Excitation of long-period free oscillations by the 2011 Tohoku earthquake: Numerical experiments and influence of source parameters

Long-period free oscillations provide robust information for the spatio-temporal characteristics of large earthquakes. In this study, we employ a normal-mode summation algorithm to generate three-component seismograms within an aspherical, anelastic, and rotating Earth model, focusing on the excitation of seismic normal modes by the 2011 Tohoku megathrust earthquake. Specifically, we analyze the effects of seismic source parameters, including fault geometry, focal depth, and rupture duration. By comparing synthetic free oscillation spectra with observed data, we validate several source mechanisms and emphasize the need for horizontal observations to improve seismic mechanism inversions. Our quantitative analyses reveal that among fault geometry parameters, dip and slip angles have a more pronounced impact on excitation amplitudes than fault strikes. Certain fault configurations enhance the detectability of specific modes, underscoring the relationship between fault geometry and mode excitation. Normal modes also exhibit varying sensitivity to focal depth, with significant excitation amplitude changes at discontinuity boundaries. Additionally, we demonstrate that while rupture duration can be inferred by minimizing differences between observed and synthetic spectra, more extensive records and modes should be included.