Liquefaction case histories from the 1987 Edgecumbe earthquake, New Zealand – Insights from an extensive CPT dataset and paleo-liquefaction trenching

Abstract

The 1987 ML 6.3 Edgecumbe earthquake triggered localized liquefaction and associated lateral spreading in the township of Whakatane in the North Island of New Zealand. Subsequent simplified CPT-based liquefaction assessments undertaken using modelled Peak Ground Accelerations (PGA) and depth to ground-water for the Edgecumbe earthquake predicts moderate-to-severe liquefaction for much of Whakatane. Variations between the predicted and observed liquefaction severities highlights areas of inconsistent prediction. Potential reasoning for the inconsistencies are subsequently examined through trenching and comparison of CPT traces at selected sites where liquefaction was predicted and was observed during the Edgecumbe earthquake, and sites where liquefaction was predicted yet not observed. Variations in depositional age, stratigraphy including thin-scale interlayering of sands, silts, and pumice, and/or sediment composition are shown to be potential contributing factors. Sites where liquefaction was observed are shown to contain thick potentially liquefiable deposits of recently deposited sand and silt, while sites where liquefaction was predicted yet not observed contain thinner and/or inter-layered liquefiable deposits of older sands and silts. The results highlight the benefit of cross-examining predicted liquefaction severities with historical case histories and outline local site conditions that may result in inconsistent liquefaction assessments.