## 2 Context of Target Area  
The city of Ulsan is located in the south-east of the Korean Peninsula. It has undergone rapid industrial development since 1960 as the central hub for heavy/chemical industries such as the automobile, shipbuilding, metal, petrochemistry, and chemical component industries. Two major contextual characteristics of Ulsan (Figure 1) are that: (1) it is connected to the fault lines that encompass the two epicenters of recent earthquakes; and (2) a portion of the central city and the industrial components were built on reclaimed land or sedimentary valleys corresponding to an active fault, called the Ulsan fault line. Figure 1 shows the structural geology of Ulsan. The Yangsan fault, a major 170 km fault line with a strike-slip form, is considered the primary contributor to the 2016 Gyeongju earthquake that hit the eastern section of Ulsan. Reclaimed land in the south of Ulsan, where the industrial facilities are mainly located, lies over the Ulsan fault (160 degree strike angle from north, 50 km length), the left branch of the Yangsan fault. Although debate remains about the status of both faults, it has been more or less confirmed that both are active [29; 30]. [PERSON] [31] estimated slip rates of 0.1-0.04 m/ka and 0.2-0.06 m/ka for the Yangsan and Ulsan faults, respectively. [PERSON] et al. [32] proposed a 0.18-0.28 mm/year slip rate for the branches of the Ulsan fault. In general, the Ulsan fault is considered more active than the Yangsan fault and will likely trigger future seismic activity.  
The Ulsan Petrochemical Complex (UPC) is equipped with \(\sim\)20-50-year-old underground pipeline, and these have become a major concern for city and national government bodies in terms of chemical leakage risks. Although safety diagnosis and a remodeling support project have been actively underway since the two earthquakes occurred, the large target area (\(\approx\)935 km\({}^{2}\)) would make a comprehensive remodeling of networks impossible. Underground buried pipelines in the UPC are up to 1150 km long, and include a 568 km chemical materials transportation line, 425 km of gas pipes, and 143 km of oil pipelines.  
Central Ulsan lies on soft ground, mainly reclamation land that is encircled by industrial facilities and houses a population of 200,000 (Figure 1b). After being subject to physical damage from the two earthquakes, and subsequent sinkhole development in recent years, the city government has investigated liquefaction over the soft reclaimed ground. As the area is built over tideland with a rock layer lying 10 to 50 m underground, the central city area and industrial parks were deemed to be at high risk for surface deformation. Although imminent risks of ground deformations have not been identified, such risks demand prompt geodetic surveys and consequent interpretation together with geophysical modelling and GIS analyses.  
Figure 1: Location and geological context of the target area. The epicenters of two major earthquakes, the involved faults lines, and two GPS stations (ULSN and WOLS) around the target area are described in terms of (**a**) the target area topography and involved contexts and (**b**) its geological context together with the locations of the industrial complex.