Topographic amplification on steep slopes during seismic events

The geomorphology of New Zealand has dictated that many transport corridors and residential areas have been formed on, or adjacent to, steep slopes and undulating terrain underlain by variable geology. Observations of the seismic behaviour of cliff-top properties in the Port Hills during the Canterbury earthquakes suggests the current approach recommended in NZS 1170.5 and the NZTA Bridge Manual may be unconservative at estimating the design peak ground accelerations (PGA’s) for sites influenced by topographic amplification. A number of sites may pose a higher risk of seismic failure, with potential to cause damage to dwellings and infrastructure, if topographic amplification effects were to be considered under the current codes. It is fundamental to design success that accurate design PGA’s are estimated for assessments of existing slopes, design of slope stabilisation works near steep slopes, and design of structures near the crests of slopes. This is particularly relevant in view of the high probability of an Alpine Fault earthquake of Mw8 and the relative vulnerability of large metropolitan areas like Wellington to seismic events. Eurocode 8 appears to be an exception to existing literature in that it is a code that provides a readily implementable criterion for estimating topographic amplification. A similar, albeit interim, simplified method for estimating topographic amplification factors in New Zealand is proposed, based on back-analysis of observed movement at the Port Hills of Christchurch during the 22 February 2011 earthquake.