Experimental Investigation of the Topographic Modification of Earthquake Ground Motion

Abstract

Topographic modification of earthquake ground motion can significantly impact ground motion amplitude and frequency content. While previous studies have investigated topographic modification of ground motion, also called "topographic effects", there are discrepancies between the results of field and numerical investigations. A new experimental approach involving physical modeling in a geotechnical centrifuge was used to study topographic effects. The centrifuge captures the complexity of a physical process and shares many of the advantages of a numerical model (e.g., material properties, instrumentation location, and ground motions can be controlled). Experimental results show that topographic amplification can exceed amplification due to subsurface geology (i.e., "site amplification"). The results also indicate the main cause of topographic amplification is slope resonance, which leads to differential movement, and the development of shear planes between the slope and surrounding landmass. Slope resonant frequencies are within the range considered in engineering practice, which can impact the design of structures near slopes. Findings from the centrifuge investigation were used to inform analyses of a ridge in the Port Hills suburb of Christchurch, New Zealand. Topographic effects were analyzed using data from a downhole array located adjacent to two cliff faces. A simplified procedure was adopted to establish a reference free field station. Results of the analyses were found to be in good agreement with the centrifuge investigation.