LETTER Earth Planets Space, 57, 203–207, 2005 Long-period ground motions from a large offshore earthquake: The case of the 2004 off the Kii peninsula earthquake, Japan

The 2004 off the Kii Peninsula earthquake excited long-period ground motions over a wide area of Honshu Island of Japan. This remarkable excitation was observed in the Osaka, Nobi, and Kanto basins as well as in the Omaezaki region. The record section indicates two types of developed long-period motions by the basin surface waves, either by the source or passage effect of the shallow and large offshore earthquake. Their combination resulted in the well-developed long-period ground motions observed within the distant basins as those during the 1985 Michoacan and 2003 Tokachi-oki earthquakes did. The distributions of pseudo-velocity response spectra confirmed this development at periods of 5–7 s in the Osaka and Nobi basins and of 7–10 s in the Kanto basin. The comparison of the distributions with the thicknesses of the sediments and the S-wave velocities of the surface layers shows that these characteristics of the long-period ground motions are closely related to the structures of the basins. The earthquake provided a timely warning of damaging long-period ground motions from future megathrust events in the Tonankai, Nankai, and Tokai regions. Key words: Long-period ground motion, large offshore earthquake, response spectrum, basin structure. 1