Seismic wave field, spatial variability and coherency of ground motion over short distances : near source and alluvial valley effects

Spatial variation of earthquake ground motion over short distances significantly affects the dynamic response of engineered structures with large dimensions. In current practices, the ground motion excitation across the foundation of a structure is assumed to be spatially uniform, which becomes inadequate for spatially extended structures in the near-fault region or on sites with lateral inhomogeneity. This PhD thesis seeks to understand the key parameters that locally control the ground motion spatial variability with the intent of putting forth practical propositions for incorporating such effects in seismic design and hazard assessment. The first part of the thesis addresses the within-event component of the standard deviation of ground-motion distribution in near source region by means of numerical simulation of ground motions for extended sources with realistic rupture kinematics. The results suggest that the within-event variability significantly depends on the rupture type, depicting an increase with distance for bilateral ruptures and a decrease for unilateral ruptures. The second part deals with the characterization of seismic wave field at the Koutavos-Argostoli site, a small-size, shallow, alluvial valley located in the seismically active Cephalonia Island in Western Greece. The seismic wave field was investigated from the recordings of a dense seismological array for a set of 46 earthquakes, with magnitude 2 to 5 and epicentral distance up to 200 km. The MUSIQUE array analysis algorithm was used to extract the phase velocity, back-azimuth, type and polarization of the dominant waves crossing the array. The results clearly indicate dominant scattering of seismic surface waves, mainly from the valley-edge directions, beyond the fundamental frequency of the valley. While Love surface waves clearly dominate the wave field close to the resonance frequency, Rayleigh waves strongly dominate only in relatively narrow frequency bands at higher frequency. Besides, an excellent consistency is observed between the dominance of the identified surface wave type in the wave field and the site amplification. The "lagged coherency" of the most energetic part of the ground motion has been quantified for each station-pair within the array. In general, spatial coherency estimated from the horizontal components exhibit decays with frequency and interstation distance. Estimates from the vertical component exhibit rather larger values at some higher frequencies. Although coherency does not show any consistent trend indicating dependence on the magnitude, back-azimuth or site-to-source distance of the event, it seems to be primarily controlled by the site geometry. Larger coherency is systematically observed when the station pair is oriented parallel to the valley axis, while lower values are observed in the perpendicular direction. This observation proves to be consistent with the MUSIQUE analysis results: the predominance of scattered surface waves propagating across the valley implies an in-phase motion along valley-parallel direction and out-of-phase motion along valley-perpendicular direction. The findings of the present research are expected to contribute in enhancing our understanding of spatial variability of ground motion and improving the coherency models used in engineering. This work also opens up new insights and many questions in need of further investigation.