Site Amplifications At the Ocean Bottom Seismograph Stations in the Sagami Bay Area

Abstract eng:
We obtained site amplifications at the K-NET ocean bottom seismographs (OBS) installed in the Sagami Bay area and other nearby sites including those in the Kanto basin by spectral inversion method. We used horizontal component S-wave Fourier spectra of ground motions from events of Mw 4.7 to 5.8 and employed the precomputed site amplification factors at a reference station in the inversion. We compared the amplification factors obtained from the inversion with the observed spectral ratios between the surface and borehole recordings as a proxy to site amplification at the KiK-net sites. We found that the spectral amplifications at higher frequencies well match the observed spectral ratios at the KiK-net sites. Also, the amplification factors are comparable with those from 1D theoretical amplifications using the Japan Seismic Hazard Information Station (J-SHIS) model and Headquarter for Earthquake Research Promotion (HERP) model at longer periods at several sites suggesting that the site amplifications obtained in this study are reasonable. The site amplifications at the OBS sites are found to be noticeably greater than those at the deep basin sites located on the land at wide frequencies; the amplification factors are about 10 or over at frequencies between 0.2 to about 10 Hz at the OBS sites. Furthermore, the JSHIS and HERP models underestimate the spectral amplifications at two OBS sites, roughly by half an order of magnitude at frequencies of 0.2 to 1.0 Hz. Very large amplifications at narrow frequency bands are not uncommon at the land stations. But it appears to be that the large amplifications at wider frequency band are quite common at the OBS sites. Since the seismometers at the OBS sites record the ground motions earlier than those at the land stations for the offshore events, the understanding of site amplification factors at the OBS sites is crucial for earthquake early warning. As the realistic velocity models are not available to model amplifications at frequencies higher than about 1 Hz, the site amplification factors estimated in this study based on the observed data are believed to be useful for appropriate correction of the ground motion data recorded at the OBS sites for estimation of source parameters like earthquake magnitude. We found that the Brune model generally describes well the source spectra obtained from inversion and the estimated magnitudes are similar to those determined by F-net with the exception of an event for which the estimated magnitude from inversion is greater by about 0.5 magnitude unit; a comparison of the observed PGAs with those from ground motion prediction equation (GMPE) for the event reveals that the GMPE considerably underestimates the data, thus suggesting for a significant inter-event residual. The Qs values obtained from the inversion are found to be comparable to the previous studies. Thus, we have concluded that the site amplifications at the OBS sites are isolated satisfactorily by the spectral inversion method with a constraint of site amplification factors at a reference station on land.

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