A fault line previously believed to be relatively small could be capable of much bigger earthquakes than originally thought.

The National Institute of Water and Atmospheric Research is heading into Cook Strait tomorrow to map the area around the swarm of earthquakes that has been rattling Wellington and Marlborough since Friday.



It is hoped the work will identify the fault line from which the quakes have been generated.



GNS Science seismologist John Ristau said that, because the quakes appeared to be happening between the known Vernon and Needles fault lines, it seemed increasingly likely that the London Hill fault was to blame.



In 2003 a Marlborough District Council geotech report described London Hill as a one of "several relatively small faults near the east coast between Seddon and the Waima River".



Ristau said it now appeared the fault line might be much longer than originally thought, which meant it could be capable of creating much larger earthquakes.



"The larger a fault line is, the larger the earthquake."



It was important scientists were able to determine exactly which fault the earthquake had occurred on, as it would allow them to establish whether other nearby faults could now produce large earthquakes as a result.



It appeared the level of stress had not greatly affected the other Wellington faults, but there were a few lines in the Marlborough region that were capable of creating quakes of magnitude 7 or greater.



"If they increase stress, it could trigger a similar-sized earthquake or even a much larger earthquake ... hopefully by the end of the week we'll be able to be a lot more definitive."



Niwa research ship Tangaroa was diverted from survey work in the nearby Pegasus Basin overnight and will spend part of tomorrow measuring whether the earthquakes have triggered any landslides in the Cook Strait Canyon.



"We have previously identified an area of potential instability in the middle of the canyon and this will give us the opportunity to see if there have been any changes," marine geologist Scott Nodder said.



An area crossing the earthquake epicentre will also be surveyed to see if the sea floor has changed.



Ristau said that mapping would help determine the fault line.



The probability of another big quake, of magnitude 6 or greater, has fallen to 4 per cent within the next day, 13 per cent in the next seven days and 33 per cent in the next year.



The probabilities would continue to fall the longer the region went without a large event, Ristau said.



Aftershocks were likely to continue for about a week.