TESTING THE ADJUSTMENT COEFFICIENTS OF SITE EFFECT DEFINED IN SEISMIC ZONATION MAP: CASE STUDY IN TIANJIN AREA

It has not been tested that the adjustment coefficients of site effect stipulated by the Chinese fifth-generation seismic zonation map are applicable universally for all regions of the country. Taking the Class III and Class IV sites in Tianjin area as an example, this study collected 137 borehole profiles and soil dynamic parameter data, and constructed their one-dimensional equivalent-linear computational models of seismic site response. Using 144 strong motion records at bedrock selected from NGA-West2 database, the PGA amplification factors (K_a) and characteristic periods of response spectrum (T_g) were calculated and compared with the mandatory values given in the seismic zonation map and the recommended values given by some previous studies. The following conclusions are obtained: The median values of K_a decrease with the increased PGAs of input ground motions, which are consistent with the change tendency of the site adjustment factor of PGA (F_a). This reflects the influence of nonlinear site response, which is more significant in the larger earthquake cases; The cumulative probabilities of K_a values reaching F_a are higher than 80% under many computational cases, indicating that the F_a values given by the fifth-generation zonation map has a high guarantee rate and the code has a certain degree of safety redundancy; The F_a values are generally close to those given by this study and the recommended values given by some previous studies. They are almost not lower than the K_\overline x\text + S values calculated by this study, indicating that the site adjustment factors of PGA given by the seismic zonation map are applicable locally for Class III and Class IV sites in Tianjin area; The median value of T_g from calculations of this study is much higher than the mandatory value in the seismic zonation map for Class III site, but close to the median value of Class IV site, suggesting that the adjustment coefficients of site effect stipulated in the seismic zoning map should account for the impact on the regional difference of site conditions in China. The achieved results from this study can provide some valuable implications for validating the currently-used seismic zoning map in other regions and preparing revisions for the next-generation map.