Earthquake researchers in Wellington have been bouncing soundwaves off the floor of Cook Strait to analyse its seismic history.
The National Institute of Water and Atmospheric Research (NIWA) says the mapping is done by towing a sound-emitting instrument behind a boat.
The sound waves penetrate the sea floor and bounce back to display the structure of faultlines.
NIWA says the team found evidence of eight large earthquakes on one of the underwater faults, the Wairau Fault.
It established that on average the fault ruptures about once every 2200 years, with the last big earthquake occurring 2000 years ago.