The area where a strong magnitude 6.0 earthquake struck on Wednesday morning has no known historical quakes above magnitude 5.0, GeoNet says.
The quake was centred in the South Island's Rangitata Gorge area where two identified active faults meet - the Ben McLeod Fault and the Lake Heron Fault.
As the magnitude was relatively small for a major fault, it was probably on a subsurface fault between those two known faults.
GeoNet said the area has no known historical quakes above M5.0.
The nearby large faults are estimated, from landform evidence, to have each ruptured several times in the recent geological past (within the last 18,000 years), but not since European records began.
The epicentre of the earthquake lay south-east of more active regions, and 70 kilometres from the Alpine Fault.
Since it struck at 9.14am, there have been at least 37 aftershocks. The largest was a M3.8 at 1.18pm and a M3.6 which occurred at 9.25am.
Fresh cracks appeared in the walls of the Mt Somers General Store following the quake.
Raewyn at the Mt Somers General Store, inland from Ashburton, said the "fairly good shake" was rather noisy inside the old building and it seemed to go on for a few minutes.
"It made my heart rate go up and you take a few deep breaths and try to calm yourself, with customers coming in you don't want to be seen as being a bit panicky about it, but everyone is safe which is the main thing, we haven't had any reports of any injuries.
"It was quite shaky, quite violent, but nothing came off the shelving, a few things moved around but nothing fell on the floor."
More than 14,000 people have reported feeling the earthquake through GeoNet's website.
The reports came from throughout the South Island with a few as far afield as Auckland.
Find out what happened by looking back over RNZ's liveblog: