During the 1999 Athens earthquake, the town of Adàmes, located on the eastern side of the Kifissos river canyon, experienced unexpectedly heavy damage. Despite the particular geometry of the slope that caused significant motion amplification, topography effects alone cannot explain the uneven damage distribution within a 300-m zone parallel to the canyon's crest, which is characterized by a rather uniform structural quality. In this article, we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion. For this purpose, we first conduct an extensive time-domain parametric study using idealized stratified profiles and Gaussian stochastic fields to characterize the spatial distribution of soil properties, and using Ricker wavelets to describe the seismic input motion; the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion. We next perform elastic two-dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings.

We analyzed temporal changes in site response associated with the strong ground motion of the 2004 M w 6.6 Mid-Niigata earthquake sequence in Japan. The seismic data were recorded at a site with accelerometers at the surface and a 100-m-deep borehole. We computed the empirical surface-to-borehole spectral ratios and used them to track temporal changes in the top 100 m of the crust. We observed that the peak spectral ratio decreases by 40%-60% and the peak frequency drops by 30%-70% immediately after large earthquakes. The coseismic changes are followed by apparent recoveries, with the time scale ranging from several tens to more than 100 sec. The coseismic peak frequency drop, peak spectral ratio drop, and the postseismic recovery time roughly scale with the input ground motions when the peak ground velocity is larger than ∼5 cm=sec (or the peak ground acceleration is larger than ∼100 Gal). Our results suggest that at a given site the input ground motion plays an important role in controlling both the coseismic change and the postseismic recovery in site response.

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of nearsurface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V s ), attenuation (Q s ), and density (ρ) profiles at the sites of interest. Based on the inversion results, V s versus Q s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surfaceto-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average V s of the upper 30 m (V s30 ) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B-C boundary site conditions (V s30 760 m=sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations.

Unusually severe structural damage was reported during the 2010 M 7.0Haiti earthquake in the vicinity of Hotel Montana, located on top of a ridge in the district of Pétionville. Prompted by the observations, U.S. Geological Survey seismic stations were deployed, and aftershock recordings indicated ground-motion amplification on the top of the hill compared to adjacent stations on reference site conditions. The presence of topographic relief has been shown to significantly aggravate the consequences of strong ground motion during past events, and topographic effects were brought forward to explain the observations. In this paper, we test the hypothesis of topographic amplification as the dominant factor that contributed to the damage concentration in the vicinity of Hotel Montana. We initially conduct numerical simulations of the ridge seismic response assuming elastic homogeneous site conditions, and show that numerical predictions of topographic amplification disagree with the field data both in amplitude and in frequency. Conversely, while 1D ground-response analyses for the site conditions at the hilltop predict amplification in the same frequency range as the field data, they significantly underestimate the recorded amplitude. We then conduct numerical simulations of the foothill ridge response to seismic motion while accounting for soil layering, and qualitatively demonstrate that the recorded amplification is most likely attributed to coupled site-topographic amplification effects, namely to seismic waves trapped in the soft soil layers of the near surface, amplified as a consequence of reverberations, and further modified due to diffraction and scattering upon incidence on the irregular ground surface. Parametric investigations of the topography-soil amplification coupling effects are then conducted, and our results show that when accounting for a hypothetical soil-bedrock interface at 100 m depth, predictions are in excellent agreement with the observed motion.

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