# Site-Specific Earthquake Ground Motions for Seismic Design of Port Facilities in Indonesia
## 3 Results and Discussion  
### EGMs at the Port Sites  
Figure 8 depicts the obtained SAs for each site class. The SA dispersion was very large for both site classes. In particular, the dispersion was large in the period band of 1.0 s or shorter for site class SD and 1.5 s or shorter for site class SE. The natural period of a quay is usually shorter than 1.5 s; therefore, this result indicated that the SA dispersion was very large in the natural period zone of the quay.  
We now discuss the reason behind this large SA dispersion. Figure 9 displays the large (red line) and small (blue line) SA results together with the average + SD SA (black line) for each site class. For the SD class, the case involving a very large SA showed very large values in the period band of 0.7-1.0 s. In the other period bands, the SAs were not large compared with the average + SD SAs. A similar pattern was seen in the SE class. The SAs were very large in the period band of 1.0-1.5 s, but did not show very large values in the other period bands. In contrast, in cases with low SAs, the SAs depicted small values compared to the average + SD SAs in the entire period range. This suggests that the amplification characteristics of the shallow subsurfaces strongly affect the SA amplitudes.  
Figure 8: SAs in site classes (**a**) SD and (**b**) SE. The blue line represents the individual SAs. The black line denotes the average SAs. The red line expresses the average + standard deviation (SD) SAs.  
Figure 10 illustrates the Fourier spectrum of the EGM incidents (blue line), the transfer function by the shallow subsurface (black line), and the Fourier spectra of EGMs at the ground surface (red line). In cases where the SA became very large, the predominant frequencies of the EGM incidents in the bedrock agreed with those of the transfer functions, resulting in large SAs at specific period bands. In cases where the SA became very small, in addition to the fact that the Fourier spectra of the EGM incidents were small, the two predominant frequencies did not agree, resulting in small SAs.  
The SA variation was very large; thus, site-specific EGMs for seismic design must be set using individual ground conditions by methods such as those shown in this study. However, in many cases, the ground data at sites of interest were not obtained in detail. In such cases, the degree of coincidence between the natural period of the port facilities and that of the shallow subsurfaces must be assessed. Attention must be paid to the decrease in the shear rigidity of soft soils during massive earthquakes because it results in an increase in the natural period of the shallow subsurface. Figure 11 shows a comparison of the transfer functions before an earthquake (red line) and during earthquake (blue line) at the points shown in Figure 9a. The natural frequency decreased from the initial value of 1.28 to 0.92 Hz during an earthquake. The SA became very large when the natural period of the port facilities coincided with that of the shallow subsurface.  
Figure 9: Comparison of the SAs in site classes (**a**) SD and (**b**) SE.