The Atlantic and Gulf Coastal Plains (CPs) in the eastern and southern United States consist of thick and soft sediments that overlie a stiff bedrock that increase the amplification and duration of earthquake ground motions. Understanding both the depth and the dynamic properties of these sediments is critical yet challenging at a regional scale. Sediment thickness and geology have been shown to be good indicators for regional characterization of shear wave velocity (V_S). However, existing site amplification models within the CPs have limited characterization and modeling of these parameters. Therefore, we have developed a new site response and hazard model that uses a geology-based V_S model and geotechnical model to characterize the CP sediments so that amplification and hazard levels can be estimated. A sensitivity study using one-dimensional linear site response analyses is conducted to determine the influence of sediment thickness and reference condition assumptions within the model. Sediment thickness values of 100, 200, and 1000 m are tested. Additionally, reference condition velocities of 2500, 3000, and 3500 m/s at the base are tested. Response spectra- and Fourier amplitude spectra-based amplification ratios are calculated. The change in sediment thickness values is found to have the largest impact on longer-period (lower-frequency) amplification ratios as the fundamental period shifts. The change in reference condition also impacts longer-period (lower-frequency) amplification ratios more significantly. Overall, the amplification ratios estimated by the model are found to be more sensitive to the change in sediment thickness than to the change in reference condition. The results of the sensitivity study indicate that caution should be taken when using the model for shallow sediment thickness values (100 m or less), as this may lead to unrealistically large amplification ratios depending on the V_S value assumed for the reference condition.