HR: 1330h
AN: U53A-03    [Abstracts]
TI: Observations of Sub-Millihertz Normal Modes Excited by The Great Sumatra-Andaman Islands Earthquake
AU: Morii, W
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: Hirose, I
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: Kano, Y
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: Kato, M
AF: Graduate School of Human and Environmental Studies, Kyoto University, Sakyo, Kyoto, 6068501 Japan
AU: Komaki, A
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: Nishiguchi, T
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: Yanagidani, T
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AU: * Kawasaki, I
EM: kawasaki@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 6110011 Japan
AB: On social side, the Great Sumatra-Andaman Islands Earthquake of 26 December 2004 devastated south Asian countries as well as other circum-Indian Ocean countries with tsunami and became one of lethal disaster in human history. On scientific side, this earthquake released the largest seismic moment in the last half century and the first such (Mw 9 class) event in the era of modern digital recordings. Low frequency seismic signal excited by this event would test and improve our knowledge on large scale Earth structure and source process of large interplate earthquake. Our targets are low frequency normal modes of the Earth, and we focus our attention on the recordings at Japanese observatories which are located 40-60 degrees away from the epicentral region. In addition to broadband seismic recordings, we use various geodetic recordings such as strainmeters and extensometers. Signal-to-noise ratio of these recordings are remarkably high, which signifies the large excitation at the source. We employ conventional spectral analysis methods such as Fourier and multi-taper methods as well as "kyosin" (resonance) method [Morii, J Geod Soc Jpn, 2001], which employs the principle of a lock-in amplifier (or a phase sensitive detector; PSD) to detect isolated spectral peaks. The latter method has an advantage in case that frequencies of target spectral peaks are known beforehand and is a noise-resistant robust detector. We are able to detect fundamental spheroidal modes up to 0S36 as distinct peak with this method. Splitting of 0S2, the lowest frequency spheroidal mode, is clearly observed with the strainmeter recordings as well as broadband seismometer recordings, for which we use as long as approximately 25 days time series. Amplitudes of 5 splitted peaks of 0S2 appear to show temporal and spatial variation. Such spatial variation is understood as a function of distance between the centroid of moment release and observatories, as the mode amplitude diminishes near its node. Temporal variation is mainly due to large aftershocks, and could bias estimates of amplitude and width (Q) of mode peaks if ignored.
DE: 1294 Instruments and techniques
DE: 7255 Surface waves and free oscillations
SC: Union [U]
MN: 2005 Joint Assembly