Scientists warn of disaster scenario in Bay of Plenty

The Bay of Plenty's earthquake and tsunami risk is far worse than previously thought - forcing scientists to radically revise threat predictions.

Scientists using state-of-the art sensors have discovered more than 1000 active faults that they have grouped into 166 potential sources of earthquakes - 152 more than previously known.

They include a huge field of faultlines capable of delivering shocks as large as the quake that devastated Edgecumbe 19 years ago.

The National Institute of Water and Atmospheric Research (Niwa) report comes after Environment Bay of Plenty stunned developers of the $300 million Papamoa Gateway project by objecting because the tsunami threat had not been properly assessed.

Five years of research into offshore earthquake faults across the Bay has dramatically raised the probability of a tsunami inundating built-up areas along the coast.

Niwa marine geologist Dr Geoffroy Lamarche suspected that the earthquake hazard in the central and eastern Bay of Plenty coastal region was now greater than previously thought.

The release of the report coincided with the 19th anniversary of the 6.5 magnitude Edgecumbe quake which was centred about 4km offshore and 8km deep.

Dr Lamarche said more than 60 of the offshore earthquake sources were likely to generate quakes bigger than the Edgecumbe shake - the most damaging to hit New Zealand in decades.

The offshore quakes would range between magnitudes 5.7 and 7.1, with more than 75 per cent of the sources able to generate earthquakes between magnitude 6.0 and 6.6. About 40 per cent were likely to produce really big quakes of magnitude 6.9 and larger.

Dr Lamarche said it was estimated that a large earthquake would occur on each fault on average every 3000 years - but there were 166 faults.

On one major fault there had been three big quakes in the past 8000 years.

In addition to the Edgecumbe fault, a further 40 earthquake sources had been identified within 20km of the Bay of Plenty coast.

At least seven of these might extend across the coastline along known active faults near Omarumutu, Opotiki, Ohope, Whakatane, and Matata, with the potential to generate quakes of magnitudes 6.6 to 7.1 at intervals of more than 1000 years.

The good news was that the expected hazard along the eastern Coromandel was probably lower.

The scientists estimated the "maximum credible magnitude" of earthquakes that could be generated at each source by looking at the size of the newly discovered faults.

They analysed more than 8000km of seismic reflection profiles, and more than 11,000sq km of multibeam bathymetric data collected since 1999 to build up a detailed picture of the seabed.

Combined with data from the Edgecumbe quake, the seismic reflection profiles showed the faults beneath the seafloor, how long they were, and how they connected.

Most of them were the same type as the one that ruptured across the Rangitaiki Plain in 1987 - "normal" earthquakes where the faults pull directly apart.

On land, similar faults are found through the Taupo Volcanic Zone, where the land is being steadily pulled apart at about an average rate of 5mm a year.

The regional council's strategic policy committee chairman, Bryan Riesterer, welcomed the report, and said the new understanding of the seismic risk would be used to improve land use planning, security for infrastructure projects, and civil defence preparations.

Niwa marine geologists used the January visit of the French oceanographic research vessel Marion Dufresne II to take core samples of seabed sediment on the fault lines.

The study did not cover the in-shore Western Bay area out to Mayor Island and Motiti Island.

Analysis of those cores would give improved estimates of the timing of uplift but would not be available until next year, Dr Lamarche said.

- BAY OF PLENTY TIMES