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The Mononobe–Okabe (MO) equation is the most widely used closed form analytical model to evaluate the seismic earth pressure in non-cohesive soil. Nevertheless, MO is known to yield non-conservative results under passive conditions, leading to unsafe design, when the wall-soil interface frictional angle is high. Consequently, although simple and easy to implement, the MO model is not recommended to use in design practice when assessing seismic passive earth pressures. A more advanced, recently published limit-equilibrium based model, known as the Log-Spiral-Rankine (LSR) method, on the other hand has shown improved results over existing earth pressure models including MO. The implementation of LSR model is, however, tedious in its current form as it involves a few iterative schemes to generate the final outcome. A semianalytical equation, combining the MO equation with a modification factor to incorporate the effects considering a curvilinear failure surface, is developed herein for evaluation of the seismic passive earth pressure in non-cohesive backfill. The proposed equation can produce much improved predictions as the LSR method, yet is as simple in mathematical form as the MO model. The proposed equation is thus suitable for use in routine design practice.