## 1 Introduction
In each time step, soil properties for the next iteration will be assumed or calculated by referring to the stress-strain relationship. Thus, stress-strain characteristics for the soil must be modeled realistically to ensure meaningful results of the analysis [17; 19]. The advantages of using the nonlinear analysis method were discussed by previous researchers [20; 21; 22]. Figure 1: Malay Basin in South China Sea [6]. The GRA method normally utilizes a shear wave propagation approach that comprises of one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) analysis. In the seismic code for the engineering structure, 1D is widely used compared to the other two methods [17; 20]. GRA involving 2D and 3D methods will normally be used under special requests since more input data will be necessary. Further discussions are provided for the 1D method. The nonlinearity of soil material is represented by using either equivalent linear or nonlinear methods. In the 1D method, there are several assumptions made where the length of the soil will be ignored, and the soil boundary is assumed horizontal but the thickness will be accounted [22]. From the literature that was reviewed, parameters of seismic waves, such as duration, amplitude, and frequency can be influenced by the soil condition and local site effect as the wave propagates upward through the soil layers from the bedrock up to the soil surface [23]. Based on [PERSON] (1996) [24], the seismic wave can either be amplified or de-amplified depending on the soil deposit parameters, such as soil density, thickness, and type. [PERSON] et al. (2000) [25] highlighted that the natural period of the site has a significant influence on the amplification of ground motion. High vibration will occur when the seismicity and site share the same frequency. Soil amplification may occur in the area of soft soil, and it is known that the offshore area normally consists of soft soil. The amplification of each site will depend on the combination of soil and thickness of each layer. The range and value of the soil amplification factor varied at different locations. It shows the importance of obtaining soil amplification factors for specific areas to avoid the under- or over-design of engineering structures. Thus, in the current study, the soil amplification factor was determined for the Malay Basin where a lot of offshore facilities are operating. Most of the offshore platforms in Malaysian water were designed based on API, ISO, and PTS codes of practice. The conventional method was also adopted by ISO 19901-2 in plotting response spectra, as shown in Figure 2. This method was adopted in the current study, in accordance with ISO 19901-2, except for the spectral accelerations of 0.2 s and 1.0 s. In constructing the response spectrum for Malaysian water using ISO 19901-2, 5% damping was assumed and the values of the spectral accelerations for 0.2 s and 1.0 s periods are provided in Figure 3. Spectral acceleration in ISO 19901-2 for 0.2 s is 0.05 g while 1.0 s is 0.02 g for the 5% damping ratio. Figure 2: Damping [26]. According to [PERSON] (2007) [27], the usefulness of response spectra as a tool in the design and analysis process increased due to the requirement of building codes and acceptable performance-based design (PBD) techniques. Thus, many studies were conducted on previous researchers' spectral responses due to seismic excitation, and the influence on the structure was also highlighted. [PERSON] (2007) [27] discussed how seismic design forces can be obtained from design response spectra for both low-rise and high-rise buildings. They also highlighted that the estimation of a smooth spectrum is reasonable if the average of the maximum and minimum curves were used. [PERSON] et al. (2005) [28] studied the comparisons of ISO and API seismic designs for three offshore platforms.