Liquefaction Hazard in the Taranaki Region

The possibility of earthquake-caused liquefaction in Taranaki is low, with just a few coastal areas potentially having the type of soil that might liquefy.

However those areas would need to be soil-tested before the potential for liquefaction can be confirmed or discounted.

In preparing the report, GNS Science examined existing land data held by South Taranaki, Stratford and New Plymouth district councils and Taranaki Regional Council as well as geological information held by GNS.

The key points in the report are:

  • Taranaki’s geology is dominated by materials that are not liquefiable. Potential liquefaction hazards in Taranaki are limited to only a few sites.
  • The potential for liquefaction hazards has been identified at Port Taranaki; and at the lower reaches of the Mohakatino, Rapanui, Tongaporutu, Mimi, Urenui, Onaero and Waitara rivers and their tributaries (in New Plymouth District); and the lower reaches of the Waitotara, Whenuakura and Patea rivers and their tributaries (in South Taranaki).
  • The hazards are based on long time periods: Earthquakes strong enough to cause liquefaction damage to land (a Modified Mercalli Intensity of 8) are expected, on average, every 150 years at Port Taranaki and between 980 and 1070 years at the river areas.
  • If liquefaction does occur, it is expected to be of low impact – nothing like what Christchurch experienced following the 2010 and 2011 quakes.

New Plymouth District Council will carry out more detailed investigations (cone penetrometer tests) at five river-mouth areas that have urban areas or residential buildings to confirm or discount the liquefaction potential. These areas are at Waitara, Onaero, Urenui, Tongaporutu and Mohakatino.

South Taranaki District Council Environmental Group Manager John McKenzie says STDC will add the GNS report to its hazard information that is used at times of development, such as when a building consent is applied for or when a land information memorandum is produced.

The New Zealand MM – or Modified Mercalli – intensity scale grades the impact of an earthquake on people, which can be more useful as an indicator of the earthquake’s significance to the community than the absolute size of the earthquake itself. More information on the MM intensity scale is available on GeoNet’s website.