[SCG50-P12] Strong S-wave attenuation in the mantle wedge beneath the NE Japan and interpretations of high-frequency wave propagation path of intraslab earthquakes
キーワード：S-wave attenuation、Intraslab earthquakes、High-frequency later phase、Scattered wave
For intraslab earthquakes at intermediate-depths in NE Japan, especially at greater than about 100-km depths, it has been often recorded that high-frequency waveform higher than 8 Hz show a remarkable delay of arrival compared with S wave which predominates in a low-frequency band. The magnitude of the delay is little in the forearc region, whereas it increases up to about 40 s in the backarc region. In this study, we investigate the details in the propagation processes of the high-frequency later phase observed for the intraslab earthquakes at the intermediate-depths. To constrain the source location of the high-frequency later phase, we employ the source-scanning algorithm (SSA) [Kao and Shan, 2004] with realistic velocity and attenuation structures, assuming that the high-frequency later phase is generated by the single scattering process. Namely, we estimate a scattering point by using mean-square amplitudes which is synthesized from two horizontal components of the observed waveforms.
Results in frequency bands of 4-32 Hz show that scattering points are distributed beneath the forearc region. The scattering points can reproduce the observed delays of amplitude amplifications associating with the high-frequency later phases. Moreover, similar locations of the scattering points are imaged along the forearc region in the entire NE Japan. These results indicate that high-frequency later phase is a wave scattered at the forearc region and that short wave-length heterogeneity generally developed in the forearc region of NE Japan.
This interpretation well matches implications from previous studies [e.g., Yomogida et al., 2004; Hasemi and Horiuchi, 2010]. Additionally, the obtained distributions of scattering points which appear to surround the highly attenuating mantle wedge suggest that attenuation structure strongly controls propagation paths of seismic wave, in particular, a high-frequency wave. In other words, the identifications of high-frequency later phase may become easy because the amplitude of direct wave crossing the high-attenuation zone decreases. To confirm this consideration, we estimate Qs along the direct wave by using the coda-normalization method [e.g., Aki, 1980]. This estimation provides 2-3 times smaller path-averaged Qs in seismic records of which rays lay on the backarc and the volcanic front than that of which rays lay on the forearc region. These results support that spatial variation in QS produces heterogenous propagations of seismic waves emitted from intraslab earthquakes. It is thus required to take scattering and attenuation structures simultaneously into account to understand complex behaviors of high-frequency waves in a future study.
Results in frequency bands of 4-32 Hz show that scattering points are distributed beneath the forearc region. The scattering points can reproduce the observed delays of amplitude amplifications associating with the high-frequency later phases. Moreover, similar locations of the scattering points are imaged along the forearc region in the entire NE Japan. These results indicate that high-frequency later phase is a wave scattered at the forearc region and that short wave-length heterogeneity generally developed in the forearc region of NE Japan.
This interpretation well matches implications from previous studies [e.g., Yomogida et al., 2004; Hasemi and Horiuchi, 2010]. Additionally, the obtained distributions of scattering points which appear to surround the highly attenuating mantle wedge suggest that attenuation structure strongly controls propagation paths of seismic wave, in particular, a high-frequency wave. In other words, the identifications of high-frequency later phase may become easy because the amplitude of direct wave crossing the high-attenuation zone decreases. To confirm this consideration, we estimate Qs along the direct wave by using the coda-normalization method [e.g., Aki, 1980]. This estimation provides 2-3 times smaller path-averaged Qs in seismic records of which rays lay on the backarc and the volcanic front than that of which rays lay on the forearc region. These results support that spatial variation in QS produces heterogenous propagations of seismic waves emitted from intraslab earthquakes. It is thus required to take scattering and attenuation structures simultaneously into account to understand complex behaviors of high-frequency waves in a future study.