11:00 AM - 11:15 AM
[SSS04-08] Extraction of S-wave site amplification factors at the S-net sites based on the spectral inversion technique
Keywords:S-net, Site amplification, Earthquake early warning, Strong motion, Japan Trench
S-net is a large-scale inline-type seafloor observation network of 150 seismograph and tsunami-meter stations for earthquakes and tsunamis along the Japan Trench (Aoi et al. 2020). The network started operation in 2016, and all the stations record and send the waveform data in real-time at the designated data management centers. One of the main objectives of operating the S-net was to enhance the Japan Meteorological Agency (JMA) earthquake early warning (EEW) and tsunami early warning. The EEW can be enhanced by faster detection of earthquakes and estimation of source parameters compared to the land-based observation network. For the reliable estimate of the earthquake source parameters such as hypocenter and magnitude, it is important to understand the subsea velocity structures and site amplification effect on the recorded motions. In the present study, we examined the strong-motion data from moderate earthquakes of magnitudes ~ Mw 3.5-6.0, recorded by S-net, and performed spectral inversion to separate the source, path, and site terms from S-wave parts of the records.
It is well known that the spectral inversion technique (e.g., Iwata and Irikura, 1988) requires a constrained condition in the inversion due to the trade-off between the source and site terms. In the present study, we used the theoretical site amplification at two KiK-net sites as constrained site amplification factors in the inversion following Yamanaka et al. (1998). The theoretical amplifications were obtained from 1D velocity structures estimated by inversion of spectral ratios between the surface and borehole records. Several KiK-net stations were also included in the spectral inversion to compare the obtained site amplifications with the observed spectral ratios as a validation of the obtained results. We used the records with vector PGA between 5 and 50 gals and hypocentral distance between 30 and 200 km. The records at S-net stations were corrected for their orientations, and standard procedures were used to process the records at the land and S-net sites. The S-onset times were obtained manually, and magnitude-dependent S-wave time windows ranging between 8 and 20 s were selected. The Fourier spectra were computed by padding zeroes for a time length of 40.96 s. The spectra were smoothed using the Parzen window of 0.2 Hz.
The extracted site amplifications at the S-net sites varied widely and exceeded 20 at 0.5, 1, and 2 s at several sites. The site amplification at the sites close to the coast off the Tohoku region were relatively lower than those at the inner sites close to the trench axis. Several sites showed peak amplification at frequencies between about 1 and 10 Hz, and the results were very similar between the outer-rise sites, the S6 segment of the network. The mean amplification factors were around ten at the period of 2 s. Although the values of the site amplification factors were similar to the H/V spectral ratios at a limited number of sites, the shapes of the H/V spectral ratio curves were similar to the shapes of the amplification curves at many sites. The obtained site amplification factors at the KiK-net sites, where the borehole sensors were in the hard layers, were remarkably similar to the spectral ratios between the surface and borehole records. The magnitudes estimated by fitting the source spectra with the omega-square models were quite comparable with the F-net catalog magnitudes. The path-averaged Qs values were also close to the results in the literature. Thus, we concluded that the extracted site amplifications were reasonable, and further analysis is ongoing to extend the results for the longer periods.
Acknowledgment
This study was supported by the "Advanced Earthquake and Tsunami Forecasting Technologies Project" of NIED and JSPS KAKENHI Grant Number JP20K05055.
References:
Aoi et al. (2020). Earth Planets Space 72, 126.
Iwata, T., and Irikura K (1988). J. Phys. Earth. 36, 155-184.
Yamanaka et al. (1998). Zisin 55, 193-202
It is well known that the spectral inversion technique (e.g., Iwata and Irikura, 1988) requires a constrained condition in the inversion due to the trade-off between the source and site terms. In the present study, we used the theoretical site amplification at two KiK-net sites as constrained site amplification factors in the inversion following Yamanaka et al. (1998). The theoretical amplifications were obtained from 1D velocity structures estimated by inversion of spectral ratios between the surface and borehole records. Several KiK-net stations were also included in the spectral inversion to compare the obtained site amplifications with the observed spectral ratios as a validation of the obtained results. We used the records with vector PGA between 5 and 50 gals and hypocentral distance between 30 and 200 km. The records at S-net stations were corrected for their orientations, and standard procedures were used to process the records at the land and S-net sites. The S-onset times were obtained manually, and magnitude-dependent S-wave time windows ranging between 8 and 20 s were selected. The Fourier spectra were computed by padding zeroes for a time length of 40.96 s. The spectra were smoothed using the Parzen window of 0.2 Hz.
The extracted site amplifications at the S-net sites varied widely and exceeded 20 at 0.5, 1, and 2 s at several sites. The site amplification at the sites close to the coast off the Tohoku region were relatively lower than those at the inner sites close to the trench axis. Several sites showed peak amplification at frequencies between about 1 and 10 Hz, and the results were very similar between the outer-rise sites, the S6 segment of the network. The mean amplification factors were around ten at the period of 2 s. Although the values of the site amplification factors were similar to the H/V spectral ratios at a limited number of sites, the shapes of the H/V spectral ratio curves were similar to the shapes of the amplification curves at many sites. The obtained site amplification factors at the KiK-net sites, where the borehole sensors were in the hard layers, were remarkably similar to the spectral ratios between the surface and borehole records. The magnitudes estimated by fitting the source spectra with the omega-square models were quite comparable with the F-net catalog magnitudes. The path-averaged Qs values were also close to the results in the literature. Thus, we concluded that the extracted site amplifications were reasonable, and further analysis is ongoing to extend the results for the longer periods.
Acknowledgment
This study was supported by the "Advanced Earthquake and Tsunami Forecasting Technologies Project" of NIED and JSPS KAKENHI Grant Number JP20K05055.
References:
Aoi et al. (2020). Earth Planets Space 72, 126.
Iwata, T., and Irikura K (1988). J. Phys. Earth. 36, 155-184.
Yamanaka et al. (1998). Zisin 55, 193-202