LIQUEFACTION INDUCED BY THE 2010-2011 CANTERBURY EARTHQUAKE SEQUENCE AND IMPLICATIONS OF RECENTLY DISCOVERED PALEOLIQUEFACTION FEATURES

Extensive liquefaction occurred in Christchurch city and the surrounding area during the 2010-2011 Canterbury Earthquake Sequence. Although small historical earthquakes are known to have induced liquefaction in the Canterbury region and liquefaction susceptibility maps of Christchurch have been available for decades, the extent of liquefaction and consequent financial loss (c. $15-20 billion) was dramatic. Given multiple active faults around the region (including the tectonic plate boundary, the Alpine Fault), it is important to assess the potential for known and unknown seismic sources to cause future liquefaction in the Christchurch area. We are searching for paleoliquefaction features in the region affected by the 2010-11 events to inform our understanding of future seismic hazards in Canterbury. Here we present our studies in the Lincoln area, 16 km southwest of Christchurch city centre, where liquefaction occurred during the 4 September 2010 (M_w 7.1; 29 km away; 0.6-0.8 g PGA) and the 22 February 2011 (M_w 6.2, 17 km away; 0.1-0.2 g PGA) earthquakes. The study area occurs on a low lying (< 6 m AMSL) floodplain-delta complex formed on the margins of Lake Ellesmere by paleo-distributory channels of the Waimakariri River, which now flows to the sea north of Christchurch. We have documented liquefaction features after each event, excavated trenches, and hand cored to 6 m at the sites in order: to assess the severity of liquefaction, to document the size and type of liquefaction features, and to search for evidence of prior events. The main liquefaction features that occurred in the 2010 and 2011 events at this site are sand volcanoes and blisters (locations where sand intruded surface soil horizons, raising the soil surface to form mounds up to 30 cm high and 1.5 m in diameter). Volcanoes were aligned along fissures forming 30 m long, 2 m wide sand ridges. Fissures parallel the abandoned river course. Volcanoes and blisters were fed by dykes (1-4 cm wide). Weathered sand dikes and a sand blow crosscut by modern sand dikes were found at some of the 2010-11 liquefaction sites, indicating at least one paleoliquefaction event. Radiocarbon ages of the paleoliquefaction features (event occurred around 800 year B.P.) and comparisons with the 2010-11 features will help assess the recurrence and severity of liquefaction at this site.