# Site-Specific Earthquake Ground Motions for Seismic Design of Port Facilities in Indonesia
## 2 Methods  
### EGM Incidents in the Bedrock  
Although Indonesia has experienced a number of huge earthquakes, only a few EGM records are available, and the EGM records for port areas are missing. Furthermore, the observed EGMs have been affected by the ground at the seismograph installation site; thus, they cannot be used for the seismic design of port facilities by simply adjusting the amplitudes of the observed EGMs. The effects of the shallow subsurfaces must be removed by a seismic response analysis, and the EGM incidents in the bedrock must be evaluated. In this study, nine EGM records (Figure 1, Table 2) were collected with the ground data of the observation sites. All were records of massive earthquakes with a moment magnitude (\(M_{w}\)) of 6.0 or larger. However, the PGAs were small due to the long hypocentral distances. Figure 1: Map. The observed EGM records contained two horizontal component records. To make the most of the limited number of EGM records, the two horizontal components were treated independently in this study. As a result, the number of EGM records was 18. Before the seismic response analysis was conducted, baseline correction was first performed, and the EGM records were tapered. Figure 2 shows the Fourier spectra of the east-west direction component of the EGMs. The Fourier spectra of MTW 1 07 S showed large amplitudes in the frequency range higher than 1 Hz because its peak ground acceleration of 0.12 g was higher than those of the other EGMs (0.015-0.04 g). Predominant frequencies covered a wide range, from 0.5 to 4 Hz, showing that EGMs with varying frequency characteristics were provided. A frequency domain seismic response analysis that considered nonlinear soil characteristics was performed to evaluate the EGM incidents in the bedrock using the EGM records at the ground surface (deconvolution). For the shear modulus and the damping coefficient dependencies on the shear strain, we referred to Seed and Idriis (2016) for sandy soil and [PERSON] and Dobry (2017) for cohesive soil. Figure 3 illustrates the nonlinear soil characteristics considered in this study. The dashed line expresses the sandy soil characteristics. The solid line denotes those of the cohesive soil. The blue line shows the shear modulus normalized by the initial value (\(G/G_{max}\)), while the red line depicts the damping coefficient. DEEPSOIL software (S \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Earthquake Events** & **Observation Station** & **Ground Motion Code** & **PGA (g)** & **Site Class** \\ \hline \multirow{2}{*}{1} & Mentawai 1, 2010 & \multirow{2}{*}{Gadut; Kuranji; Unand} & MTW1 10 G & 0.037 & SC \\  & (Mw 7.8—focal depth: 20.1 km) & & MTW1 10 K & 0.026 & SD \\  & & MTW1 10 U & 0.037 & SD \\ \hline \multirow{2}{*}{2} & Mentawai 2, 2010 & \multirow{2}{*}{Gadut; Kuranji; Unand} & MTW2 10 G & 0.039 & SC \\  & (Mw 6.3—focal depth: 26 km) & & MTW2 10 K & 0.025 & SD \\  & & MTW2 10 U & 0.043 & SD \\ \hline \multirow{2}{*}{3} & Southern Sumatra, 2007 & \multirow{2}{*}{Sikuai Island} & \multirow{2}{*}{SS1 07 S} & \multirow{2}{*}{0.037} & \multirow{2}{*}{SD} \\  & (Mw 8.4—focal depth: 34 km) & & & & \\ \hline \multirow{2}{*}{4} & Mentawai 1, 2007 & \multirow{2}{*}{Sikuai Island} & \multirow{2}{*}{MTW1 07 S} & \multirow{2}{*}{0.110} & \multirow{2}{*}{SD} \\  & (Mw 7.9—focal depth: 30 km) & & & & \\ \hline \multirow{2}{*}{5} & Mentawai 2, 2007 & \multirow{2}{*}{Sikuai Island} & \multirow{2}{*}{MTW2 07 S} & \multirow{2}{*}{0.014} & \multirow{2}{*}{SD} \\  & (Mw 7.0—focal depth: 22 km) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: EGM records. Figure 2: Fourier spectra. The collected ground data only provided the unit weight and the N-values obtained by the standard penetration test. The S-wave velocity of each soil layer was calculated using the correlation equation proposed by [PERSON] and [PERSON] [29]. Table 3 shows the soil conditions of the Sikuai Island station as an example. For this site, the hard clay silt of the sixth layer was treated as the bedrock.