Study On Spatial Variation Of Ground Motions Based On Array Records

The spatial vairatoin of sesmic ground motions has an important effect on the response of large span structures. Therefore, based on new recognitions and new data, establishing a reasonable description to seismic ground motions’spatial variability and a space correlated multi-point ground motion input method which considers ground motion’s nonstationary characteristic in frequency domain mean a lot to seismic analysis of large span structures. In this study, the spatial variation of seismic ground motions at different directions based on accelerations recorded by Zigong Array(Wenchuan earthquake) and UPSAR Array(San Simeon earthquake, Parkfield earthqauke) were analyzed through the application of digital signal processing technology. Then, the variation mechanisms of the coherency functions at various directions along with the separation distance and frequency changes were analyzed. Based on the analysis results, a synthetic method for spatial correlative multi-support ground motion which considers ground motion’s nonstationary characteristic in frequency domain was developed. This method can provide a useful basis for research and engineering practice. The achievements are summarized as follows:1. The analysis results, spatial variation of seismic ground motions under the condtions of large-earthqauke far-field and middle-earthquake near-field, based on the accelerations recorded by different arrays indicate that:(1) Under the same site conditions, within the local area, the variation of different station’s Fourier specta of accelerations are related to the distance from fault. The farther away from the fault, the lower of the amplitude spectrum and the smaller the coefficient of variation, but this change won’t be affected by focal mechanism.(2) Different stations’records changes in amplitudes are related to frequemcy, directions and height differences between stations. The variation of horizontal record amplitudes in lower frequency is generally consistent, but it shows quite differences in higher frequency. And the increase of the relative position of station will amplify the amplitude. The variation of vertical record amplitudes stays the same in whole frequency range.(3) The degressive trend of the coherency function become obvious with the separation distance elongation and frequency gradually increasing, however, the rates of change in low frequency range are quite different from that in high frequency range. There is a "corner frequency", which increases with the epicenter distance increasing.(4) The variation of coherence function is affected by the site condition at high-frequency range, but the site condition’s influence on its variation trend is not significant. On the contrary, variation trend of the coherence function is well affected by topographical condition, yet its influence decreases with the separation distance elongation.2. The "corner frequency" value that affected by epicentral distance is presented based on the analysis results of spatial variation of gournd motion accelerations with the distance and frequency changes, and also based on the results of existing coherence function models’advantages and disadvantages. A new coherency function model is established, and the fitting paramaters suit for the conditions of large-earthqauke far-field and middle-earthquake near-field presented as well.3. Using the wavelet transform method, a time-frequency envelop function was presented, based on that, a synthetic method for spatial correlative multi-support ground motion which considers ground motion’s nonstationary characteristic in frequency domain was preliminarily studied. This method wil well respond to ground motions frequency variability.