Micromorphological analysis of liquefaction features in alluvial and coastal environments of Christchurch, New Zealand

Abstract

We conducted micromorphological analysis of liquefaction features (dikes, sills, sand blows) associated with the 2010-2011 Canterbury Earthquakes Sequence and dikes of an earlier generation of liquefaction (paleoliquefaction). Our aim was to determine if diagnostic features existed that would allow robust discrimination of the two generations of liquefaction features to support paleoliquefaction studies and earthquake hazard assessment. We found that there were similarities in sedimentary fabric between all forms and ages of liquefaction but that paleoliquefaction could be distinguished on the basis of pedogenic features. We studied liquefaction and paleoliquefaction at two sites of distinctive sedimentary setting: the first the floodplain of the Halswell River at Greenpark in the Lincoln-Taitapu area, and the second, coastal sand dunes in Wainoni and QEII Park in Christchurch. Analysis was carried out on thin sections prepared from resin-impregnated blocks of soil hosting the liquefaction features. The modern liquefaction fabric in the alluvial system is characterised by a single grain micro-structure, from moderate to well sorted with many simple packing voids and planar voids. Similarly, modern liquefaction fabric in the coastal dune system is characterised by a very well sorted single grain micro-structure. In both environments it was possible to see evidence of porewater-induced structures such as water escape structures, cutans and silt caps. The paleoliquefaction fabric is characterised by similar microstructure type but differs from modern liquefaction by the presence of pedogenic features in the form of infilling of planar voids, silt coating, hypocoating of voids and excrement features. Our results demonstrate how the thin section analysis can be a valuable post field-campaign tool to support and better define evidence of sequences of liquefaction events. In the future, more quantitative applications of this technique could be focussed, for example, on determining the resolution of the technique for discriminating multiple generations of paleoliquefaction.