Controls on patterns of liquefaction in a coastal dune environment, Christchurch, New Zealand

Abstract

Liquefaction features in the geological record are important off-fault markers of moderate to large (>5 Mw) paleoearthquakes. The study of contemporary liquefaction features provides a better understanding of where to find past (paleo) liquefaction features which, if correctly identified and dated, can provide information on the occurrence, magnitude and timing of past earthquakes. This is particularly important in areas with blind active faults. This paper describes liquefaction features in the coastal setting of Christchurch (South Island of New Zealand), and explores the role of liquefaction and fluidization in the surface soil profile and their role in controlling surface ejection. The paper also compares the styles of liquefaction surface manifestation in the alluvial and coastal settings, and the role played by the sedimentary architecture in both environments. This analysis contributes to our understanding of which of the two environments provides a better target for paleoliquefaction studies, and which geomorphic setting within those environments is most likely to host paleoliquefaction features. The coastal setting (in particular, young coastal areas, <300 years old) is especially prone to liquefaction because near surface soils are dominated by well-sorted sandy aeolian deposits and a shallow water table exists. Fluidization within the near surface sandy layer that liquefied and in horizons above, due to upward moving pore water, resulted in a very disrupted soil stratigraphy, making it difficult to identify paleoliquefaction features. Therefore, young coastal areas are not the best target for identifying individual paleoliquefaction events. In comparison, the alluvial setting is characterized by sandy point-bar and channel deposits capped by cohesive overbank and abandoned-channel deposits. As a consequence, sand dikes that form in the alluvial settings are well defined, with preservation of the original soil stratigraphy and distinct cross cutting relationships. These conditions make the alluvial setting, along with older coastal deposits (>1000 years old), a better target for paleoliquefaction studies.