This study presents the geotechnical evaluation of an existing tank structure under earthquake loads, based on TBDY 2018, international standards, and the published models in the literature. Due to the presence of liquefaction potential and clay layers thicker than 35 m with low shear strength in the field, non-linear site-specific analyses were performed in the time domain with and without pore pressure models. Soil softening and liquefaction act as a natural isolator, damping surface accelerations significantly, but they have caused high lateral displacements (2.0-2.3 m) and vertical settlements (58-128 cm) in the free field. It is estimated that the differential settlement at the tank structure is approximately 70-80 cm at the maximum and 5-10 cm at the lowest that is needed to be considered in pipe connections with the tank at the site. Under the additional earthquake loads from the superstructure, the bearing capacity and sliding control on the bottom of the tank structure and under the fill reinforced with geogrid were found to be safe.