Presenter: Yu-Hua Liou
Date: 2017/05/18
Abstract
Site effect is one of the most important factors dominating ground motion prediction. For engineering usage, the ground motion prediction equation (GMPE) usually includes the site-effect factor (e.g. Vs30, Z1.0). The site-effect, however, is considered as non-linear response, and could be critical for the aspect in broadband waveform modeling for ground motion prediction through simulation. We analyze data of three large earthquakes (M L 6.2-6.5) from CWB borehole network to estimate the site amplification factors and the transfer functions for sediments over the basement. A ratio method is used to acquire empirical amplification factors (EAF) for PGA, PGV and frequency spectra from 0.1 to 20 Hz. The relationship between the EAF and Vs30 value were analyzed as well. To obtain the transfer function from borehole stations to surface ones, two deconvolution methods, water-level deconvolution and Projected Landweber deconvolution are applied. The results show that nonlinear relationship between PGA (or PGV) amplification and Vs30, suggested that the amplification of PGA or PGV from borehole to surface was not dominated by shallow structure (i.e., top 30 m of strata). On the contrary, the relationships between EAF for frequency spectra range of 0.1 to 20 Hz and Vs30 could be classified into two site types which are strongly correlated to Vs30 values of sites. For Type I sites, the EAF increased with increasing frequency, and the Vs30 of these sites are generally over 760 (m/s). Type II sites which Vs30 are generally lower than 760 (m/s), the peak value of EAF shown around frequency range of 1 to 3 Hz attributed to the soft-soil conditions. The empirical amplification and transfer function analysis can be applied to obtain the GMPE for basement rock and also be adopted in ground motion simulation.
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