6) ANOMALOUSLY SPLIT MODE 3 S 2 4) OBSERVATIONS 3) INTEREST of MODE OBSERVATIONS 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, Paris, France. Contact: 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. Some modes are very interesting due to their sensitivity to the outer and inner cores UNM station. Spectrum of 10 days recording (Roult et al., 2008) 2) DATA 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. inner core outer core vertical components (10days) horizontal components (10days) The symbol size is proportional to measurement quality All the anoumalously split modes, as 3 S 2, 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]. 2 0 S 2 mode, ECH (France, G) 3 S 2 mode, COLA (Alaska, IU) 0 S 0 mode, CAN (Australia, G) S 0 mode, BBSR (Atl. Oc., IU) 3 S 1 mode, KMI (China, IU) S20S2 53.9’ 2S12S1 0T20T2 0S30S3 35.6’ 20.9’ dr=0.05mm 0S00S0 0S40S4 1S21S2 0T40T40T30T3 0S50S5 2S21S32S21S3 3S23S2 0S60S6 1 S 5 2 S 4 0S70S7 0S80S80S90S90 S 10 1S41S4 1S01S0 4S21S84S21S8 2S32S3 1 S 7 2 S 6 1S91S9 3S13S1 0 S 12 0 S 11 2 S 5 1 S 6 CMB mantle ICB 0S30S3 3S13S1 3S23S2 ___ compressional --- shear ___ density m = S 2 mode, ECH (France, G) 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. 5) FUNDAMENTAL MODES O S 2 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). We plot all observed eigenfrequencies versus the station latitude (red dots and vertical red lines corresponding to the individual singlet mean value) 8) RESULTS 7) SYNTHETICS SPECTRA 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). 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. Roult G., J.Roch and E. Clévédé, 2008, A new catalog of eigenfrequencies from the 26 th December 2004 Sumatra-Andaman mega-event, PEPI, submitted All FDSN (GEOSCOPE, IRIS,...) stations used in this study. Too few were able to provide long time series without any problem (gaps, glitches…). The data are best fitted for the Tsai et al. complex rupture (5 sources) than for the Harvard mechanism.