The continuation time of the seismic motion in the 2011 off the Pacific coast of Tohoku Earthquake was extraordinary long, more than three minutes, which had never been experienced. This paper describes the liquefaction behavior of soil under seismic motions having a long continuation time. The paper includes case studies on the liquefaction prediction method, verification of an empirical equation for the correction factor on the irregularity of seismic wave proposed by authors, and discussions on the effect of the continuation time on liquefaction phenomenon. The main results are follows.
　It is revealed that the liquefaction prediction method gives a conservative prediction for seismic motions having a long continuation time and the method is applicable to the practical design.
　The empirical equation for the correction factor on the irregularity of seismic motion can estimate the correction factors obtained from the laboratory experiments using the seismic motions having a long continuation time.
　When the continuation time becomes long, the degree of liquefaction, that is the magnitude of liquefaction damage, becomes large and the soil behavior becomes less ductile.