A magnitude 4.9 earthquake at Arthur's Pass overnight was followed by a swarm of aftershocks, all centred on a similar area to the large 1994 earthquake.
GNS said it could be a delayed aftershock from 1994.
That quake, magnitude 6.7, caused 70 landslides and blocked the Otira zig-zag highway.
GNS duty seismologist Caroline Holden said what happened overnight was a classic main shake and aftershock sequence, the largest of which was a magnitude 4. "There is nothing unusual in the sequence."
She did not believe it impacted on the Alpine Fault as it was located further away, where the 1994 large jolt was centred, about 25km south-east of Arthur's Pass, she said.
It was fairly close to State Highway 73, close to a peak called Nervous Knob.
The overnight seismic activity could even be a delayed aftershock from 23 years ago, she said.
The first quake, which was felt by many on the West Coast through to Canterbury, was measured at magnitude 4.9. Arthur's Pass resident John Charles said he felt it as "a bit of a jolt", and wondered if it was at Blenheim or Kaikoura again.
"I thought that was a reasonable one." He did not feel any of the aftershocks.
• 2.59am: 2.8, 25km south-east.
• 3.46am: 2.9.
• 3.49am: 3.4.
• 4.28am: 3.5.
• 4.29am: 4.0.
- Greymouth Star