Abstract
Long-term continuous observation of ambient vibration/seismic motion is underway at the crest of an aged arch dam. The 2011 off the Pacific coast of Tohoku Earthquake occurred in the course of this long-term continuous observation, and the records of main shock, preshocks, lots of aftershocks were obtained, together with ambient vibration records before/after such events. Analyses of such observed data revealed the following;
1) System frequency identification method was applied, using ambient vibration data, which made clear that the proper frequency of the dam during about half a year from summer to winter became lower together with the dam surface temperature, suggesting that the both parameters are correlated positively with each other. On the other hand, it is inferred that the level change of water in the dam reservoir did not affect the above-mentioned proper frequency variation so much, as far as the example shown in this paper was concerned.
2) During the main shock of the 2011 off the Pacific coast of Tohoku Earthquake, observed record with very large maximum acceleration of about 630gal was obtained at the crest of the dam. The duration of the seismic motion of the main shock ranged about three minutes.
3) Spectral analyses of the main shock of the 2011 off the Pacific coast of Tohoku Earthquake and of ambient vibrations before/after the main shock revealed that the predominant frequency during the main shock became remarkably lower than the ones from the ambient vibrations before/after the main shock, and that the predominant frequency from the ambient vibration after the main shock returned back to the one before the main shock on the whole.
4) The phenomenon mentioned in 3) was confirmed to reappear also in the large-scale aftershock occurred on the 7th of April, 2011.
