Observation of split modes from the 26th December 2004 Sumatra-Andaman mega-event
Résumé
The high quality of the records obtained after the huge Sumatra-Andaman earthquake of December 2004 from the broad-band seismological stations of all FDSN networks offers unique opportunity for studying the free oscillations of the Earth and consequently the behavior of some particular modes. The observation and clear identification of singlets of the gravest seismic normal modes (frequencies lower than 1 mHz) and of some particular anomalously split modes should offer strong constraints on Earth structure. We carefully analyzed 23 different modes and report the detailed results of 17 of them, showing an enhancement on the eigenfrequencies determination thanks to the high SNR (Signal to Noise Ratio) of the recordings. We analyzed vertical component recordings but also additional horizontal components. We present modal frequency measurements of some spheroïdal fundamental gravest modes, of some particular higher modes such as inner core modes exhibiting normal or anomalous splitting. The results are compared with the theoretical eigenfrequencies computed for the PREM model, taking into account both rotation and ellipticity effects of the Earth and lateral heterogeneities of the SAW12D model ( Li and Romanowicz, 1996). The misfit between measured and theoretical eigenfrequencies point out that some physical parameters are not well constrained. The new catalogue of modal eigenfrequencies provided from the Sumatra-Andaman 2004 event analysis offers a plethora of perspectives; it is an important step towards the resolution of deep Earth parameters.