Abstract
Liquefaction has been reported following upwards of 13 recent and historical earthquakes in New Zealand. Collating reports outlining the extents of liquefaction manifestation following these events provides insights into the distributions of sediments with low cyclic resistances to liquefaction, and enables liquefaction hazard assessments to be cross-examined. Collated reports following the 1987 Edgecumbe earthquake indicate that localized liquefaction manifestations occurred proximal to the Whakatane River in Whakatane. However, analysis of an extensive geotechnical dataset using back-calculated peak ground accelerations and depth to ground water models show that the simplified methodologies predict widespread and severe liquefaction for much of Whakatane. Comparison of observed and predicted manifestations with local geomorphic variability indicates areas of inconsistent prediction occur within the distal flood-plain, while manifestations are accurately predicted within point-bar and paleo-channel deposits. Paleo-liquefaction trenching confirms an absence of liquefaction features in areas where liquefaction was predicted yet not observed, and provides a methodology by which predicted liquefaction extents can be moderated. This paper highlights the potential applications of incorporating historical extents of liquefaction manifestation, with geomorphic mapping, and paleo-liquefaction trenching into liquefaction hazard assessments.
1 INTRODUCTION
Liquefaction and associated lateral spreading poses a significant hazard to the built environment, as highlighted following the 2010-2011 Canterbury Earthquake Sequence (CES), and 2016 Kaikoura earthquake (Quigley, 2017; GEER, 2017). Liquefaction during the CES heightened awareness of the consequences of liquefaction to the built environment and associated financial losses. As a result, revisions to the Resource Management Act (1991) have been proposed which will require councils to better understand natural hazards in their area, including producing relevant hazard maps as part of their planning requirements. Liquefaction hazard maps are typically derived from the identification of low lying Quaternary aged alluvial deposits from geological maps, then further refined using simplified CPT- and SPT-based liquefaction triggering procedures on available geotechnical data. Historical cases of liquefaction are often not considered in the development of these maps, nor are the resultant maps cross-checked with observed performances from historical earthquakes.
Historical records indicate that upwards of 13 earthquakes have triggered liquefaction within parts of New Zealand prior to the CES, resulting in damage to the built environment (indicated in Figure 1; e.g. Fairless and Berrill, 1984). Much of the post-event literature indicates that localized liquefaction occurred proximal to waterways during these events, however recent liquefaction hazard assessments suggests that soils with low cyclic resistances to liquefaction are widespread throughout the urban centres in New Zealand. Recent studies following the 2010-2011 Canterbury earthquake sequence (CES) have shown inconsistencies between the predicted and observed extents of liquefaction manifestation (i.e. van Ballegooy, 2016). Many of the areas of over-prediction have been shown to correspond with silty back-swamps distal the rivers within the region (Beyzaei et al., 2017). The potential over-prediction of liquefaction hazards may result in unnecessary restrictions on land development and retrofitting of existing infrastructure, and may direct efforts away from areas that are truly susceptible to liquefaction.
