Liquefaction analysis for selected locations in the Napier-Hastings area, New Zealand

Griffin, A.G. 2024 Liquefaction analysis for selected locations in the Napier-Hastings area, New Zealand. Lower Hutt, NZ: GNS Science. GNS Science report 2024/07. 34 p.; doi: 10.21420/RA04-M404

Abstract

The Hikurangi Subduction Zone (HSZ) off the east coast of the North Island is regarded as capable of generating great earthquakes (magnitude &#8805;8). Such an event would likely produce a cascading array of hazardous consequences for the Hawke’s Bay region, especially on the alluvial plains in the vicinities of the cities of Hastings and Napier (the region’s most populous city). The largest historic seismic event to affect the region was the 1931 Hawke’s Bay (Napier) earthquake, which caused widespread damage, including liquefaction. An often under-recognised earthquake-related phenomenon, liquefaction has been documented from several large earthquakes around New Zealand since European settlement (~AD 1840), but it was the 2010–2011 Canterbury Earthquake Sequence that brought it to the fore of public awareness. Liquefaction susceptibility assessments have now become a standard part of understanding the hazards and mitigating the risks arising from large to great earthquakes. Liquefaction is a phenomenon whereby ground-forming materials lose much of their strength. It a phenomenon associated with strong earthquake shaking of poorly consolidated fine-grained (silt to sand) sediments that are saturated with groundwater. Liquefaction susceptibility is characteristic of relatively young (less than ~10,000 years old) sediments underlying coastal plains and river or stream valleys, typically deposited in low energy settings such as lagoons, estuaries and low-gradient fluvial systems. Ejection of liquefied sediment onto the ground surface can cause damaging amounts of differential ground deformation, including vertical settlement and lateral displacement, to the detriment of human constructions on or in the ground, such as buildings, pipework and engineered (and non engineered) fills. A previous liquefaction susceptibility assessment in 2017 used geological and geomorphological maps to refine and improve existing liquefaction susceptibility maps for the Hawke’s Bay region. To enhance the confidence in the 2017 maps as an input to cascading risk modelling for HSZ earthquake scenarios, additional selected publicly available cone penetration tests (CPTs) were analysed for liquefaction susceptibility, using the existing parameters from the 2017 work and new knowledge gained since then. This report details the workflow and results from liquefaction-triggering assessments on the 17 selected CPT sites in the Napier and Hastings areas. Assuming all the requirements for soil to liquefy are met, the probability of liquefaction occurring (and the severity of it) is dependent on the earthquake location and magnitude, and the consequent peak ground acceleration (PGA) values at the CPT site. The results from this study suggest that liquefaction is likely to occur at each of the 17 CPT sites in all four earthquake scenarios, at PGAs &#8805;0.2 g, and that the severity of damage will vary from little (LSN <10) to moderate to major (LSN 30–40). Extrapolating liquefaction susceptibility between CPT sites is not encouraged, as soil conditions can vary within a few metres, and therefore affect the probability of liquefaction occurring. (auth)