## 2 Materials and Methods
### Phase One: Development of Seismic Microzonation Map
#### 2.1.4 Ground Response Analysis (GRa)  
In this study, one-dimensional nonlinear analyses were carried out by using the Nonlinear Earthquake Site Response Analysis of Layered Deposits (NERA) program. GRa in this study was conducted for 11 sites with a total of 19 boreholes for 6 seismic events and 5 return periods. The overall methodology for GRa is depicted in Figure 4. In NERA, the first step is to decide the time step increment, desired maximum acceleration, and maximum frequency cut-off. In the current study, the time step increment was set to 0.01 while the desired maximum acceleration was set to ensure that the PGa value at the bedrock reflected the value. The maximum frequency cut-off was set at 10 Hz. All of this information is the input in the worksheet 'earthquake'. Along with that, the selected seismic time history was inserted.  
The second step was to provide the soil parameters in the worksheet 'profile'. However, soil material type, thickness of a layer, total unit weight, and shear wave velocity were added. A sample of worksheet profiles is presented in Table 4. There were 2 soil material types available, '1' represents clay while '2' represents sand [35]. The number of time-sub increments in the worksheet 'Iteration' was set at the minimum value and the outcrop was set for the type of input motion. In the worksheet 'Spectra', a number of sublayers were set as 1 to represent the surface or seabed while the ratio of critical damping was set at 5%, as suggested in the ISO19901-2 [26].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Depth (m)** & **Thickness** & **Soil** & **Vs** \\ \hline
**From** & **To** & **(m)** & **Description** & **(m/s)** \\ \hline
0.0 & 6.9 & 6.9 & Very soft to soft clay & 117.764 \\
6.9 & 9.8 & 2.9 & Stiff silt & 193.169 \\
9.8 & 14.9 & 5.1 & Stiff clay & 162.077 \\
14.9 & 21.6 & 6.7 & Firm to stiff clay & 150.722 \\
21.6 & 24.0 & 2.4 & Very stiff silt & 282.296 \\
24.0 & 27.2 & 3.2 & Firm clay & 208.693 \\
27.2 & 29.8 & 2.6 & Very stiff silt & 231.147 \\
29.8 & 38.9 & 9.1 & Stiff clay & 200.705 \\
38.9 & 44.8 & 5.9 & Stiff to very stiff clay & 204.381 \\
44.8 & 50.0 & 5.2 & Very stiff silt & 280.731 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Soil model for D1 used in the present study.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Coordinate** & \multicolumn{5}{c}{**PGa (g)**} \\ \hline
**Lat.** & **Long.** &
\begin{tabular}{c} **100-Year** \\ **Return Period** \\ \end{tabular} & **200-Year** & **500-Year** & **1000-Year** & **2500-Year** \\
**Return Period** & **Return Period** & **Return Period** & **Return Period** & **Return Period** \\ \hline
3.0 & 103.5 & 0.02 & 0.03 & 0.04 & 0.05 & 0.07 \\
3.5 & 103.5 & 0.02 & 0.03 & 0.03 & 0.05 & 0.07 \\
4.0 & 103.5 & 0.02 & 0.02 & 0.03 & 0.04 & 0.06 \\
4.5 & 103.5 & 0.02 & 0.02 & 0.03 & 0.04 & 0.05 \\
5.0 & 103.5 & 0.02 & 0.02 & 0.03 & 0.03 & 0.05 \\
5.5 & 103.5 & 0.02 & 0.02 & 0.02 & 0.03 & 0.04 \\ \hline \hline \end{tabular}
\end{table}
Table 3: PGa values for offshore Peninsular Malaysia [34].  
After everything was assigned, the first step was to use the process earthquake data command where, here, the time history input was processed to achieve the desired PGa value. The second step was executed by using the 'calculate' step-by-step command; in this stage it involved the NERA read profile, material curve, and then the execute the key calculation to produce the acceleration, velocity, and displacement in the worksheet 'Iteration'. The third step was to produce the output by using the command 'Calculate Output'. In this stage, the output for spectra in terms of acceleration was obtained. The maximum acceleration at sublayer 1, produced in the Iteration worksheet, was recorded as it portrayed the value of the acceleration at the seabed or surface. The movements of the wave propagation in vertical motion throughout the soil layers and the spectra for the surface were produced.