# Site-Specific Earthquake Ground Motions for Seismic Design of Port Facilities in Indonesia
## 1 Introduction  
Indonesia is an earthquake-prone country, and Sumatra is one of its high-seismicity regions. The seismic activity in Sumatra is characterized by two geological features: the Sunda megathrust, which is an inter-plate subduction zone between the Indo-Australian and Eurasian plates, and the Sumatra fault zone that divides Sumatra into two parts [1]. Indonesia's infrastructure (e.g., port facilities) has been damaged by earthquakes, such as the Sumatra-Andaman Islands earthquake in 2004 [2], the South Sumatra earthquake in 2007 [3], the West Sumatra earthquake in 2009 [4], and the Mentawai earthquake in 2010 [5]. Port facilities are installed on soft soils in coastal areas; therefore, they are vulnerable to earthquakes. Earthquakes that occurred in other areas, such as the 1995 Kobe earthquake [6], the 1999 Kocai earthquake [7], and the 2020 Samos earthquake [8], also caused severe damage to port facilities and led to the stagnation of the social economy. Evaluating the rational earthquake ground motions (EGMs) for seismic design is necessary to mitigate seismic disasters in high-seismicity countries, such as Indonesia. The seismic design code applied to port facilities in Indonesia is SNI 1726:2019, \"Seismic Design Criteria for Buildings and Other Structures\" (SNI) [9]. The code provides EGMs based on ASCE 7-16 [10] and the 2017 Indonesian Seismic Hazard Maps (ISHM) [11]. We provide a brief overview of how EGMs for seismic design are set up in the SNI. The ISHM was developed using an enhanced probabilistic seismic hazard analysis (PSHA). In the PSHA, the EGMs corresponding to a certain exceedance probability were evaluated by referring to the historical seismic data around the target area from 1900 to 2016. The distances between the target points and the reference fault planes were also referenced. Only one exceedance probability of the EGM is referred to in the SNI (2% in 50 years, with 2475 years as a return period) [9]. The exceedance probability corresponds to that of the EGM to assess the structural performance level of life safety protection in ASCE 7-16. The representative values of the EGMs shown in the ISHM are peak ground accelerations (PGAs) and spectral accelerations (SAs) with periods of 0.2 s and 1.0 s, respectively, at rock site. The waveforms used in the PSHA are not revealed. EGM amplification occurs at sites with a sedimentary layer, and the degree of amplification differs depending on the ground condition. The shallow subsurface is classified into six site classes in the SNI (Table 1). Site class SF refers to extremely soft ground on which the construction of structures is prohibited; therefore, it is not subject to seismic design. The SAs of other site classes are obtained by multiplying the amplification factor with the SA of the rock site. The amplification factor for each site class is based on the work of [PERSON] and [PERSON] [12]. [PERSON] and [PERSON] [12] simply modeled the ground using data on the average shear wave velocity of the top 30 m (\(Vs30\)) and conducted a seismic response analysis. The amplification factor for seismic design was then determined as the average value of the analysis result for each ground condition. The EGMs for seismic design shall be determined as site-specific ones [13; 14]. Research examples on site-specific EGMs include those of [PERSON] et al. [15], [PERSON] et al. [16], and [PERSON] [17] for the EGMs of Korea, Vietnam, and Japan, respectively. No case study on site-specific EGMs has yet been conducted to replace the EGMs in the SNI. The SNI EGMs might be reasonable in terms of the PGAs for sites with thin sedimentary layers because they were determined by a PSHA considering Indonesian seismicity, and the EGM amplification only had a slight effect. However, its validity has not been confirmed for the SAs at sites with sedimentary layers. First, the ground models of [PERSON] and [PERSON] [12] were overly simplified, and the ground data referred to were those in the United States, Japan, and other areas outside Indonesia.