Spatial Coherency Analysis Of Ground Motions Based On Strong-motion Arrays

The “spatial coherency” represents that the amplitude and phase of ground motion at two different observation sites are significantly different,but,are closely related between sites within a smaller area.Spatial variability of ground motions is caused by four distinct phenomena,i.e.,the incoherence effect?the wave-passage effect?the site-response effect and attenuation effect,where the attenuation effect is often ignored.Thus,non-uniform seismic ground motion input for the large-size structures and multi-support structures such as long span bridges and large underground structure,is usually mandatory.During the past forty years,most studies on spatial coherency of ground motions predominately used three methods,i.e.,empirical method,semi-empirical method and theoretical method,respectively.The objective of this paper is to analyze the spatial coherency of ground motions recorded at dense observation arrays from home and abroad through the empirical method.Not only the dependency of ground-motion coherency on surface distance and vibration frequency were discussed,but also the dependency on underground depth was discussed emphatically.And the effects of magnitude,epicentral distance and classifications of site on spatial coherency were investigated.Besides,partial coherency obtained in this study was compared with two models widely used in the literature.The strong-motion data recorded by KiK-net in Japan,EUROSEISTEST array in Greece,Xiang-tang array and SMART-1 array in China were calculated to analyze the spatial coherency.The main work and conclusions are listed as follows:(1)To investigate the effects of underground depth,epicentral distance,magnitude and classifications of site on spatial coherency,the lagged coherency of ground motions are calculated based on the strong-motion data recorded by KiK-net.A remarkable downward tendency of the ground-motion coherency was generally observed with the increased frequency or the depth beneath ground.And the faster decrease of coherency appeared as the frequency increased when recordings at deeper borehole sites were considered.For vertical components,the downward tendency of ground motion coherency was similar to horizontal components at low frequencies,however,at high frequencies,a distinct crest appeared.Besides,the numerical value of coherency of ground motions at site B is larger than that at site C.The value of coherency at site D is the smallest,but the attenuation is the fastest.In addition,at lower frequencies,the influence of epicentral distance was little.But at higher frequencies,it was observed that the coherency decreased as the frequency increased when the epicentral distance increased.The effect of magnitude on spatial coherency can be ignored.(2)In order to analyze the spatial coherency of ground motions recorded at the dense observation array EUROSEISTEST located in Greece,the coherence of ground motion at both horizontal and vertical components was investigated.A remarkable downward tendency of the ground-motion coherency was generally observed with the increase of the frequency or the depth beneath ground.The coherency of ground motions at horizontal components was more significant than that at vertical components.The effect of epicentral distance on spatial coherency was observed merely at lower frequencies,the value of coherency increased a little as the epicentral distance decreased.And the effect of magnitude on coherency can be ignored at low frequencies.Finally,for the horizontal components,the ground-motion coherency at low frequencies observed in our study showed approximately good consistency with Ding's empirical model.Nevertheless,the result at high frequencies fitted badly with both empirical models.For the vertical components,the ground-motion coherency observed in our study did not have any consistency with both models irrespective of frequency.(3)We used strong-motion data recorded by Xiang Tang array to analyze the dependency of ground-motion coherency on depth and other factors,and used data recorded by SMART-1 array to investigate the dependency of ground-motion coherency on the distance on surface.The downward tendency of coherency was observed with the increase of the frequency,depth beneath ground and distance on surface.The coherency of vertical components was different from that of horizontal components at high frequencies(>10Hz),the oscillation of coherency is larger.Besides,the spatial coherency was observed isotropous,which needs to be studied further.