New Liquefaction Hazard Assessment report – what does it mean for your property?

A new updated report has been released detailing the risk of earthquake-induced liquefaction in Hawke’s Bay. The report gives greater insight into how properties may be affected in the event of an earthquake.

Since 1840, Liquefaction effects have been reported during four of the seven earthquakes having produced ‘strong shaking’ in Hawke’s Bay (Hawke’s Bay Civil Defence, 2017). There are areas of Hawke’s Bay underlain by potentially liquefiable soils, and there are several earthquake faults within or near the area capable of generating strong earthquakes. Low lying areas of Hawke’s Bays, particularly near the coast, and reclaimed land are particularly susceptible to liquefaction.

The 2017 report refines previous work being more specific and is a worthwhile reference for property owners, tenants, developers and investors concerned with potential seismic risks. The report is available here and a FAQ sheet available here. Councils are currently reviewing the updated hazard information and how it will be incorporated into building and resource consent applications and District plans.

The Hawke’s Bay Civil Defence Emergency Management Group in conjunction with the region’s five Councils commissioned the GNS Science report, which builds upon the historic GNS Liquefaction study carried out in the Hawke’s Bay region in 1999.

The report concludes that there is little change in mapped liquefaction susceptibility to Wairoa, Napier, Hastings, and Havelock North with a reduction in the mapped liquefaction hazard in parts of Taradale and a significant reduction in the hazard assessed in Central Hawkes Bay.

Liquefaction has been widely recognized as one of the principle earthquake hazards (Department of Civil Engineering, Universirty of Canterbuy, 2014). ‘Liquefaction is the process where, during earthquake shaking, sand and silt grains in wet soil are rearranged and the water in the spaces between the grains is squeezed. Pressure builds up until the silt and sand grains float in the water, and the soil behaves more like a liquid than a solid. The pressurized water is forced up to the ground surface through the easiest path it can find – often through cracks and crevasses in the ground or concrete. The water takes silt and sand with it, forming boils or volcanoes. The ground surface above liquefied soil often tilts and sinks.’ (ECAN, 2017) As a result of earthquake induced liquefaction, buildings, roads, pipes and other associated infrastructure can be damaged by the tilting or sinking of the ground.

‘Lessons learned about liquefaction after the Canterbury earthquakes has allowed scientists to significantly refine and improve existing liquefaction susceptibility maps for the Hawke’s Bay region, using existing geological, geomorphological, groundwater and geotechnical conditions’ (Hawkes Bay Emergency Management, 2017). The report noted that liquefaction effects had been reported in the Hawke’s Bay during historical earthquake events with ‘ground damage including sand boils, water ejection, subsidence and fissuring and lateral spreading’ (New Zealand Herald, 2017).

Hawke’s Bay is considered one of the most seismically active, with several large and damaging earthquakes having occurred since records began. ‘Hawkes Bay is located on the Australian Plate, about 150km west of the Hikurangi Trough, which marks the subduction boundary between the Pacific and Australian Plates’. (Hawkes Bay Emergency Management, 2017). This means that Hawke’s Bay is located in a zone of high deformation, with a high susceptibility to earthquakes. The 1931 earthquake is considered the most notable of these events causing considerable damage, killing 256 people and significantly altering the landscape.

1.0      References

Department of Civil Engineering, Universirty of Canterbuy. (2014). Key Facotrs in the Liquefaction-induced damange to buildings and infrastructure in Christchurch: Preliminary findings. 2014 NZSEE Conference.

ECAN. (2017, December 19). Environment Canterbury Regional Council. Retrieved from https://www.ecan.govt.nz/your-region/your-environment/natural-hazards/earthquakes/liquefaction/

Hawke’s Bay Civil Defence. (2017, 12 5). Hawke’s Bay Liquefaction Hazard Report – Frequently Asked Questions. Retrieved from http://www.hbemergency.govt.nz/assets/Hazard-Information-Portal/Liquefaction-Risk-FAQs.pdf

Hawkes Bay Emergency Management. (2017, December 19). Media Releases. Retrieved from http://www.hbemergency.govt.nz/community/media-releases

New Zealand Herald. (2017, November 19). Retrieved from http://www.nzherald.co.nz/hawkes-bay-today/news/article.cfm?c_id=1503462&objectid=11950300

 

 

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