PALEO-LIQUEFACTION IN CHRISTCHURCH NEW ZEALAND

Liquefaction during the 2010 M_w 7.1 Darfield earthquake and large aftershocks (Canterbury earthquake sequence, CES) caused severe damage to land and infrastructure in Christchurch, New Zealand. Mapping the distribution of liquefaction features within Avonside, eastern Christchurch following the 22 February 2011 M_w 6.2 earthquake indicates features formed sub-parallel to the closest section of the Avon River, or internal paleo-cut bank. Features were also in highest abundance within 40 m of the river or paleo- bank. Subsurface trenching across aligned fissures at two sites within Avonside revealed multiple generations of CES liquefaction ‘feeder’ dikes that cross-cut the Holocene-to-Recent fluvial and anthropogenic stratigraphy, and partially intrude oxidized and weathered dikes and sills interpreted as pre-CES ‘paleo-liquefaction’. Cross-cutting relationships and ¹⁴C ages constrain the paleo-liquefaction to post 1660 AD to pre ca. 1905 AD at one site, and post 1435 AD to pre ca. 1910 AD at the other. M_w estimates and attenuation relationships are applied to derive approximate PGA_7.5 for five well documented historical earthquakes causing regional damage between 1869 and 1922. Only the 1869 M_w ~4.8 Christchurch earthquake produces a median modelled PGA_7.5 that exceeds the empirically derived threshold for minor liquefaction of PGA_7.5 0.06 g. The recognition of paleo-liquefaction in late Holocene sediments is consistent with hazard model-based predicted return times of PGA above the liquefaction triggering threshold in Christchurch. Residential development in eastern Christchurch from ca. 1860 to 2005 almost certainly took place on top of sedimentary units containing evidence for paleo-liquefaction. This highlights the potential of paleoliquefaction studies, in addition to geotechnical testing, to contribute to land use planning in areas underlain by liquefaction susceptible sediments.