Abstract
The characteristics of the long-period ground motion with a period range from 2 to 20 s observed in the southwestern Kanto district, Japan, are discussed. Dispersive wave trains are found in seismograms observed during a moderate earthquake near the Izu-Oshima Island. The observed group velocities are in good agreement with the ones calculated for Love wave in subsurface structures derived from seismic refraction work in the area. It is interpreted that the Love wave radiated from the source is amplified, then it propagates in sediments of the plain. Normal mode summation technique and 2D finite difference method are combined to forward modeling of the Love-wave propagation in the sediments. The subsurface structure from the source to the margin of the plain is modeled by a multi-layered structure derived from the dispersion analysis. A two-dimensional model is constructed from the margin to sites on the plain, considering the result of the seismic surveys. The observed displacements in the first 60 s are well simulated. However, later arrivals cannot be explained at all. This is probably related to the simple assumption of the subsurface structure from the source to the margin of the Kanto Plain in the Sagami Bay.
