Study reveals 166 offshore earthquake sources in Bay of Plenty

The number of known offshore earthquake sources in the Bay of Plenty has been drastically revised.

In a report to Environment Bay of Plenty, NIWA has identified 166 offshore earthquake sources in the region. A national 'probabilistic seismic hazard assessment' published in 2000 incorporated just 14 offshore earthquake sources for the bay.

"We are recommending that the earthquake and tsunami hazard be re-evaluated based on this new information," says NIWA marine geologist Dr Geoffroy Lamarche. "We suspect our new results mean the earthquake hazard in the central and eastern Bay of Plenty coastal region is greater than previously thought, while the expected hazard in eastern Coromandel is probably lower."

Bryan Riesterer, Environment Bay of Plenty strategic policy committee chairman, has welcomed the report. "It is important that local authorities have an accurate understanding of the seismic potential of our region, both on-shore and off-shore," he says. "This understanding will translate into more knowledgeable land use planning, improved security for infrastructure projects, and better informed planning processes for our civil defence functions."

Five years of intensive investigation

NIWA’s results are the product of a major five-year study of active offshore faulting in the area between Cape Runaway and the Coromandel Peninsula, from the coast to approximately 100 kilometres offshore.

"We have had to interpret more than 8000 kilometres of seismic reflection profiles, and more than 11 000 square kilometres of state-of-the-art multibeam bathymetric data which we've collected since 1999. And we've combined that with almost 20 years of archived data," says Dr Lamarche.

The seismic reflection profiles give Dr Lamarche and his colleagues at NIWA a picture of the faults beneath the seafloor, how long they are, and how they connect up with each other. The scientists actually identified more than 1000 active fault traces, which they grouped into the 166 earthquake sources based on the potential linkage between the faults, kilometres beneath the seafloor.

The majority of the faults are of the same type as that which ruptured across the Rangitaiki Plain in the 1987 Edgecumbe earthquake.

NIWA made very precise measurements of the movement of some faults near the surface of the seabed. They used these measurements and the age of the faulted sediments to estimate earthquake 'recurrence' – how often (as a long-term average) an earthquake occurs on a given fault.

NIWA scientists have also estimated the 'maximum credible magnitude' of earthquakes which could be generated by each source. They did this by looking at the size of the newly discovered faults in comparison with other offshore faults around the world and with faults on shore in the central North Island.

Big quakes possible

The maximum credible magnitude for quakes from the offshore earthquake sources range between magnitudes 5.7 and 7.1. More than 75 percent of the earthquake sources could generate earthquake magnitudes of 6.0 to 6.6, while 40 percent are associated with magnitudes of 6.9 and larger. To put this in perspective, the 1987 Edgecumbe earthquake was a magnitude 6.5.

While this may sound alarming, NIWA estimates the recurrence intervals for the offshore earthquake sources range up to 58,000 years. The average is about 3000 years.

Forty-one earthquake sources close to shore

There are 41 earthquake sources identified within 20 kilometres of the Bay of Plenty coast. At least seven of these may extend across the coastline along known active faults near Omarumutu, Opotiki, Ohope, Whakatane, and Matata. They could generate earthquakes up to magnitudes 6.6 to 7.1 but the recurrence intervals are more than 1000 years.

The report was funded by Environment Bay of Plenty. The fundamental science behind it was funded by the Foundation for Research, Science & Technology.