## 2 Methodology
### Assessment of Pohang Earthquake-Induced Liquefaction at Youngil-Man Port Using the UBCSAND2 Model  
#### 2.3.1 Study Area and Ground Motion of the Pohang Earthquake  
The Pohang earthquake occurred on 15 November 2017, causing damage to the caisson-type quay wall structures and reclaimed area at the Youngil-man port, which is located about 6 km from the epicenter, as shown in Figure 10. After the earthquake, [PERSON] et al. [11] conducted a field study and indicated that the settlement of reclaimed area was approximately 10-20 cm, the caisson horizontal displacement was in a range of 5-15 cm, and the caisson settlement was less than 10 cm. Additionally, many sand boils could be found in this area, as shown in Figure 10.  
Figure 10: Locations of the Youngil-man port, liquefaction observations, and seismic motion recording stations relative to the epicenter of the 2017 Pohang earthquake. Figure 9: \(CSR\)-\(N_{\text{liq}}\) curves obtained from the UBCSAND2 model for Pohang sand and laboratory CDSS tests. During the Pohang earthquake, ground motions were recorded at four stations--PHA2, CHS, DKJ, and HAK--with distances from the epicenter of 9, 26, 29, and 23 km, respectively, as shown in Figure 10. The seismic motions were obtained from the National Earthquake Comprehensive Information Center [26]. Table 2 summarizes the information from the four selected stations: peak ground acceleration (PGA)--north-south (NS), east-west (EW), and up-down (UD)--locations, distances from epicenter, and soil classifications. The site condition in Pohang is classified as \(S_{\text{B}}\) based on the Korea Building Code [27]. This means that an average shear wave velocity for the ground of up to a depth of 30 m (i.e., \(V_{\text{s}}\)[30]) is 760-1500 m/s. We note that the horizontal peak accelerations in the EW and UD directions were significantly lower than those recorded in the NS direction for all of stations. Furthermore, the peak accelerations decreased remarkably when the stations' distances from the epicenter increased. From Figure 10, the observed site (i.e., Youngil-man port) locates nearly 6 km from the epicenter, which is closer than all the recording stations. However, there is no seismic instruments available in this area because they are only installed in the rock formation that is visible on the surface (i.e., outcrop). If near-field effects are considered, the ground motions should be modified based on distances from the epicenter to the selected station, and the observed site. This kind of modification requires a modern and complex model based on fault rupture surfaces, geometries, and energy radiation distributions. For example, the Southern California Earthquake Center Broadband Platform (SCEC-BPP) was successfully used to address the near-field effects of the 1971 San Fernando earthquake-induced damages in Lower and Upper San Fernando dams [28; 29]. For the Pohang earthquake, neither reliable research nor available data have been performed to study the near-field effects to the recorded PGAs. Therefore, this study applied a simple method to scale the PGAs from the available data provided by the National Earthquake Comprehensive Information Center [26]. A relationship between recorded PGAs and distances from the epicenter of different recording stations was established in Figure 11. Based on the logarithm curve, the PGA of 0.350 g at Youngil-man port was obtained. Then, the acceleration-time history recorded at PHA2 station was scaled up to 0.35 g and it is considered as the target earthquake given as outcrop motion at the observed site. For FLAC analyses [30], the seismic input must be applied at the base of the model while the target earthquake at Youngil-man port is the outcrop motion.