Taranaki liquefaction report – fact sheet

 In May 2012 South Taranaki, Stratford  New Plymouth District Council will and New Plymouth district councils, carry out more detailed investigations Taranaki Regional Council, Powerco (cone penetrometer tests) at five river- and Transpower funded jointly a study mouth areas with residential buildings by GNS Science into liquefaction to confirm or discount the liquefaction hazards in Taranaki. GNS studied hazards. These areas are at Waitara, existing land data held by the councils Onaero, Urenui, Tongaporutu and in conjunction with geological Mohakatino. information held by GNS to identify the  In South Taranaki the river margins of potential for liquefaction and ground Patea, Whenuakura and Waitotara are damage in the region. not subject to development pressure.  GNS Science has found that Taranaki’s However STDC will add the GNS geology is dominated by materials that Science report to its hazard information are not liquefiable. Potential that is used at times of development, liquefaction hazards in Taranaki are such as when a building consent is limited to only a few sites. applied for or when a land information memorandum is produced.  The potential for liquefaction hazards has been identified at Port Taranaki; and at the lower reaches of the What is liquefaction? Mohakatino, Rapanui, Tongaporutu, Mimi, Urenui, Onaero and Waitara  Liquefaction occurs when waterlogged rivers and their tributaries (in New sediments are agitated by seismic Plymouth District); and the lower shaking. This separates the grains from reaches of the Waitotara, Whenuakura each other, reducing their load-bearing and Patea rivers and their tributaries (in capacity. Buildings and other structures South Taranaki). can sink into the ground or tilt over, while underground pipes and tanks may  The hazards are based on long time rise to the surface. periods: Earthquakes strong enough to cause liquefaction damage to land (a  When the vibrations stop the sediments Modified Mercalli Intensity of 8) are settle again, squeezing groundwater out expected, on average, every 150 years of fissures and holes in the ground to at Port Taranaki and between 980 and cause flooding. The aftermath of 1070 years at the river areas. liquefaction can leave large areas covered in a deep layer of mud.  If liquefaction does occur, it is expected to be of low impact – nothing like what Christchurch experienced following the 2010 and 2011 quakes.

May 2013