The spatial variation of seismic ground motions has an important effect on the response of lifelines, such as bridges and pipelines; it may induce significant additional loads in the structure than the ones obtained if the structure's supports are subjected to the same seismic motions. The spatial variation of seismic ground motions results from the apparent propagation of the waveforms on the ground surface and the differences in their shape at the various locations of the supporting structure.
In a previous study, the PI developed an alternative methodology for the investigation of spatially variable seismic ground motions recorded at dense instrument arrays. It models the seismic motions as superpositions of sinusoidal functions, described by their amplitude, frequency, wavenumber and phase. The application of the methodology to data recorded at the SMART-1 array in Lotung, Taiwan, revealed that the common, coherent component approximates to a very satisfactory degree the actual motions, and that the spatial variability is caused by arrival time delays at the array stations associated with the upward traveling of the waves through the site topography, and by variabilities in both the amplitudes and phases of the motions around the common component characteristics.
The developed methodology considers simultaneously the amplitude and the phase variation in seismic motions. Its application to recorded data recognized correlation patterns between amplitude and phase variabilities around a coherent approximation of the motions; this observation may lead to significant developments in the description of spatial variability. However, the methodology has been applied to one site only, and, for the generalization of its results, it is necessary that it be applied at different sites subjected to various earthquakes.
Therefore, the present study uses extensive additional sets of data recorded at rock and soil sites. Differentiation is made in terms of the earthquake magnitude: small and large events (e.g., main event and aftershocks) at the same site are investigated. The effect of the water table at the sites ('wet' and 'dry' sites) is also taken into consideration.
