[S-EM12] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM12-P08] Interpretation of anomalous phases observed around Akan caldera

*Tomohiro Inoue¹, Koki Aizawa¹, Hiroshi Ichihara², Takeshi Hashimoto³ (1.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 2.Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, 3.Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University)

Keywords:Magnetotellurics, anomalous phases, 3-D resistivity modeling

Magnetotelluric (MT) data is sometimes accompanied by “anomalous” impedance phases in the off-diagonal components that deviate from the normal quadrants. This phenomenon is called the phases out-of-quadrant (POQ). The POQ is difficult to explain in terms of simple 1- or 2-D structure. However, the POQ can contribute to modelling 3-D resistivity structure. The POQ appeared in the Zyx component at the Akan caldera in eastern Hokkaido. Inoue (2023, Ph.D. dissertation) performed a 3-D resistivity inversion with the POQ and imaged an oblique conductive column (OCC) tilted west from the Mt. Meakandake volcano. The conductive column well explained the POQ at the northwestern foot of the volcano. Similar POQ can be seen also at a few MT sites on the southern foot of Mt. Meakandake. However, the relationship between the POQ in this area and the above-mentioned OCC was not clear because of sparse data coverage. Therefore, we conducted some additional MT measurements in Rikubestu and Ashoro areas on the western side of the Akan caldera in 2023. We acquired the time series of five components (two electric fields + three magnetic fields) for about three days using the ADU07e system (Metronix Ltd.) at nine sites. At seven sites, we also measured electric fields alone using the Elog-1k system (NT System Design Ltd.). In calculating the response functions, we used the BIRRP (Chave and Thomson, 2004) code and applied the remote reference processing.
We found POQs in the period bands longer than about 500 s for the Zyx component from Ashoro to Rikubetsu regions. In these regions, induction vectors pointed to the east in the same period bands. On the other hand, the major axes of the phase tensor ellipse in Rikubetsu were mainly in the east-west directions, whereas the major axes in Ashoro were in the northwest-southeast directions. The Phi2 around Rikubetsu showed more or less 45^o, while those around Ashoro indicated about 20^o, suggesting a resistive layer underneath.
Based on the 3-D resistivity structure (Inoue, 2023; Ph.D. dissertation) together with generic model that an OCC buried in a resistive host rock can generate POQs (Inoue and Hashimoto, in revision), the POQ on the northwestern side (Rikubetsu region) of the Akan caldera is likely to occur due to the OCC extending westward from Mt. Meakandake. Meanwhile, the difference in the phase tensor and Phi2 between Ashoro and Rikubetsu suggests structural differences between these regions. We suspected that the POQ in Ashoro was produced by different mechanism other than the OCC model. This study discusses the generation mechanism of the POQ based on the updated 3-D conductivity model in which the additional data is incorporated.

Acknowledgments: This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).