Scientists working for New Zealand’s National Institute of Water and Atmospheric Reseach (NIWA) have today released a report from a two-year mapping project for the West Coast Regional Council, which wanted to assess tsunami and earthquake risk for the coastal communities in the area. The report has identified 10 active marine faults in a 320km stretch from Hokitika to Farewell Spit. They are the type of faults that generate tsunamis and the problem is that it is unknown when they will rupture.
The faults run parallel to the coastline within 30km of land, some being only a few kilometres from the shore. They vary in length from just 10km to the massive Kongahu Fault at 120km. The report includes three new faults with the informal names of Farewell, Elizabeth and Razorback. These 10 faults divide the 250km-long Cape Foulwind Fault into five segments.
The faults are large and could potentially cause severe quakes. Philip Barnes, a Niwa marine biologist, said yesterday that the largest could generate major quakes of magnitudes ranging from 6.5 to 7.8. Having said this, Barnes also added some good news by saying that they had extremely long recurrence intervals so would only rupture once every 7,500 to 30,000 years, but not before adding “we’ve got no idea when the last earthquake occurred on any of them. For all we know, that may be very close.”
Barnes said that they were compressional faults. These are faults that would lift the seabed when they ruptured. Recent quakes in Canterbury and Seddon showed that all the faults interacted with other nearby faults, which means that marine faults off Hokitika could potentially activate the nearby Alpine Fault. This could be bad news because the Alpine Fault is New Zealand’s largest at 600km and ruptures every 330 years on average.
Francesca Ghisetti, the co-author of the report said that research was built on work carried out by previous scientists and data gathered over several decades from oil exploration off the West Coast. “However, this is the first time the offshore data have been analysed to determine whether these faults are active and the potential earthquake hazard they pose,” she said.
Port Wellington (NZ) earthquake, July 2013. Image via Getty.
