Abstract
We investigated the causes of large ground motions with the seismic intensity scale of 7 at K-NET Tsukidate and KiK-net Haga during the Tohoku earthquake. We showed that the vibrations pointed by Motosaka et al. as partial uplifting and slipping of the foundation of the seismometer and the nonlinearity of soil amplified 10 times at 4Hz for NS component and 15 times at 8Hz for vertical component at Tsukidate. We also pointed out that similar vibrations which are not ground motions amplified twice at 2 Hz for horizontal components and 10 times at 4 Hz for vertical component at Haga. The seismic intensity scale calculated from ground motions without such vibrations was estimated to be 6 upper at Haga by the equivalent linear analysis.
