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Research papers, Lincoln University

Within four weeks of the September 4 2010 Canterbury Earthquake a new, loosely-knit community group appeared in Christchurch under the banner of “Greening the Rubble.” The general aim of those who attended the first few meetings was to do something to help plug the holes that had already appeared or were likely to appear over the coming weeks in the city fabric with some temporary landscaping and planting projects. This article charts the first eighteen months of Greening the Rubble and places the initiative in a broader context to argue that although seismic events in Christchurch acted as a “call to palms,” so to speak, the city was already in need of some remedial greening. It concludes with a reflection on lessons learned to date by GTR and commentary on the likely issues ahead for this new mini-social-environmental movement in the context of a quake-affected and still quake-prone major New Zealand city. One of the key lessons for GTR and all of those involved in Christchurch recovery activities to date is that the city is still very much in the middle of the event and is to some extent a laboratory for seismic and agency management studies alike.

Research papers, University of Canterbury Library

Damage distribution maps from strong earthquakes and recorded data from field experiments have repeatedly shown that the ground surface topography and subsurface stratigraphy play a decisive role in shaping the ground motion characteristics at a site. Published theoretical studies qualitatively agree with observations from past seismic events and experiments; quantitatively, however, they systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. We have hypothesized in previous work that this discrepancy stems from idealizations of the geometry, material properties, and incident motion characteristics that most theoretical studies make. In this study, we perform numerical simulations of seismic wave propagation in heterogeneous media with arbitrary ground surface geometry, and compare results with high quality field recordings from a site with strong surface topography. Our goal is to explore whether high-fidelity simulations and realistic numerical models can – contrary to theoretical models – capture quantitatively the frequency and amplitude characteristics of topographic effects. For validation, we use field data from a linear array of nine portable seismometers that we deployed on Mount Pleasant and Heathcote Valley, Christchurch, New Zealand, and we compute empirical standard spectral ratios (SSR) and single-station horizontal-to-vertical spectral ratios (HVSR). The instruments recorded ambient vibrations and remote earthquakes for a period of two months (March-April 2017). We next perform two-dimensional wave propagation simulations using the explicit finite difference code FLAC. We construct our numerical model using a high-resolution (8m) Digital Elevation Map (DEM) available for the site, an estimated subsurface stratigraphy consistent with the geomorphology of the site, and soil properties estimated from in-situ and non-destructive tests. We subject the model to in-plane and out-of-plane incident motions that span a broadband frequency range (0.1-20Hz). Numerical and empirical spectral ratios from our blind prediction are found in very good quantitative agreement for stations on the slope of Mount Pleasant and on the surface of Heathcote Valley, across a wide range of frequencies that reveal the role of topography, soil amplification and basin edge focusing on the distribution of ground surface motion.

Images, UC QuakeStudies

A photograph of the Observatory tower at the Christchurch Arts Centre. There are cracks in the masonry of the tower near where the two storeys join. The cracks formed as a result of the 4 September 2010 earthquake. Wire fencing has been placed at the entrance to the courtyard in front as a cordon.