The Little Red Hill Seismic Experimental Study: Topographic Effects on Ground Motion at a Bedrock-Dominated Mountain Edifice

The kinematic response of a bedrock-dominated mountain edifice to seismic shaking near Lake Coleridge in the Southern Alps of New Zealand was examined. A small, elongated, and bedrock-dominated mountain ridge (Little Red Hill) was equipped with a seismic array. Seven EARSS instruments (Mark L-4-3D seismometers) were installed along the ridge crest, the flank, and at 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 were used to provide information on amplification and deamplification effects, as well as on the frequency response at different parts on the edifice. The ground-motion records were analyzed using three different methods: comparisons of peak ground accelerations, power spectral density analysis, and standard spectral ratio analysis. 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. The field experiment also highlighted the extreme variations in amplification with regard to closely located sites but at contrasting topographic locations. 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. 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 seismically generated block fields at Little Red Hill. It is concluded that topography, geometry, and distance to the seismic source play key roles in causing amplification effects of seismic ground motion and supporting dilation and degradation of rock mass across bedrock-dominated mountain edifices.