Scientists mapping the seabed under Cook Strait have found last month's magnitude 6.5 earthquake was triggered by previously unknown faults beneath the earth's crust.
GNS duty seismologist Caroline Little said an initial magnitude 5.7 quake on 19 July happened on the interface between the Australian and Pacific plates. Subsequent ones were on invisible faults that have not been mapped.
Ms Little said scientists from GNS and NIWA are now studying these faults and there are likely to be other, similar faults in Cook Strait.
More than 1000 aftershocks have been recorded in the region during the past three weeks. There remains a 3% chance of an aftershock of magnitude 6.0 or higher.