Research on Polarization of Earthquake Ground Motion and Application to Strong Motion Prediction

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Accelerograms of 17 earthquakes recorded by a dense array system (Chiba array) have been analyzed to investigate polarization characteristics of ground motions. The followings are concluded.1.Polarization directions of low frequency ground motions are almost the same for all observation points of the array, but those of high frequency ground motions are very different from one another.2.Each of the observation points has its preferred direction of ground shaking at high frequency.The local site effect on polarization characteristics is caused by the directional difference in seismic response of surface layrs, which may be due to anisotropy of the surface layrs.4.Polarization of ground motion becomes weak with the increase in earthquake magnitude, hypocentral distance and frequency.Average polarization characteristics of the array site vary with the frequency of ground shaking and do not directly reflect the radiation pattern of seismic waves at the earthquake source.The above items 4 and 5 should be noted in synthesizing ground motions due to large earthquakes from records of small earthquakes by the empirical Green's function method.