Estimation of Long Period Ground Motion

This is a Group Summary following the CUREe-Kajima Meeting at Tokyo during September 2 - 3, 1992. The subject of the present report is the long-period strong motion with periods around 8s often observed in the Tokyo-Yokohama area. The goal of the project is to simulate the observed motions using realistic earthquake source model and three dimensional structure of the Kanto basin. In the beginning, this goal looked too ambitious to attain in the short project period. To our surprise, we seem to have attained it, because we now have a clear, quantitative understanding of both amplitude and duration of the long-period motion in the Tokyo-Yokohama area.

The understanding was achieved primarily because we used three distinctly different methods to deal with the complex wave field. They are (1) the Gaussian Beam method for surface waves in laterally smoothly varying media, (2) the T-matrix, generalized reflection/transmission matrices method, and (3) the Boundary Element method (BEM) for laterally sharply varying media. The methods (1) and (3) were applied to the 3D basin structure, while the method (2) were restricted to the 2D case.

We found that the Gaussian Beam method, which is more than thousand times faster in computation than others, can correctly simulate the effects of the location, focal depth and focal mechanism of the earthquake source, as well as the effects of focusing and defocusing due to lateral variation of the basin structure. This method accurately reproduces both amplitude and phase for early arriving surface waves including the focusing effect of the low velocity Quaternary basin in the Sagami bay which acted like a lens to focus surface wave energy and to cause amplification of ground motions at Tokyo and Yokohama.