THE GRAVITY EFFECT OF CORE MODES FOR A ROTATING EARTH

Previous work has shown that the surface gravity effect associated wit h core modes (undertones) for a non-rotating Earth model is at or belo w the nanogal level of detectability for a large earthquake excitation . The assumption in the non-rotating case is that each undertone consi sts of a single spherical harmonic. In this study we add the Coriolis force to the equations governing the oscillations in a rigid shell wit h the Boussinesq approximation. By including up to 20 additional harmo nics, we find that there are many solutions that are still dominated b y low-degree spherical harmonics when judged by the distribution of ki netic energy among the harmonics. For these modes the contribution of higher degrees to the eigenfunctions is not substantial, which suggest s that the addition of rotation will probably not significantly change the gravity effects computed without rotation. This conclusion remain s tentative until verified in a fuller treatment that explicitly evalu ates the excitation for a rotating Earth model.