Vietnam is located in a strong earthquake zone with many areas having a ground acceleration (a_g) of greater than 0.08 g (TCVN 9386:2012). According to Vietnamese standard TCVN 9386:2012, the calculation and design of construction grade III or higher in areas where have ground acceleration a_g ≥ 0.08 g must include seismic design. In calculations of earthquake-resistant pile foundation, the bearing capacity of the pile including the tip resistance strength (q_b) and endurance strength (f_i) need to be multiplied by attenuation coefficients (g_eq1 and g_eq2). They are the most important parameters and depend on the soil types, saturation conditions, and earthquake intensity. The article introduces a method to determine the attenuation coefficients according to the standard TCVN 10304:2014. In addition, analyzing the theoretical basis to give the expression to determine attenuation coefficients based on the durability factor (I_red) and pore water pressure ratio (R_u) of saturated sand under the earthquakes. Furthermore, the paper presents a method to determine R_u from the results of standard penetration test (SPT) by combining the method of Seed and Alba (1986) with the method of Marcuson and Hynes (1990). In which, Seed and Alba's method was used to determine factor of safety against liquefaction (F_SL), and then Marcuson and Hynes' method was used to determine R_u from F_SL. The application in Nhon Hoi Economic Zone shows that: The silty fine grained sand, which is medium dense and saturated, has a attenuation coefficient of tip resistance g_eq1 = 0.74÷0.76 and attenuation coefficient of friction g_eq2 = 0.90; The fine grained sand, which is dense and saturated, has g_eq1 = 0.79÷0.82 and g_eq2 = 0.94.