The Aki-Larner method is extended to allow computation of seismic motions in three-dimensional (3-D) irregularly layered subsurface structures. Applying it to the incidences of plane S waves to two types of sedimentary basin models, we find various effects of 3-D structures on seismic motions such as the rapid growth of surface waves, the appearance of surface waves in localized areas, and the strong spatial variations of waveforms which result from the generation of many surface waves at the periphery. As a result of them, seismic motions in 3-D structures cannot be reproduced correctly in amplitude and duration by the simulation using 2-D subsurface structures.

In this paper, the Aki and Larner method is extended to make it possible to compute seismic motions in a three-dimensional framework. We present several numerical examples of seismic motions in three-dimensional (3-D) sedimentary basins with an arbitrarily-shaped interface, due to a rectangular fault source. These numerical examples are then compared with a 3-D wavefield with a flat-layer subsurface structure quantitatively, and with a two-dimensional (2-D) wavefield with irregular-layer subsurface structures qualitatively. We found the seismic motions of the 3-D sedimentary basin models to be larger in amplitude and longer in duration than those of the flat-layer models, and the waveform duration except for the onset portions of a 3-D wavefield to be quite different from those of a 2-D wavefield. Moreover, we investigate the effects of such parameters as dip angle, epicenter location, and irregular interface shapes, on flat free surface responses. By comparing 3-D and 2-D wavefield seismic motions with irregular-layer subsurface structures due to dip-slip faults, we find that seismic motions in a 3-D wavefield are more varied(depending on the effects of the above parameters) than those of a 2-D wavefield. From these results, we see that the effects of a seismic source and a 3-D irregular subsurface structure must be taken into account when predicting strong ground motions.

Seismic wave records at the surface of sedimentary plain are mainly composed of main phase and surface waves. The informations of input wave record in base rock and S wave velocity distribution to base rock are enough to consider the main phase.And moreover the informations about underground structure to bed rock of all sections of plain are necessary to discuss surface wave which travels from surrounding rock range to observation point. For this items of the Osaka plain, there were some crumbs of informations. In 1988, we tried two explosions at a manmade "Hokko" island in Osaka bay. This paper shows the results: five ground structures and a countour map of depth to base rock of the Osaka plain.Key Word: Surface wave, Sedimentrary plain, Underground structure of the Osaka plain.

No abstract

No abstract