6) ANOMALOUSLY SPLIT MODE 3 S 2

1. A new catalog of eigenfrequencies from the 26th December 2004 Sumatra-Andaman mega-event and first perspectives Julien Roch, Geneviève Roult and Eric Clévédé Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France. Contact: roch@ipgp.jussieu.fr 3) INTEREST of MODE OBSERVATIONS 1) INTRODUCTION The high-quality records obtained for the Sumatran-Andaman event of 2004, December 26th from all FDSN stations, allows to determine precise eigenfrequencies of some singlets from the gravest modes and from some « anomalous » modes. These data will help to better constrain density models of the deep Earth and increase our knowledge on some parameters for the whole Earth. 4) OBSERVATIONS 8) RESULTS 3S2 0S3 3S1 0S2 mode, ECH (France, G) 0S0 mode, CAN (Australia, G) 1S2 mode, ECH (France, G) -2 -1 0 +1 +2 -2 -1 0 +1 +2 2 mantle CMB ___compressional ---shear ___ density outer core ICB 2) DATA 3S2 mode, COLA (Alaska, IU) 3S1 mode, KMI (China, IU) 1S0 mode, BBSR (Atl. Oc., IU) inner core -2 -1 0 +1 +2 -1 0 +1 Some modes are very interesting due to their sensitivity to the outer and inner cores 0S3 0S0 20.9’ dr=0.05mm 0S2 53.9’ 35.6’ 0S4 3S1 0S5 7) SYNTHETICS SPECTRA 2S2 1S3 0T2 1S2 2S1 Using the HOPT method (Lognonné, 1991; Lognonné et Clévédé, 2002; Millot-Langet et al., 2003), we compute synthetics spectra with the PREM model taking into account both rotation and ellipticity and a 3D elastic model of the mantle (SAW12D). 0T3 0T4 All FDSN (GEOSCOPE, IRIS,...) stations used in this study. Too few were able to provide long time series without any problem (gaps, glitches…). 0S7 0S12 0S6 0S8 0S9 0S10 0S11 We got data more than 1 year after the Sumatra-Andaman event, via the NetDC procedure from 320 operational FDSN stations. We downloaded all data spanning the period 2004 December 25th to 2005 February 1st. Our goal was to obtain records of one month in continue, in order to improve the resolution. Unfortunately, lots of records have numerous gaps or glitches, it means that for some stations we got more than 30 files with gaps exceeding sometimes 5 minutes. We finally decided to restrict ambition to the 15 days continuous series, that it to say a selection of only 151 stations and 247 different recordings. We also compute synthetics spectra using a complex source (Tsai et al. 2005), and the Harvard CMT source, and we compare the results. The blue beachballs indicate the focal mechanism of the five Tsai et al. sources used to represent the fault rupture. The red one is the Harvard CMT solution. 2S3 2S5 1S6 3S2 1S5 2S4 1S0 1S4 4S2 1S8 1S7 2S6 1S9 UNM station. Spectrum of 10 days recording (Roult et al., 2008) 6) ANOMALOUSLY SPLIT MODE 3S2 5) FUNDAMENTAL MODES OS2 vertical components (10days) horizontal components (10days) The symbol size is proportional to measurement quality We plot all observed eigenfrequencies versus the station latitude (red dots and vertical red lines corresponding to the individual singlet mean value) 9) CONCLUSION The FDSN networks provide high quality records of the Sumatra mega-event and long-period spectra allowing a precise determination of eigenfrequencies for some modes and individual singlets. At present 17 modes have been precisely analyzed from both vertical and horizontal components (0S0, 1S0, 2S0, 3S0, 4S0, 0S2, 0S3, 0S4, 0S5, 2S1, 3S1, 11S1, 1S2, 3S2, 13S2, 2S3, 0T2). Our results show sometimes interesting discrepancy with prediction and known features. We are completing our collection in order to determine a density profile model and to improve the resolution of both attenuation and density in the Earth, parameters not well presently constrained. They will contribute to a better knowledge of the internal structure of the Earth. -2 -1 0 +1+2 m = -2 -1 0 +1 +2 All the anoumalously split modes, as 3S2, are sensitive to the inner core. The five black thin lines indicate the PREM-re theoretical eigenfrequencies. Two singlets are particularly well resolved, the singlet m=-2 and the ‘supposed’ singlet m=0. Despite few measurements for the singlet m=+1, we calculate the splitting ratio (r=1.70) that can be compared to the previously estimated 1.58 splitting ratio [He and Tromp, 1996]. Our results exhibit no large discrepancies with the theoretical values (black lines) computed in the PREM-re (PREM model [Dziewonski and Anderson, 1981] with rotation and ellipticity) [Millot et al., 2003], The observed splitting ratio r (ratio of the observed splitting width to the splitting width predicted due to the effects of rotation and hydrostatic ellipticity) is indicated (top left). The data are best fitted for the Tsai et al. complex rupture (5 sources) than for the Harvard mechanism. Roult G., J.Roch and E. Clévédé, 2008, A new catalog of eigenfrequencies from the 26th December 2004 Sumatra-Andaman mega-event, PEPI, submitted