We studied the source model and the underground velocity structure model for quantitative estimation of long-period (3-20s) ground motion in the Oita Plain due to a hypothetical Nankai earthquake. First, the deep subsurface velocity structure of the Oita sedimentary basin was validated by a long-period ground motion simulation of the 2000 western Tottori earthquake records. The simulated waveforms reproduced the amplification and duration of the observed waveforms and peak periods of pseudo response spectra at strong motion stations both in and out the Oita Plain reasonably well. Then we combined the subsurface velocity structure with a crustal velocity structure and performed long-period ground motion simulations of a Nankai earthquake by a three-dimensional finite-difference method. The source model has a total area of 34,000 km2 with total seismic moment of 6.24×1021 Nm (MW 8.5). In addition to the scenario in which rupture propagates from the east (east-hypo model) that have been generally accepted, we investigated another from the west (west-hypo model). For the east-hypo model, the maximum amplitudes of the simulated horizontal ground motion in the Oita plain was three to four times as large as that on a rock site beside the plain. Especially in the bay area, the maximum ground motion reached 100cm/s and the pseudo velocity response spectra at period 6-8s were more than 400cm/s. On the other hand, the ground motion simulated by the west-hypo model was roughly one fifth smaller than that by the east-hypo model. It is also pointed out that in the east-hypo model simulation, the seismic waves generated by the two asperities near to the Oita plain are enlarged due to the directivity effect and amplified and prolonged by the sedimentary basin structure.
