Observation of the seismic anisotropy effects on free oscillations below 4 mHz

Abstract

We present observations of significant fundamental spheroidal‐toroidal mode coupling at frequencies below 4 mHz in the early part of vertical component records from seismic stations on near‐equatorial source‐receiver propagation paths after the 26 December 2004 and 28 March 2005 great Sumatra earthquakes. Since the mixed‐type coupling induced by rotational Coriolis force are very weak at these selected equatorial‐path stations, we investigate what effects mimic the strong Coriolis effects at frequencies below 4 mHz, suggesting that local azimuthal anisotropy in the upper mantle is the most likely explanation for the strong anomalous coupling we observed. In addition, strong anisotropy coupling observed in the frequency band of 1–4 mHz is always characterized by anomalous small amplitudes of coupled spheroidal modes on the vertical component of seismograph, suggesting that excitation of quasi‐toroidal modes by azimuthal anisotropy associates with geometric nodes of fundamental spheroidal modes.