If a replaced sand layer by which clay was replaced in order to improve a breakwater foundation liquefies during an earthquake, a breakwater should subside and may be unable to secure required height to protect the hinterland from Tsunami. As compared with the 1995 Hyogoken-nanbu Earthquake with such subsidence damage, scenario earthquakes in recent years has long continuation time, and there are many scenario earthquakes which have a larger power spectrum intensity (PSI) used for the index of earthquake motion scale.
　In this research, shake table tests of the large model in 1g gravitational field using two earthquake motions were conducted in order to understand deformations of the breakwaters and influences of characteristics of earthquake motions. One motion is the vertical array acceleration record obtained at Port Island in Kobe city during the 1995 Hyogoken-nanbu earthquake. The other motion is a scenario motion assumed for the earthquake along the Nankai Trough, whose continuation time is long but maximum acceleration is not so large.
　In the experimental result, the breakwater subsided with the rise of excess pore water pressure in the replaced sand layer and the subsidence velocity was accelerated once liquefaction took place in a part of replaced sand layer. Although value of excess pore pressure ratio causing the breakwater subsidence was the same in the experiments with the two earthquake motions, process of excess pore water pressure generation and subsidence rate were affected by waveform characteristics of the earthquake motions. There was significant correlation between the PSI and the amount of subsidence; the larger PSI an input earthquake motion had, the larger subsidence was.