Abstract
A geomagnetic observatory named SFEMS is being operated on the deep seafloor in the northwest Pacific since August, 2001. SFEMS is capable of measuring both scalar and vector geomagnetic fields as well as the seafloor instrument’s precise attitudes, which makes it a powerful tool in detecting the so-called oceanic dynamo effect. It was found that SFEMS captured clear magnetic signals generated by the giant tsunamis of the 2011 Tohoku Earthquake even for an epicentral distance of larger than 1500 km. Here we report estimates of the focal mechanism of a closer tsunamigenic earthquake in January, 2007 on the seaward slope of the Kuril Trench using tsunami-generated variations in the observed downward magnetic component. Three-dimensional solutions of the tsunami-generated magnetic components were calculated by a new numerical code based on non-uniform thin-sheet approximation and particle motions of seawater using the linear Boussinesq approximation. As a result, a southeast dipping fault alone reproduced the dispersive nature of the downward magnetic component, while any northwest dipping faults could not. This implies that the tsunami-generated electromagnetic fields are useful for determination of focal mechanisms of tsunamigenic earthquakes, since fault dips are one of the most difficult source parameters to estimate even in modern seismology.
Highlights
A large tsunamigenic earthquake occurred along the Kuril-Kamchatka trench in January, 2007 (Table 1)
The scalar and vector measurements of the geomagnetic field were conducted by an Overhauser proton precession magnetometer and a three-component fluxgate-type variograph with resolutions of 0.1 nT and 0.01 nT, respectively
SeaFloor ElectroMagnetic Station (SFEMS) is equipped with a fibre optical gyro, a two-component horizontal tilt-meter with a resolution of 0.9 arcsec and a thermometer with a resolution of 0.01 °C for post-retrieval calibration
Summary
For kinetic simulation of tsunami propagation, we solved the linear Boussinesq momentum equation by modifying the following finite-difference code: Cornell Multi-grid Coupled Tsunami Model (COMCOT15, Version 1.7). COMCOT originally employed the linear shallow water momentum equation in the spherical coordinate system and uses an explicit leapfrog finite-difference method for its time evolution. The linear shallow water momentum equations can be given as follows;. We solved the linear shallow water momentum equations (2) and (3) on the lateral boundaries to obtain boundary values of Ψ using Eq (6). The induction equation for the magnetic field, b, in the frequency domain is given by;. Using the horizontal electric field within the thin sheet, E* derived, the horizontal components of the tsunami-generated magnetic field on the seafloor is given by;. The vertical magnetic component can be calculated by taking curl of E*
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