HR: 1340h
AN: S13D-1082    [Abstracts]
TI: High-frequency envelope inversion analysis of the 2003 Tokachi-Oki, JAPAN, earthquake (Mw8.0)
AU: * Nakahara, H
EM: naka@zisin.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-Ku, Sendai, 980-8578 Japan
AB: The 2003 Tokachi-Oki earthquake (Mw 8.0) took place on September 26, 2003 at the plate interface between the subducting Pacific plate and the Hokkaido island, northern Japan. The focal depth is around 30km and the focal mechanism is thrust type. This earthquake caused 2 missings, more than 100 injures, 2000 collapsed houses, and so on. Slip distribution on the fault plane was already estimated by inversion analyses of low-frequency seismograms. However, source characteristics for the earthquake in frequencies higher than 1 Hz is not so far clarified. In this study, we execute an envelope inversion analysis based on the method by Nakahara et al. (1998) and clarify the spatial distribution of high-frequency seismic energy radiation on the fault plane of this earthquake. We use three-component sum of mean squared velocity seismograms multiplied by a density of earth medium, which is called envelopes here, for the envelope inversion analysis. Three frequency bands of 1-2, 2-4, and 4-8 Hz are adopted. We use envelopes in the time window from the onset of S waves to the lapse time of 128 sec. Green functions of envelopes representing the energy propagation process through a scattering medium are calculated based on the radiative transfer theory, which are characterized by parameters of scattering attenuation and intrinsic absorption. We use the values obtained for eastern Hokkaido (Hoshiba, 1993). We assume the fault plane as follows: strike=$249^{\circ}$, dip=$15^{\circ}$, rake=$130^{\circ}$, length=150km, width=165km with reference to a waveform inversion analysis in low frequencies (e.g. Yagi, 2003). We divide this fault plane into 110 subfaults, each of which is a 15km x 15km square. Rupture velocity is assumed to be constant. Seismic energy is radiated from a point source as soon as the rupture front passes the center of each subfault. Time function of energy radiation is assumed as a box-car function. The amount of seismic energy from all the subfaults and site amplification factors for all the stations are estimated by the envelope inversion method. Rupture velocity and the duration time of a box-car function should be estimated by a grid search. Theoretical envelopes calculated with best-fit parameters generally fit to observed ones. The rupture velocity and duration time were estimated as 3.0 km/s and 6 sec, respectively. The high-frequency seismic energy was found to be radiated mainly from two spots on the fault plane: The first one is the deeper part beneath the initial rupture point and the second is the southern shallow part of the fault plane. Radiated energy was estimated to be $7.2 \times 10^{16}J$ in the 1-8Hz band. Acknowledgements: We used strong-motion seismograms recorded by the K-NET and KiK-net of NIED, JAPAN.
DE: 7203 Body wave propagation
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting