| The Noto Hanto Earthquake, registered by the Japan Meteorological Agency as magnitude (Mj) 6.9, occurred on March 25, 2007 offshore west of the Noto Peninsula facing the Sea of Japan. Two dams, Hakkagawa Dam and Oya Dam, are located within 50 km from the epicenter. We conducted a field survey to investigate the effects of the earthquake on these dams. Hakkagawa Dam, which is a concrete gravity dam, is located 14 km from the epicenter. The seismometer installed at the dam foundation recorded a peak horizontal acceleration of over 200 gal, which is one of the largest acceleration motions ever recorded at a dam foundation in Japan. The increase in drainage amount and deformation of dam body due to the earthquake was slight. The different behavior between each dam block during the earthquake may have caused friction acting between the contraction joint faces. Such energy dissipation may have resulted in the low amplification and high damping against dam vibration mode shown in the spectral analyses of recorded earthquake motions. Oya Dam, which is a rockfill dam, is located 48 km from the epicenter. The amount of seepage through the impervious earth core changed due to the earthquake and the monitored data was evaluated. Earthquake-induced settlement was 25 mm at the crest of the maximum cross section. The amount of settlement was evaluated and compared with cases of other embankment dams in Japan and other countries and regions, through the relationship between crest settlement and peak horizontal acceleration at dam foundations. Besides the above, Oya Dam was struck by a large earthquake of Mj 6.6 in 1993, just after completion. The performance of the dam during the two large earthquakes was compared. |
