Characterisation of modern and paleo-liquefaction features in Christchurch, New Zealand following the 2010-2011 Canterbury earthquake sequence

The liquefaction of late Holocene fluvial and estuarine deposits during the 2010 Darfield Mw 7.1 earthquake and subsequent aftershocks caused severe land and infrastructure damage in eastern Christchurch, New Zealand. Two sites in the suburb of Avonside that liquefied up to eight different times in earthquakes with PGA ≥ 0.1 were chosen for detailed investigation. Trenching of surface ejecta preserved at one site from the June 13 (2 events) and Dec 23 (2 events) earthquakes revealed four distinct fine-sand ejecta packages overlain by silt drapes, indicating different liquefaction episodes could be preserved within the geologic record. Subsurface trenching investigations at both sites revealed that the sub-vertical planar feeder dykes and lateral spreading cracks that fed the surface vents were re-activated during each liquefaction episode, with silt drapes also separating events. The location of these modern feeder dykes and cracks were also found to correspond with paleo-liquefaction features including oxidized and bioturbated fine grained sand sills and dykes that were cross-cut by modern liquefaction features and anthropogenic pits. One of these dykes was traced up the trench walls where it was truncated by a paleo-flood deposit. The presence of paleo-liquefaction confirms that previously undocumented liquefaction has occurred within eastern Christchurch prior to the Darfield earthquake sequence. The alignment of the modern and paleo-liquefaction features suggests re-use of the same zone of weakness, therefore mapping the distribution of liquefaction provides important information on the likely location of future surface ejection events. It is anticipated that this may aid the re-zoning of land for re-building Christchurch, and improve recurrence interval models for liquefaction.