A great number of earth-fill dams suffered significant damage during previous major earthquakes in Japan including the 2011 off the Pacific Coast of Tohoku Earthquake and the 2016 Kumamoto Earthquakes. One of the major factors is a relatively low degree of compaction of the dam body, which results into low undrained shear strength prone to severe deterioration during cyclic loading (liquefaction). A new simplified and pragmatic seismic stability analysis that takes into consideration these factors is proposed, combining a modified Newmark slip stability analysis and a pseudo-static non-linear FEM analysis, both taking into account the continuous degradation of undrained stress-strain properties during seismic load based on the cumulative damage and total stress concepts. Working examples are presented and confirm significant effects of compaction on the dam seismic stability. A case study of an actual case, the collapse of Fujinuma dam by the 2011 Great East Japan Earthquake, is presented in a companion paper.