Liquefaction and associated lateral spreading during the 1987 ML 6.3 Edgecumbe earthquake caused severe damage to parts of Whakatane, New Zealand. Recent studies utilizing extensive CPT investigations indicate that much of the Whakatane Central Business District is underlain by sediments with a low cyclic resistance to liquefaction. However, no evidence of liquefaction was observed in this area following the Edgecumbe earthquake. In this project, evaluation of the extensive existing CPT dataset was undertaken using estimated PGA for the Edgecumbe earthquake and modelled groundwater depths to examine the extent of liquefaction predicted from the CPT-based simplified methods. The predicted Liquefaction Severity Index (LSN) parameter at the CPTs was then compared with that documented from historical reports and resident recollections (Figure 1).
Lateral-spreading fissures associated with the 1987 Edgecumbe earthquake were exposed in trenches at sites known to have liquefied (the yellow shaded areas in Figure 1). The fissures cross-cut fluvial stratigraphy predominantly composed of fine-sand with thin lenses of medium sand, and were sourced from silty fine sand (Figure 2). No evidence of pre-historic liquefaction was observed, indicating that the sediments had not liquefied since their deposition and prior to the Edgecumbe earthquake.
Figure 1: LSN calculated from PGA estimated for the 1987 Edgecumbe earthquake. Documented extents are indicated by black and yellow.
Trenching at sites where severe liquefaction manifestation was predicted, yet not observed, revealed sediments comprising fine sand to silt interbedded with medium to coarse sand with pumice granules. The CPT investigation tool does not capture fine-scale inter-layering, nor does it discriminate between the coarse grained sandy soils with or without pumice granules, this may account for some of the variability between predicted and observed liquefaction. No liquefaction features were observed in the subsurface, indicating that the sediments have not liquefied since their deposition. The interlayered coarser sediment provides high hydraulic connectivity between the fine sand layers, which may enable pore-water dissipation and thus may be another reason for the variability between predicted and observed liquefaction.
This study highlights potential reasoning for the inconsistencies between predicted and observed liquefaction in Whakatane for the 1987 Edgecumbe earthquake. Future work will include cyclic simple shear testing of reconstituted bulk samples from the trenches to further examine the liquefaction resistance of these sediments.
This project is funded by QuakeCoRE (project #16060).
Figure 2: Interpreted field photograph of an exposed lateral spreading fissure.
