ANALYSIS AND SIMULATION OF SEISMIC GROUND MOTIONS OBSERVED BY AN ARRAY IN A SEDIMENTARY BASIN

Abstract

The effects of the irregularity of a shallow subsurface interface on seismic motions are investigated using aftershock records of the western Nagano Prefecture earthquake of September 14, 1984, observed by an array in' the Kyoto basin extending from the rock outcrop near the eastern margin to the center. Comparison between low-pass-filtered seismograms at the basin sites indicates the presence of spatial waveform variation and elongation of duration of the S-wave portion (from the onset of S wave to a later part where amplitudes decrease to a certain low level). The phase and group velocities of the wavelets contained in the S-wave portion suggest that the above characteristics of seismic motion in the Kyoto basin are primarily caused by the excitation of surface waves at the eastern margin of the basin. This means that the prediction of seismic motions in a basin-like structure requires the consideration of irregular interfaces. To reproduce the recorded seismograms in the Kyoto basin, the Aki and Lamer method is extended to allow the computation of seismic responses of a two-dimensional basin-like structure with multiple irregular interfaces. The computed shear waves exhibit spatial waveform variation and elongation of duration due to surface waves generated at the eastern margin of the basin. However, the amplitude of the computed surface waves are less than the observed amplitude. To obtain better agreement, the other effects such as those of three-dimensional subsurface structure and non-plane wave front of incident motions should be included in the synthetic seismograms.