5:15 PM - 6:45 PM
[SEM12-P05] Examination of subsurface resistivity in Kozushima Island using electric field variations generated by the 2011 Tohoku tsunami
Keywords:Kozushima, tsunami-induced electromagnetic field, the 2011off the Pacific coast of Tohoku Earthquake, resistivity
During tsunamis, electromagnetic (EM) field variations observable occur through electromagnetic induction as electrically conductive seawater moves within the Earth's main magnetic field (e.g., Tyler, 2005). In the case of the tsunami generated by the 2011 Tohoku earthquake, numerous tsunami-generated EM (TGEM) fields were observed on the seafloor and on land (Minami and Toh, 2013; Minami et al., 2017). In Kozushima, one of the islands in the Izu islands, electric field variations with peaks occurring almost simultaneously with the peak of tidal gauge data at Kozushima (~0.8m) have been observed (Nakatani, 2015, Bachelor's Thesis, Tokyo Gakugei University). While TGEM field variations observed on the seafloor are hardly affected by subsurface resistivity structures (Shimizu and Utada, 2015), those observed on islands are significantly influenced by subsurface resistivity structures, as reported by a numerical simulationstudy (Shibahara, 2022, Bachelor's Thesis, Kobe University). In this study, we attempted to infer the subsurface resistivity structure of Kozushima using electric field variation data assumed to be generated by tsunamis.
During the observation on Kozushima, conducted as part of the "International Frontier Research on Earthquake" by Dr. Yoshitaka Orihara and Mr. Yoichi Noda, two long baselines, oriented southeast-northwest (2.382km) and northeast-southwest (2.137km), were deployed, and two components of the electric potential difference were observed. In this study, we converted these potential difference data into electric field data for north-south and east-west components by dividing the two potential data by the baseline lengths and attempted to reproduce them through numerical calculations while varying the subsurface resistivity structure. For electric field calculations on Kozushima, we first generated the time evolution of the flow velocity field due to tsunamis using the tsunami calculation code COMCOT (Wang and Liu, 2006), and then performed electromagnetic field calculations using the time-domain finite-element TGEM simulation code TMTGEM (Minami et al., 2017) with the velocity field as input. Satake et al.'s (2013) source model was used for the tsunami calculation. For subsurface resistivity structures of Kozushima, EM fields were calculated using uniform resistivity structures of 1000, 100, and 10 Ωm, as well as a structure with a low resistivity region along the southeastern coastline of the island. In the EM field calculations, pseudo-observation points were placed at intervals of 238.2m along the southeast-northwest baseline and 213.7m along the northeast-southwest baseline, and electric fields calculated at these pseudo-observation points were integrated in the directions of the baselines to calculate potential differences comparable to observations.
In the electric field variations calculated in this study, when a uniform structure was used underground, the larger the subsurface resistivity value becomes, the larger the amplitude of the electric field does. Additionally, variation amplitudes of electric fields calculated with a uniform resistivity structure becomes large in the north-south direction and small in the east-west direction, which is attributed to the arrival of the tsunami from the east side of the island. In this study, to reproduce the east-west component of the observed electric field variation, we placed a low-resistivity region in the shape of a flipped L along the eastern edge of the island, referring to Ichihara and Mogi (2009). This L-shaped low resistivity zone successfully matched the phases of the observed electric field variatio to the obesrved data. However, since the observed electric field variation is stronger in the east-west direction than in the north-south direction, there is room for improvement in the subsurface resistivity structure of Kozushima. Based on the results of this study, it is found that there is a resistivity structure that enhances east-west electric field variations even when the tsunami reaches the island from the eastern side. In future studies, we plan to further investigate subsurface resistivity structures that explains electric field observations better and, on the other hand, consider the possibility that electric field variations on Kozushima was caused by factors other than tsunamis.
During the observation on Kozushima, conducted as part of the "International Frontier Research on Earthquake" by Dr. Yoshitaka Orihara and Mr. Yoichi Noda, two long baselines, oriented southeast-northwest (2.382km) and northeast-southwest (2.137km), were deployed, and two components of the electric potential difference were observed. In this study, we converted these potential difference data into electric field data for north-south and east-west components by dividing the two potential data by the baseline lengths and attempted to reproduce them through numerical calculations while varying the subsurface resistivity structure. For electric field calculations on Kozushima, we first generated the time evolution of the flow velocity field due to tsunamis using the tsunami calculation code COMCOT (Wang and Liu, 2006), and then performed electromagnetic field calculations using the time-domain finite-element TGEM simulation code TMTGEM (Minami et al., 2017) with the velocity field as input. Satake et al.'s (2013) source model was used for the tsunami calculation. For subsurface resistivity structures of Kozushima, EM fields were calculated using uniform resistivity structures of 1000, 100, and 10 Ωm, as well as a structure with a low resistivity region along the southeastern coastline of the island. In the EM field calculations, pseudo-observation points were placed at intervals of 238.2m along the southeast-northwest baseline and 213.7m along the northeast-southwest baseline, and electric fields calculated at these pseudo-observation points were integrated in the directions of the baselines to calculate potential differences comparable to observations.
In the electric field variations calculated in this study, when a uniform structure was used underground, the larger the subsurface resistivity value becomes, the larger the amplitude of the electric field does. Additionally, variation amplitudes of electric fields calculated with a uniform resistivity structure becomes large in the north-south direction and small in the east-west direction, which is attributed to the arrival of the tsunami from the east side of the island. In this study, to reproduce the east-west component of the observed electric field variation, we placed a low-resistivity region in the shape of a flipped L along the eastern edge of the island, referring to Ichihara and Mogi (2009). This L-shaped low resistivity zone successfully matched the phases of the observed electric field variatio to the obesrved data. However, since the observed electric field variation is stronger in the east-west direction than in the north-south direction, there is room for improvement in the subsurface resistivity structure of Kozushima. Based on the results of this study, it is found that there is a resistivity structure that enhances east-west electric field variations even when the tsunami reaches the island from the eastern side. In future studies, we plan to further investigate subsurface resistivity structures that explains electric field observations better and, on the other hand, consider the possibility that electric field variations on Kozushima was caused by factors other than tsunamis.