The Mw 7.0 Miyagi-Oki earthquake, which occurred on 26 May 2003, was well recorded by the KiK-net and K-net networks. A large number of stations recorded very high peak ground accelerations above 0.5g and large peak ground velocities above 0.5 m/sec. These high ground-motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high-frequency ground-motion content associated with this intraslab earthquake. The objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration ≥0.3g. Linear site response is first estimated from observed weak motion (aftershock) records. In this case, we use a spectral inversion method, without reference stations, to separate the source, path, and site-response effects. The resulting weak motion analysis for the source, path, and site response agree with other previous studies. The mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency-dependent radiation pattern. The comparison of the site amplification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the Mw 7.0 event. The results also suggest a correlation between low near-surface material velocity and the degree of nonlinearity.