New research has revealed that some of the West Coast is sandwiched between enormous offshore fault lines and the Alpine Fault.
They are the type that generate tsunamis, which is bad news for coastal townships because it is unknown when they will rupture.
National Institute of Water and Atmospheric Research (Niwa) scientists today released a report from a two-year mapping project for the West Coast Regional Council, which wanted to assess earthquake and tsunami risk for its coastal communities.
The report identifies 10 active marine faults in a 320-kilometre stretch from Hokitika to Farewell Spit.
That includes three new faults, informally named the Farewell, Elizabeth and Razorback faults, and divides the 250km-long Cape Foulwind Fault into five segments.
The faults run parallel to the coastline within 30km of land, some only a few kilometres offshore, and vary in length from 10km to the longest, Kongahu Fault, at 120km.
Niwa marine geologist Philip Barnes said yesterday the largest could generate major quakes of magnitude-6.5 to 7.8.
National tsunami modelling, prompted by the 2004 Boxing Day Indian Ocean tsunami, showed no marine faults off the West Coast so the new research filled in a huge gap in understanding seismic hazards, he said.
The latest information would be used to update the country's seismic hazard work.
While the faults were relatively large and capable of causing severe quakes, Barnes said the good news was they had extremely long recurrence intervals so would only rupture once every 7500 to 30,000 years.
"But we've got no idea when the last earthquake occurred on any of them. For all we know, that may be very close."
They were compressional faults, which typically would lift the seabed when they ruptured, he said.
Barnes said work on the recent quakes in Canterbury and Seddon showed all faults interacted with nearby faults, which meant marine faults off Hokitika could stir up the nearby Alpine Fault.
The Alpine Fault is New Zealand's largest fault, which spans 600km from Fiordland to Marlborough and ruptures every 330 years on average.
No marine faults were found south of Hokitika down to northern Fiordland and none were further out to sea than 30km.
The report's co-author, Francesca Ghisetti, of geological consultancy company TerraGeoLogica, said the research built on work by previous scientists and data from the oil industry's exploration off the West Coast over several decades.
"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.