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[J07-P-12] Site amplification at Nankai seafloor observation network DONET1 in Japan evaluated by spectral inversion
Seafloor strong motion observation networks such as DONET and S-net have been deployed in the subduction zones of Japan. Their observations are expected to be useful for the real-time analyses on the magnitude estimation and the strong-motion forecast for subduction earthquakes. For these applications, site amplifications at the seafloor stations need to be evaluated because site amplifications caused by thick low-velocity sediments in the subduction zones can strongly affect the real-time analyses. In this study, we focus on DONET1, which are operated by NIED and JAMSTEC in the Nankai subduction zone, Japan, and evaluate their site amplification factors by the spectral inversion method.
We estimate site amplifications by separating source, propagation path, and site characteristics from observed Fourier amplitude spectra based on the spectral inversion technique (Iwata and Irikura 1986). The frequency band of this analysis is 0.2–10 Hz. We use the vector summation of two horizontal components of Fourier amplitude spectra, which are obtained from S-wave records of 20.48 s. The amplitude spectra are smoothed by the filter of Konno and Ohmachi (1999). We use records of strong motion seismographs at DONET1 and records of broadband seismograph at F-net of NIED. To solve a problem of trade-off between the source spectra and site effect, we assume the F-net KMT station as a reference rock site whose amplification factor is 2, following the procedure of Moya and Irikura (2000).
The spectral inversion indicates that all DONET1 stations, especially near-land stations (Nodes A and B) and near-trench stations (Node C), have larger site amplification factors than the reference station. The site amplification factors at the stations of Nodes A and B are ~10 in the broad frequency band (0.2–5 Hz). The site amplification factors at the stations of Node C have a sharp peak at ~5 Hz with the value of ~20.
We estimate site amplifications by separating source, propagation path, and site characteristics from observed Fourier amplitude spectra based on the spectral inversion technique (Iwata and Irikura 1986). The frequency band of this analysis is 0.2–10 Hz. We use the vector summation of two horizontal components of Fourier amplitude spectra, which are obtained from S-wave records of 20.48 s. The amplitude spectra are smoothed by the filter of Konno and Ohmachi (1999). We use records of strong motion seismographs at DONET1 and records of broadband seismograph at F-net of NIED. To solve a problem of trade-off between the source spectra and site effect, we assume the F-net KMT station as a reference rock site whose amplification factor is 2, following the procedure of Moya and Irikura (2000).
The spectral inversion indicates that all DONET1 stations, especially near-land stations (Nodes A and B) and near-trench stations (Node C), have larger site amplification factors than the reference station. The site amplification factors at the stations of Nodes A and B are ~10 in the broad frequency band (0.2–5 Hz). The site amplification factors at the stations of Node C have a sharp peak at ~5 Hz with the value of ~20.