Electrokinetic effect of the Loma Prieta earthquake calculated by an entire-Earth FDTD solution of Maxwell's equations

We report what we believe to be the first three-dimensional computational solution of the full-vector Maxwell's equations for hypothesized pre-seismic electromagnetic phenomena propagated within the entire Earth-ionosphere cavity. Periodic boundary conditions are used in conjunction with a variable-cell finite-difference time-domain (FDTD) space lattice wrapping around the complete Earth-sphere and extending ±100 km radially from sea level. This technique permits a direct time-domain calculation of round-the-world ULF/ELF propagation accounting for arbitrary horizontal as well as vertical geometrical and electrical inhomogeneities/anisotropies of the excitation, ionosphere, lithosphere, and oceans. In this study, we model electrokinetic currents at depths of 2.5 km and 17 km near the hypocenter of the Loma Prieta earthquake and compare the FDTD-calculated surface magnetic field to analytical results and measurements previously reported in the literature. We accommodate the complete physics introduced by impulsive electromagnetic wave propagation through the conductive Earth, and hence illustrate the importance of solving the full Maxwell's equations when modeling current sources within the Earth's crust. Our calculated spectra agree qualitatively with those reported by Fraser-Smith et al. (1990).