HR: 1340h
AN: T33B-0532    [Abstracts]
TI: High Attenuation Zone Beneath the Subducting Pacific Plate?
AU: * Shito, A
EM: azusas@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, 305-0006 Japan
AB: Interesting systematic change in waveform observed in seismic networks on Japan islands from events in Java - Tonga subduction zones (the epicentral distances are from 40-80 degrees). Some waveforms sampling eastern region (oceanward) of Japan subduction zones from Tohoku to Izu have been broadened and the amplitude has been decayed. These characteristics can be due to anelasticity in the propagating media. We estimated anelasticity using two kinds of differential attenuation (δ t^*) method; S-S₀ method and S-P method. The methods are based on forward modeling of waveform by using absorption band model. A reference signal is modified synthetically with changing parameter Q to find best fit with observed signal. The S-S₀ method determines differential attenuation of S-wave to a reference S₀-wave. The S-P method determines differential attenuation of S-wave to P-wave. In the modeling, we used normalized amplitude. Therefore, we only take account of dispersion but amplitude decay. Two types of waveform data set were recorded by tiltmeters installed at stations of the High Sensitivity Seismograph Network of Japan (Hi-net), and broad band seismograph network (F-net). The both networks are operated by National Research Institute for Earth Science and Disaster Prevention (NIED). The tiltmeters can be used as broadband seismometers with horizontal components and have dense spatial distribution. The broad band seismograph network is sparser than that of tiltmeters, but each station has three components including vertical component. Using data from five events, a high attenuation region is detected in the upper mantle beneath subducting Pacific plate in Japan subduction zones from Tohoku to Izu. If we assume that all the absorption occurred only in the upper mantle (400 km thick), the estimate value of Q_s is 40-60. The spatial distribution of the high attenuation zone is correspondent to that of a low velocity zone suggested by Obayashi et al. [2005]. The origin of the high attenuation and low velocity zone will be discussed in the future work.
DE: 7203 Body waves
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005