Even though the time series of the sliding mass move- ment is uncertain, the seismological data have provided a clue to understanding the timing of the events. Fig. 16 shows the strong motion records observed at the KiK-net IWTH25 underground, at the audit passage in the Kur- ikoma Dam and in the Aratosawa Dam, respectively. The absolute timing of each record is matched for comparison. The scale of amplitude for the latter part, that is, from 40 to 160 s, is magnified to those of the earlier part in order to identify the aftershocks visibly. As the detailed locations of the observation points shown in Fig. 8, KiK-net IWTH25 is located 13 km north of the Aratosawa landslide. The Kurikoma Dam is about 6 km east of the landslide and the Aratosawa Dam is about 1 km south of it. When a sliding mass hits the opposite side bank, severe ground motion might occur due to the impact. If a seismograph picks up such a motion, the amplitude will surely become larger and such a phase will appear earlier in the short distance to the landslide site. If the propagat- ing velocity of the impact-induced ground motion is assumed to be 3 km/s, the corresponding phase will appear first in the Aratosawa Dam records and then in the Kurikoma Dam records 2 s later. Finally, it will appear in the KiK-net IWTH25 records 4 s later. After a detailed examination, we concluded that the wave group of D, F and K in the figure almost satisfies such requirements. According to these records, the block that started to slip due to the main shock slipped for about 1 min if the wave group of D were attributable to the hit of the sliding body on the opposite bank. As it moved 320 m in 60 s, the average velocity of the mass movement is about 20 km/h. In this study, we just use a travel time of remarkable phase. Although, in a sense, it is a very primitive study, this estimation proposes one possibility of the landslide in time history.