There are a number of papers and sources for how to determine the seismic coefficient in slope stability analyses. For example, the Hynes-Griffin and Franklin paper recommends a seismic coefficient equal to 0.5*PGA (peak ground acceleration). As well, the Open Pit Slope Design book by Read and Stacey provides a chart for converting PPV to seismic coefficient.
The following references are recommended:
Hynes-Griffin, M. E., and Franklin, A. G. 1984. ‘‘Rationalizing the Seismic Coefficient Method.’’ U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, Misc. Paper GL-84-13.
Baker, R., Shukha, R., Operstein, V., and Frydman, S. "Stability charts for pseudo-static slope stability analysis." Soil Dynamics and Earthquake Engineering. 26(9): 813-823.
Woodward, P. K., and Griffiths, D. V. 1996. "Comparison of the pseudo-static and dynamic behaviour of gravity retaining walls." Geotechnical and Geological Engineering. 14: 269-290.
Read, J., and Stacey, P. F. 2009. "Guidelines for Open Pit Design." CSIRO Publishing, Melbourne.