Topographic Amplification of Ground Motions in Mt. Pleasant, Christchurch, New Zealand: An Experimental Study

摘要

This paper presents an experimental quantification of ground motion topographic amplification in the Mt. Pleasant and Heathcote Valley area of Christchurch, New Zealand, where the largest ground motions were recorded during the 2011 Christchurch earthquake. The Lyttelton volcanic group, overlain by only a few metres of Aeolian soils, dominates the geology of the area, whereas the thickness of sediments in the valley can be up to 40 metres. A linear array of nine three-component seismometers recorded a number of distant earthquakes in March 2017, which were examined using the standard spectral ratio (SSR) technique with two different reference stations, the median reference method (MRM) and the horizontal-to-vertical spectral ratio (HVSR) method. The SSR method with two different reference stations consistently demonstrated that the ground motions on Mt. Pleasant were amplified in a frequency band consistent with previous studies; the amplitude of this amplification was also comparable with previous theoretical and numerical studies. However, MRM was not able to demonstrate the effect of surface topography in this case study, most likely because of the strong basin amplification dominating the median reference motions. We found that horizontal ground motions are moderately polarised along the ridge-parallel direction, which is inconsistent with many previous studies that reported the polarity of horizontal motions along the ridge-perpendicular direction. The reason for the observed inconsistency is currently unclear, and this requires further study.