10:00 〜 10:15
[SEM14-15] Resistivity structure of the Lau basin as inferred from M2 tidal magnetic variation at the seafloor
キーワード：電磁気、海洋潮汐、比抵抗、トンガ、ラウ海盆
Electrically conductive seawater moving in the ambient Earth’s magnetic field generates electric currents in the ocean and associated magnetic variation. Ocean tide is one of significant sources of the phenomena which leads to the tidally-induced magnetic variation depends on the subseafloor resistivity structure. Previous studies inferred global one-dimensional resistivity structures from tidal magnetic signals observed at satellite altitudes (Grayver et al. 2016; 2017), while other studies proved with forward calculations that tidal magnetic variation at the seafloor is also sensitive to the subseafloor resistivity structure (Schnepf et al. 2016; Zhang et al. 2019). The seafloor tidal magnetic data has, however, never been used in inversions before; it is mainly due to difficulty in combining strong local bathymetry effects and the spatial heterogeneity of the source flow field in the ocean.
In this study, we developed a three-dimensional (3-D) inversion code for seafloor tidal magnetic data based on the tetrahedral-mesh generator of Minami et al. (2017) and the 3-D CSEM inversion code of Minami et al. (2018). The mesh generator of Minami et al. (2017) was tailored for the numerical simulation of tsunami-induced magnetic variation, thereby is capable of including local bathymetry near observation sites and heterogenic source field in the ocean. We applied the developed inversion code to the M2 tidal period components in the magnetic data obtained at two west-east survey lines crossing the spreading axis at the Lau basin with the source of the tidal model of Egbert and Erofeeva (2002). The inversion was performed with the amplitude data of M2 vector magnetic signals at 11 sites. In the inversion, we successfully incorporated the precise geometry of the subducting Pacific slab in the tetrahedral mesh based of the slab2 model (Hayes et al. 2018).
The obtained model shows two notable features: a broad low resistivity zone extending from just below the seafloor to the depth around 75 km and a low resistivity zone beside the restive slab at the depth from 100 to 200 km. The former feature is not consistent with the previous MT studies (e.g. Matsukura, 2014, Master thesis of Kobe University) and implies high sensitivity of the tidal magnetic field to the resistivity of the oceanic crust. The latter feature is similar with the previous model by Matsukura (2014), but more conductive with the resistivity value of ~ 1 S/m. This conductive zone possibly delineates the dehydration process from the subducting Pacific plate below the Lau basin. We are now trying to perform joint inversions using both MT and tidal magnetic data at the Lau basin, where we currently face difficulty in different requirements for shallow resistivity from MT and tidal magnetic data: MT data requires higher resistivity than tidal magnetic data. In the presentation, we plan to report the inversion result using tidal magnetic variation at the Lau basin and compare it with preliminary results of the joint inversion using both MT and tidal magnetic data.
In this study, we developed a three-dimensional (3-D) inversion code for seafloor tidal magnetic data based on the tetrahedral-mesh generator of Minami et al. (2017) and the 3-D CSEM inversion code of Minami et al. (2018). The mesh generator of Minami et al. (2017) was tailored for the numerical simulation of tsunami-induced magnetic variation, thereby is capable of including local bathymetry near observation sites and heterogenic source field in the ocean. We applied the developed inversion code to the M2 tidal period components in the magnetic data obtained at two west-east survey lines crossing the spreading axis at the Lau basin with the source of the tidal model of Egbert and Erofeeva (2002). The inversion was performed with the amplitude data of M2 vector magnetic signals at 11 sites. In the inversion, we successfully incorporated the precise geometry of the subducting Pacific slab in the tetrahedral mesh based of the slab2 model (Hayes et al. 2018).
The obtained model shows two notable features: a broad low resistivity zone extending from just below the seafloor to the depth around 75 km and a low resistivity zone beside the restive slab at the depth from 100 to 200 km. The former feature is not consistent with the previous MT studies (e.g. Matsukura, 2014, Master thesis of Kobe University) and implies high sensitivity of the tidal magnetic field to the resistivity of the oceanic crust. The latter feature is similar with the previous model by Matsukura (2014), but more conductive with the resistivity value of ~ 1 S/m. This conductive zone possibly delineates the dehydration process from the subducting Pacific plate below the Lau basin. We are now trying to perform joint inversions using both MT and tidal magnetic data at the Lau basin, where we currently face difficulty in different requirements for shallow resistivity from MT and tidal magnetic data: MT data requires higher resistivity than tidal magnetic data. In the presentation, we plan to report the inversion result using tidal magnetic variation at the Lau basin and compare it with preliminary results of the joint inversion using both MT and tidal magnetic data.