Wellington earthquakes may have occurred on unmapped fault line

The Cook Strait earthquakes may have occurred on an unmapped fault line, seismologists believe.

GNS duty seismologist Dr John Ristau said the "big one" people were most concerned with was an earthquake along the Wellington fault - different to the fault where the earthquakes over the weekend occurred.

"Niwa has mapped quite a number of faults out in Cook Strait but we still need to have a closer look at the data to actually refine the location to which fault it might be on if it's even on a known active fault."

There were about 24 known active faults in the Cook Strait and five in the area where the earthquakes occurred. However, the weekend's quakes could have occurred on a fault yet to be mapped, Dr Ristau said.

The faults in Cook Strait were not large enough to generate magnitude-8 earthquakes, but were capable to magnitude 7-7.5 size earthquakes, he said.

It was fortunate that the weekend's earthquakes were offshore and away from population centres, he said.

"To get something devastating like Christchurch the earthquake would likely need to be at least magnitude-7 - similar to the initial Darfield earthquake which started the entire Canterbury sequence."

The damage seen following Christchurch's earthquakes was due to a "perfect storm" of earthquake conditions, Dr Ristau said.

"It had much larger ground accelerations than what you normally would have anticipated for a earthquake of its size."

Also, although the Christchurch earthquakes were not as strong as Sunday's, they were a lot shallower, he said.

"With Christchurch, with where the earthquake was and the direction that the energy was radiated from the earthquake, it was basically aimed straight at Christchurch.

"Then because much of Christchurch is built on very soft soils, that just tended to amplify everything."

The weekend's earthquakes would also have needed to have been bigger than magnitude 7.5 to generate a tsunami, Dr Ristau said.

However, some coastline around Cook Strait had areas identified as potential landslide risk. A smaller earthquake could cause a landslide which could then generate a tsunami.

"The tsunami could be quite devastating to the south coast of the North Island and north coast of the South Island," Dr Ristau said.