This dissertation details work aimed at better understanding topographic effects in earthquake ground motions. The experiment, conducted in Central-Eastern Utah, used frequent and predictable seismicity produced by underground longwall coal mining as a source of low-intensity ground motions. Locally-dense arrays of seismometers deployed over various topographic features were used to passively monitor seismic energy produced by mining-induced implosions and/or stress redistribution in the subsurface. The research consisted of two separate studies: an initial feasibility experiment (Phase I) followed by a larger-scale main study (Phase II). Over 50 distinct, small-magnitude (M[subscript 'L'] < 1.6) seismic events were identified in each phase. These events were analyzed for topographic effects in the time domain using the Peak Ground Velocity (PGV), and in the frequency domain using the Standard Spectral Ratio (SSR) method, the Median Reference Method (MRM), and the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The polarities of the horizontal ground motions were also visualized using directional analyses. The various analysis methods were compared to assess their ability to estimate amplification factors and determine the topographic frequencies of interest for each feature instrumented. The MRM was found to provide the most consistent, and presumably accurate, estimates of the amplification factor and frequency range for topographic effects. Results from this study clearly indicated that topographic amplification of ground motions does in fact occur. These amplifications were very frequency dependent, and the frequency range was correctly estimated in many, but not all, cases using simplified, analytical methods based on the geotechnical and geometrical properties of the topography. Amplifications in this study were found to generally range from 2 to 3 times a reference/baseline site condition, with some complex 3D features experiencing amplifications as high as 10. Maximum amplifications occurred near the crest of topographic features with slope angles greater than approximately 15 degrees, and the amplifications were generally oriented in the direction of steepest topographic relief, with some dependency on wave propagation direction. / text

Topography effects, the modification of seismic motion by topographic features, have been long recognized to play a key role in elevating seismic risk. Site response, the modification of ground motion by near surface soft soils, has been also shown to strongly affect the amplitude, frequency and duration of seismic motion. Both topography effects and 1-D site response have been extensively studied through field observations, small-scale and field experiments, analytical models and numerical simulations, but each one has been studied independently of the other: studies on topography effects are based on the assumption of a homogeneous elastic halfspace, while 1-D site response studies are almost exclusively formulated for flat earth surface conditions. This thesis investigates the interaction between topographic and soil amplification, focusing on strong ground motions that frequently trigger nonlinear soil response. Recently, a series of centrifuge experiments tested the seismic response of single slopes of various inclination angles at the NEES@UCDavis facility, to investigate the effects of nonlinear soil response on topographic amplification. As part of this collaborative effort, we extended the search space of these experiments using finite element simulations. We first used simulations to determine whether the centrifuge experimental results were representative of free-field conditions. We specifically investigated whether wave reflections caused by the laminar box interfered with mode conversion and wave scattering that govern topographic amplification; and whether this interference was significant enough to qualitatively alter the observed amplification compared to free-field conditions. We found that the laminar box boundaries caused spurious reflections that affected the response near the boundaries; however its effect to the crest-to-free field spectral ratio was found to be insignificant. Most importantly though, we found that the baseplate was instrumental in trapping and amplifying waves scattered and diffracted by the slope, and that in absence of those reflections, topographic amplification would have been negligible. We then used box- and baseplate-free numerical models to study the coupling between topography effects and soil amplification in free-field conditions. Our results showed that the complex wavefield that characterizes the response of topographic features with non-homogeneous soil cannot be predicted by the superposition of topography effects and site response, as is the widespread assumption of engineering and seismological models. We also found that the coupling of soil and topographic amplification occurs both for weak and strong motions, and for pressure-dependent media (Nevada sand), nonlinear soil response further aggravates topographic amplification; we attributed this phenomenon to the reduction of apparent velocity that the low velocity layers suffer during strong ground motion, which intensifies the impedance contrast and accentuates the energy trapping and reverberations in the low strength surficial layers. We finally highlighted the catalytic effects that soil stratigraphy can have in topographic amplification through a case study from the 2010 Haiti Earthquake. Results presented in this thesis imply that topography effects vary significantly with soil stratigraphy, and the two phenomena should be accounted for as a coupled process in seismic code provisions and seismological ground motion predictive models.

A field experiment was conducted at near Lake Coleridge in the Southern Alps of New Zealand, focusing on the kinematic response of bedrock-dominated mountain edifices to seismic shaking. The role of topographic amplification of seismic waves causing degradation and possible failure of rock masses was examined. To study site effects of topography on seismic ground motion in a field situation, a small, elongated, and bedrock-dominated mountain ridge (Little Red Hill) was chosen and equipped with a seismic array. In total seven EARSS instruments (Mark L-4-3D seismometers) were installed on the crest, the flank and the base of the 210 m high, 500 m wide, and 800 m long mountain edifice from February to July 2006. Seismic records of local and regional earthquakes, as well as seismic signals generated by an explosive source nearby, were recorded and are used to provide information on the modes of vibration as well as amplification and deamplification effects on different parts of the edifice. The ground motion records were analyzed using three different methods:comparisons of peak ground accelerations (PGA), power spectral density analysis (PSD), and standard spectral ratio analysis (SSR). Time and frequency domain analyses show that site amplification is concentrated along the elongated crest of the edifice where amplifications of up to 1100 % were measured relative to the motion at the flat base. Theoretical calculations and frequency analyses of field data indicate a maximum response along the ridge crest of Little Red Hill for frequencies of about 5 Hz, which correlate to wavelengths approximately equal to the half-width or height of the edifice (~240 m). The consequence of amplification effects on the stability and degradation of rock masses can be seen: areas showing high amplification effects overlap with the spatial distribution of seismogenic block fields at Little Red Hill. Additionally, a laboratory-scale (1:1,000) physical model was constructed to investigate the effect of topographic amplification of ground motion across a mountain edifice by simulating the situation of the Little Red Hill field experiment in a smallscale laboratory environment. The laboratory results show the maximum response of the model correlates to the fundamental mode of vibration of Little Red Hill at approximately 2.2 Hz. It is concluded that topography, geometry and distance to the seismic source, play a key role causing amplification effects of seismic ground motion and degradation of rock mass across bedrock-dominated mountain edifices.