Long-period ground motion of the Tokyo bay area observed from the 2011 off the Pacific coast of Tohoku earthquake, foreshock and aftershocks

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The characteristics of long-period ground motion of Tokyo bay area were studied using the records of the 2011off the Pacific coast of Tohoku earthquake, foreshock and aftershocks. The long-period pulse with period about 20 seconds is significant in velocity seismograms and displacement seismograms of main shock. There is no significant peak in the velocity response spectrum of main shock between 5 to 10 seconds. The velocity response spectra of foreshock and aftershocks located in Miyagi-oki and Ibaraki-oki area also have no significant peak. On the other hand, the spectra of the event located in northern Nagano area have significant peak in periods of 5 to 7 seconds. The excitation of long-period ground motion is different in accordance with the epicenter regions. The phase velocities in period range between 7 to 8 seconds evaluated from the 2011 off the Pacific coast of Tohoku earthquake are faster than those of the 2007 Niigataken Chuetsu-Oki earthquake. Long-period ground motions were excited in Tokyo metropolitan area during the 2007 Niigataken Chuetsu-Oki earthquake. This result suggests that the incident wave field has a relation to the excitation of long-period ground motions.