The amount and significance of observed spatial variations of the phasing of seismic strong ground motion are explored using synchronized accelerograms from a surface array of digital strong motion accelerometers. A dynamic response ratio is defined for a discrete linear structural system in order to measure the effect of spatial variations of the ground motion on the response of large structures with multiple seismic wave inputs. The response with differential phasing may differ significantly to that with traditional rigid base inputs. Measurements show that phase shifts in the ground motion recorded across the SMART 1 array in Taiwan can produce up to a 25 per cent reduction in the inphase response spectrum at 5 Hz for a structure spanning a distance of 200 m. The out-of-phase dynamic response ratio is 0.20 to 0.40 for a flexible structure with a period of 1 sec and a support spacing of 200 m.

In order to analyze more fully the effect of multi-support input motions, the concept of a seismic phase response spectrum is developed to complement the usual Housner (amplitude) response spectrum. An efficient estimation algorithm is outlined using complex demodulation. Seismological phase response spectra would seem to have wide application for engineering design.

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