Hashimoto (2024). However, data quality is not very high due to artificial noise, leaving our results inconclusive.
According to the resistivity model presented by Yamaya et al. (2009), a very conductive part is inferred right beneath the summit lava dome. Airborne magnetic surveys have also indicated a tilted, columnar re-magnetizing region beneath the dome (Shibuya, 2022). Furthermore, broadband MT surveys suggest the presence of a conductive zone extending from Tarumae Volcano toward the depths in the northwest (Yamagiwa, 2022). If these anomalies are inter-connected, they could be a structure that concentrates induced currents, potentially leading to the POQs in long-period bands and localized geomagnetic variations. However, this hypothesis has yet to be verified. Future efforts should include forward modeling to simulate how such distinctive structures reproduce the observed POQ, followed by a detailed comparison with the observational data.
Acknowledgments: We express our sincere thanks to Profs. R. Yoshimura and M. Hata of Kyoto University for providing the fluxgate sensors. We extend our deepest gratitude to T. Fujita, N. Fujiwara, Y. Fukuyasu, Y. Hirado, N. Izumi, A. Onishi, S. Nakaya, Y. Tsutsui, and H. Ogawa for their assistance in the fieldwork.