Whakatane earthquake study will help urban design

A new earthquake study of Whakatane will encourage more effective and informed planning of land use and future developments in the town’s urban area, Environment Bay of Plenty says.

For the study, the Institute of Geological and Nuclear Sciences Ltd analysed various seismic threats and worked out their likely impact on the soils on which the town is built. It also investigated how earthquakes affect different designs of buildings.

Environment Bay of Plenty project engineer Peter Blackwood says the results will be very useful when designing high buildings and critical structures such as bridges, hospitals, schools and flood protection works. He also hopes it will encourage residents to be better prepared for earthquakes. “Earthquakes can happen at any time. We need to be ready for them.”

Mr Blackwood says, while the study reinforced information on the location and nature of the Whakatane fault, its main objective was to determine the nature of the underlying soils and how earthquakes transmitted their energy to the surface. It paid special attention to the threats of amplified ground shaking, landslips, fault hazards and liquefaction. Liquefaction can occur when strong shaking results in the soil losing its strength and behaving more like mud or wet sand.

Russ Martin, civil defence emergency management consultant, gave a detailed presentation on the study’s findings to last week’s operational services committee. Mr Martin told the committee the study’s authors considered three scenario earthquakes to describe the effects in Whakatane:

The least event would be a large earthquake 20 to 50 kilometres away that would generate MM (Modified Mercalli scale) 6 intensity shaking. This type of earthquake occurs about every 10 years. It would be strongly felt, as weak soils near the surface would amplify the shaking, but damage would be minor and there would not be significant liquefaction.

Light to moderate damage and a few casualties could occur in an earthquake nearby in the Taupo Volcanic Zone or one of the other Shear Belt faults such as Waimana or Waiohau. Shaking could reach MM7 to MM8 intensity, similar to the 1987 Edgecumbe Earthquake. Its recurrence interval is around 100 years.

The most damaging event would be an earthquake on the Whakatane fault through the town. Shaking would be very strong at MM9 to MM10, with widespread liquefaction and ground damage to the town, rockfalls and landslides on the cliffs and road cuts. A 2-3 metres high fault scarp would divide the town, with land dropping on the western side of the fault. There would be extensive building damage and some casualties. Damage to infrastructure (pipes and bridges) would be extensive. However, this is a rare earthquake, expected to occur on average about every 1,600 years.

Mr Martin said the study revealed that, in Whakatane’s central urban area, former beach sands lie less than 10 metres below the surface and so may be within reach of building piles. When the overlying floodplain silts and sand dune deposits are weak or loose, the soil has less resistence to liquefaction.

A positive finding was that in the areas of former land dunes, such as the Whakatane Hospital, the water table is low and the potential for liquefaction reduced. The study has classified the ground in Whakatane according to the new building code into “rock”, shallow or stiff soil, and deep or soft soil categories.

Environment Bay of Plenty has passed the study’s results to Whakatane District Council.

Be prepared for earthquakes

Restrain loose objects and valuables in your home

Set up and maintain emergency kits with first aid, food, torches, battery radios, purification tablets, and clothing

Store water in containers

Work out an emergency action plan, including a family assembly point

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