2018 Volume 18 Issue 4 Pages 4_91-4_110
Shitoki Dam is a rockfill dam of 83.5 meters in height located in Fukushima Prefecture, Japan. It suffered severe earthquake motions during the 2011 off the Pacific coast of Tohoku earthquake and the 2011 Fukushima-ken Hamadori earthquake. The long-term observation records at the dam site provide valuable information about the wave propagation at a large strain level in and around the dam body. We conducted Normalized Input-Output Minimization (NIOM) method for the observation records and examined the propagation time of seismic waves in the dam body. The results revealed that (1) the S-wave propagation time increased from 0.153 s to about 0.23 s during the 2011 off the Pacific coast of Tohoku earthquake due to nonlinear behavior of the dam body; (2) this propagation time corresponds to the 53% reduction of the shear moduli of the dam body at the strain level of 2 to 3 ×10-4; (3) a similar increase and decrease in the S-wave propagation time were observed during the Fukushima-ken Hamadori earthquake; and (4) the damping ratio in the dam body also increased due to the principal motions of the mainshocks and decreased after them in the similar manner as seen in the results of the propagation time.
