TAG - Geological Society of Australia

school children. Here, along the base of the cliff sat a line ofabandoned houses where roofs were punched with holes andencircled by newly fallen rubble. Atop the cliff, another cluster ofabandoned houses stood at strange angles, rooflines askew.Evidently, the location of houses below these cliffs (comprised ofa series of ash layers, basalt and ignimbrites) was not consideredto be a problem at the time of construction. This scene of rockfall-relateddamage continued throughout the suburb of Sumner,which was badly affected by the 21 February 2011 earthquake.Another major cause of building damage was liquefactionand lateral spreading in riverbank gravels along the HeathcoteEstuary and Avon River. In Avonside, street after street of houseshad been abandoned as ground ruptures affected the buildings’foundations and/or structural integrity. The uneven road was thefirst indication of subsidence, although it was not until we tooka closer look at street level that we saw why these houses wereempty. In one instance, a surface rupture ran across the frontyard of one house, affecting the foundations, and then continuedthrough the side fence to damage the neighbouring house. It wastragic to see these charming houses with their leadlight windows(built from the 1880s to 1920s) in such a depressing state. Itseemed the old weatherboard houses survived the lateral spreadingbetter than the brick houses. Bricks made of Sumner rockhave a flat top and base and therefore provided no flexibilityduring the earthquake.Next we were shown a study site adjacent to the Avon Riverwhere Masters student Sarah Baston is looking into theliquefaction history. Her trench revealed multiple generations ofliquefaction and paleoliquefaction, indicating that there waspre-Darfield seismicity in the area. In total, 150 houses were lostto cliff collapse and rockfalls, while roughly 6000 houses werelost from liquefaction and lateral spreading.We then wound our way through the back streets towardsthe CBD. At one stage a road closure and road repairs made usturn around. Finally in the CBD, we noted many of the historicbuilding frontages were boarded up and fenced off at the awningfor pedestrian safety. Orange bollards and signs declared ‘RoadClosed’ or ‘No Public Access’. There were excavators clearing awayrubble in empty building sites, and then there were whole blocksof damaged buildings, some with scaffolding, others covered withtarpaulins. One building had been recently demolished, leavinga pile of jagged rubble and dust. We were able to take a shortwalk down to the Christchurch Cathedral site cordoned off byhigh fences. Scaffolding was present where the steeple oncestood and another wall had started to crumble away. It wasobviously once a magnificent building and as it was located atthe heart of the city, it represented much more than just acathedral. Places where building damage is most severe aroundChristchurch have been mapped as ‘Red Zones’. These zonesclosely correlate with gravel point bars along the Avon River,which are sources of liquefaction throughout Christchurch.Group photo in front of Lake Poerua, Inchbonnie, near to surface expression of the Alpine Fault. Photo courtesy Emma Mathews.TAG December 2012| 39