Exploring methods to assess paleoliquefaction in the Canterbury area

Abstract

We have used the natural laboratory for liquefaction studies provided by the 2010-2011 earthquake sequence in Canterbury to assess the best approach to identify good sites and analyse prehistoric liquefaction in Canterbury and other similar environments worldwide. For that purpose, we tested several exploratory techniques and disciplines such as: geomorphic mapping; paleoseismic trenching; Ground Penetrating Radar (GPR); hand piston coring; and high resolution photography and X-ray of cores.

While a multi-technique approach to identify sites and analyse paleoliquefaction is best, we found that: (a) geomorphic mapping is the best method to identify potential paleoliquefaction sites during the initial exploratory study phase (liquefaction during the 2010-2011 earthquake sequence occurred at specific geomorphic features, e.g., meander point bar environments); (b) paleoseismic trenching is the best method for analysis given that it best exposes liquefaction features (also through trenching, the size and geometry of the 2010-2011 liquefaction features was observed, which helped assessing what other the type of exploration methods could be appropriate) ; (c) GPR can be useful for imaging prehistoric liquefaction features and can thus help study site selection (dependant on soil and surface conditions) and, if soil conditions are appropriate, it is a great complementary method to trenching studies; (d) hand piston coring can be useful to identify occurrence of paleoliquefaction at a site (although the area of exposure is small in the cores, soft sediment deformation, sand dikes and rip-up clasts can sometimes be identified), and very useful to complement trenching studies during the analysis phase (e.g., for extending the depth of stratigraphy below the water table and finding the source layer for liquefaction features); and (e) high resolution photography and X-ray of cores are techniques that enhance detectability of liquefaction features in cores and can be used as a quick and effective tool to assess presence of liquefaction at a site. 

During the study, we identified a paleoliquefaction event that occurred between AD 1019 and AD 1337 in Lincoln. Based on the coincidence in time with known paleo-eathquakes, that event is likely to be caused by large earthquakes on the Alpine Fault or the Porters Pass Fault. Alternatively, liquefaction between AD 1019 and AD 1337 in Lincoln could have been caused by rupture of a closer, hidden fault.