3:45 PM - 4:00 PM
[SEM16-08] The dependence of the tsunami-genereted electromagnetic fields due to tsunamis on the subsurface resistivity
Keywords:tsunami, electrical conductivity
The electromagnetic (EM) field variations due to large tsunamis have been observed at seafloor stations and land ones near the sea. Since the subsurface resistivity may affect the tsunami EM field variations, the phenomena could be applied to the investigation of the resistivity structure as well as prediction of tsunami arrivals and heights. Shimizu and Utada (2015) revealed that the EM field variations due to long waves are less affected by the subsurface resistivity at the flat seafloor in the period range of tsunamis. However, the dependence of the EM field observed on land on the subsurface resistivity has not been clarified and needs further investigation.
In this study, we investigated the subsurface resistivity dependence of the tsunami-generated EM field on the island by numerical calculations for the following two cases. In Case 1, plane waves propagate in a flat seafloor with a constant depth and pass through a simple conical island; in Case 2, the 2011 Tohoku tsunami reached Chichijima Island. In both cases, we assumed a simple semi-infinite homogeneous resistivity structure below the island and the adjacent seafloor, and used the tsunami EM field simulation code TMTGEM (Minami et al. 2017). In the EM field calculations for Case 1, we tested two radii of the island of 3, 6km and three homogeneous subsurface resistivities of 1000, 100, and 10 Ωm. We found in the result of Case 1 that the amplitude of the vertical component of the magnetic field (Bz) varies by a maximum of 10% and the horizontal component of the electric field (Eh) varies by a maximum of 40%, in terms of those for 1000 Ωm. The variations in the amplitude were not affected by the radius of the island. In Case 2, i.e. the case of Chichijima, we used the fault model of the 2011 Tohoku earthquake estimated by Tatehata et al. (2015) as the tsunami source for tsunami simulation and tested three subsurface resistivities in the same manner as Case 1. The results of Case 2 show that the amplitude of Eh varies by a maximum of 60%, and that of Bz varies by a maximum of 30%, in terms of those for 1000 Ωm. The results both in Case 1 and 2 indicate that the tsunami-generated Eh on the island is affected by the subsurface resistivity and may be possible to utilize in exploring the resistivity structure of islands. We succeeded in explaining the dependence of the amplitude of Eh on the subsurface resistivity by a simple stationary parallel circuit model. This circuit model implies that the electric field variation observed at the island does not depend on the resistivity deeper locations than the island.
In this study, we investigated the subsurface resistivity dependence of the tsunami-generated EM field on the island by numerical calculations for the following two cases. In Case 1, plane waves propagate in a flat seafloor with a constant depth and pass through a simple conical island; in Case 2, the 2011 Tohoku tsunami reached Chichijima Island. In both cases, we assumed a simple semi-infinite homogeneous resistivity structure below the island and the adjacent seafloor, and used the tsunami EM field simulation code TMTGEM (Minami et al. 2017). In the EM field calculations for Case 1, we tested two radii of the island of 3, 6km and three homogeneous subsurface resistivities of 1000, 100, and 10 Ωm. We found in the result of Case 1 that the amplitude of the vertical component of the magnetic field (Bz) varies by a maximum of 10% and the horizontal component of the electric field (Eh) varies by a maximum of 40%, in terms of those for 1000 Ωm. The variations in the amplitude were not affected by the radius of the island. In Case 2, i.e. the case of Chichijima, we used the fault model of the 2011 Tohoku earthquake estimated by Tatehata et al. (2015) as the tsunami source for tsunami simulation and tested three subsurface resistivities in the same manner as Case 1. The results of Case 2 show that the amplitude of Eh varies by a maximum of 60%, and that of Bz varies by a maximum of 30%, in terms of those for 1000 Ωm. The results both in Case 1 and 2 indicate that the tsunami-generated Eh on the island is affected by the subsurface resistivity and may be possible to utilize in exploring the resistivity structure of islands. We succeeded in explaining the dependence of the amplitude of Eh on the subsurface resistivity by a simple stationary parallel circuit model. This circuit model implies that the electric field variation observed at the island does not depend on the resistivity deeper locations than the island.